529 lines
20 KiB
Perl
529 lines
20 KiB
Perl
.\" $NetBSD: 2.t,v 1.3 2003/08/07 10:30:54 agc Exp $
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.\"
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.\" Copyright (c) 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This document is derived from software contributed to Berkeley by
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.\" Rick Macklem at The University of Guelph.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)2.t 8.1 (Berkeley) 6/8/93
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.\"
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.sh 1 "Not Quite NFS, Crash Tolerant Cache Consistency for NFS"
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.pp
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Not Quite NFS (NQNFS) is an NFS like protocol designed to maintain full cache
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consistency between clients in a crash tolerant manner.
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It is an adaptation of the NFS protocol such that the server supports both NFS
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and NQNFS clients while maintaining full consistency between the server and
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NQNFS clients.
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This section borrows heavily from work done on Spritely-NFS [Srinivasan89],
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but uses Leases [Gray89] to avoid the need to recover server state information
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after a crash.
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The reader is strongly encouraged to read these references before
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trying to grasp the material presented here.
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.sh 2 "Overview"
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.pp
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The protocol maintains cache consistency by using a somewhat
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Sprite [Nelson88] like protocol,
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but is based on short term leases\** instead of hard state information
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about open files.
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.(f
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\** A lease is a ticket permitting an activity that is
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valid until some expiry time.
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.)f
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The basic principal is that the protocol will disable client caching of a
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file whenever that file is write shared\**.
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.(f
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\** Write sharing occurs when at least one client is modifying a file while
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other client(s) are reading the file.
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.)f
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Whenever a client wishes to cache data for a file it must hold a valid lease.
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There are three types of leases: read caching, write caching and non-caching.
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The latter type requires that all file operations be done synchronously with
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the server via. RPCs.
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A read caching lease allows for client data caching, but no file modifications
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may be done.
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A write caching lease allows for client caching of writes,
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but requires that all writes be pushed to the server when the lease expires.
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If a client has dirty buffers\**
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.(f
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\** Cached write data is not yet pushed (written) to the server.
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.)f
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when a write cache lease has almost expired, it will attempt to
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extend the lease but is required to push the dirty buffers if extension fails.
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A client gets leases by either doing a \fBGetLease RPC\fR or by piggybacking
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a \fBGetLease Request\fR onto another RPC. Piggybacking is supported for the
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frequent RPCs Getattr, Setattr, Lookup, Readlink, Read, Write and Readdir
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in an effort to minimize the number of \fBGetLease RPCs\fR required.
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All leases are at the granularity of a file, since all NFS RPCs operate on
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individual files and NFS has no intrinsic notion of a file hierarchy.
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Directories, symbolic links and file attributes may be read cached but
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are not write cached.
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The exception here is the attribute file_size, which is updated during cached
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writing on the client to reflect a growing file.
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.pp
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It is the server's responsibility to ensure that consistency is maintained
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among the NQNFS clients by disabling client caching whenever a server file
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operation would cause inconsistencies.
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The possibility of inconsistencies occurs whenever a client has
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a write caching lease and any other client,
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or local operations on the server,
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tries to access the file or when
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a modify operation is attempted on a file being read cached by client(s).
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At this time, the server sends an \fBeviction notice\fR to all clients holding
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the lease and then waits for lease termination.
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Lease termination occurs when a \fBvacated the premises\fR message has been
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received from all the clients that have signed the lease or when the lease
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expires via. timeout.
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The message pair \fBeviction notice\fR and \fBvacated the premises\fR roughly
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correspond to a Sprite server\(->client callback, but are not implemented as an
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actual RPC, to avoid the server waiting indefinitely for a reply from a dead
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client.
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.pp
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Server consistency checking can be viewed as issuing intrinsic leases for a
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file operation for the duration of the operation only. For example, the
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\fBCreate RPC\fR will get an intrinsic write lease on the directory in which
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the file is being created, disabling client read caches for that directory.
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.pp
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By relegating this responsibility to the server, consistency between the
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server and NQNFS clients is maintained when NFS clients are modifying the
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file system as well.\**
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.(f
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\** The NFS clients will continue to be \fIapproximately\fR consistent with
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the server.
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.)f
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.pp
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The leases are issued as time intervals to avoid the requirement of time of day
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clock synchronization. There are three important time constants known to
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the server. The \fBmaximum_lease_term\fR sets an upper bound on lease duration.
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The \fBclock_skew\fR is added to all lease terms on the server to correct for
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differing clock speeds between the client and server and \fBwrite_slack\fR is
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the number of seconds the server is willing to wait for a client with
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an expired write caching lease to push dirty writes.
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.pp
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The server maintains a \fBmodify_revision\fR number for each file. It is
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defined as a unsigned quadword integer that is never zero and that must
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increase whenever the corresponding file is modified on the server.
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It is used
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by the client to determine whether or not cached data for the file is
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stale.
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Generating this value is easier said than done. The current implementation
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uses the following technique, which is believed to be adequate.
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The high order longword is stored in the ufs inode and is initialized to one
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when an inode is first allocated.
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The low order longword is stored in main memory only and is initialized to
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zero when an inode is read in from disk.
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When the file is modified for the first time within a given second of
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wall clock time, the high order longword is incremented by one and
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the low order longword reset to zero.
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For subsequent modifications within the same second of wall clock
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time, the low order longword is incremented. If the low order longword wraps
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around to zero, the high order longword is incremented again.
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Since the high order longword only increments once per second and the inode
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is pushed to disk frequently during file modification, this implies
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0 \(<= Current\(miDisk \(<= 5.
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When the inode is read in from disk, 10
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is added to the high order longword, which ensures that the quadword
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is greater than any value it could have had before a crash.
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This introduces apparent modifications every time the inode falls out of
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the LRU inode cache, but this should only reduce the client caching performance
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by a (hopefully) small margin.
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.sh 2 "Crash Recovery and other Failure Scenarios"
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.pp
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The server must maintain the state of all the current leases held by clients.
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The nice thing about short term leases is that maximum_lease_term seconds
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after the server stops issuing leases, there are no current leases left.
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As such, server crash recovery does not require any state recovery. After
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rebooting, the server refuses to service any RPCs except for writes until
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write_slack seconds after the last lease would have expired\**.
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.(f
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\** The last lease expiry time may be safely estimated as
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"boottime+maximum_lease_term+clock_skew" for machines that cannot store
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it in nonvolatile RAM.
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.)f
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By then, the server would not have any outstanding leases to recover the
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state of and the clients have had at least write_slack seconds to push dirty
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writes to the server and get the server sync'd up to date. After this, the
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server simply services requests in a manner similar to NFS.
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In an effort to minimize the effect of "recovery storms" [Baker91],
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the server replies \fBtry_again_later\fR to the RPCs it is not
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yet ready to service.
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.pp
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After a client crashes, the server may have to wait for a lease to timeout
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before servicing a request if write sharing of a file with a cachable lease
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on the client is about to occur.
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As for the client, it simply starts up getting any leases it now needs. Any
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outstanding leases for that client on the server prior to the crash will either be renewed or expire
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via timeout.
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.pp
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Certain network partitioning failures are more problematic. If a client to
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server network connection is severed just before a write caching lease expires,
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the client cannot push the dirty writes to the server. After the lease expires
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on the server, the server permits other clients to access the file with the
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potential of getting stale data. Unfortunately I believe this failure scenario
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is intrinsic in any delay write caching scheme unless the server is required to
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wait \fBforever\fR for a client to regain contact\**.
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.(f
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\** Gray and Cheriton avoid this problem by using a \fBwrite through\fR policy.
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.)f
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Since the write caching lease has expired on the client,
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it will sync up with the
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server as soon as the network connection has been re-established.
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.pp
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There is another failure condition that can occur when the server is congested.
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The worst case scenario would have the client pushing dirty writes to the server
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but a large request queue on the server delays these writes for more than
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\fBwrite_slack\fR seconds. It is hoped that a congestion control scheme using
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the \fBtry_again_later\fR RPC reply after booting combined with
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the following lease termination rule for write caching leases
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can minimize the risk of this occurrence.
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A write caching lease is only terminated on the server when there are have
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been no writes to the file and the server has not been overloaded during
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the previous write_slack seconds. The server has not been overloaded
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is approximated by a test for sleeping nfsd(s) at the end of the write_slack
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period.
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.sh 2 "Server Disk Full"
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.pp
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There is a serious unresolved problem for delayed write caching with respect to
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server disk space allocation.
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When the disk on the file server is full, delayed write RPCs can fail
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due to "out of space".
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For NFS, this occurrence results in an error return from the close system
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call on the file, since the dirty blocks are pushed on close.
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Processes writing important files can check for this error return
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to ensure that the file was written successfully.
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For NQNFS, the dirty blocks are not pushed on close and as such the client
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may not attempt the write RPC until after the process has done the close
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which implies no error return from the close.
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For the current prototype,
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the only solution is to modify programs writing important
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file(s) to call fsync and check for an error return from it instead of close.
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.sh 2 "Protocol Details"
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.pp
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The protocol specification is identical to that of NFS [Sun89] except for
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the following changes.
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.ip \(bu
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RPC Information
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.(l
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Program Number 300105
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Version Number 1
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.)l
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.ip \(bu
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Readdir_and_Lookup RPC
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.(l
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struct readdirlookargs {
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fhandle file;
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nfscookie cookie;
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unsigned count;
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unsigned duration;
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};
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struct entry {
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unsigned cachable;
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unsigned duration;
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modifyrev rev;
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fhandle entry_fh;
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nqnfs_fattr entry_attrib;
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unsigned fileid;
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filename name;
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nfscookie cookie;
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entry *nextentry;
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};
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union readdirlookres switch (stat status) {
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case NFS_OK:
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struct {
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entry *entries;
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bool eof;
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} readdirlookok;
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default:
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void;
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};
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readdirlookres
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NQNFSPROC_READDIRLOOK(readdirlookargs) = 18;
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.)l
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Reads entries in a directory in a manner analogous to the NFSPROC_READDIR RPC
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in NFS, but returns the file handle and attributes of each entry as well.
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This allows the attribute and lookup caches to be primed.
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.ip \(bu
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Get Lease RPC
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.(l
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struct getleaseargs {
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fhandle file;
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cachetype readwrite;
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unsigned duration;
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};
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union getleaseres switch (stat status) {
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case NFS_OK:
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bool cachable;
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unsigned duration;
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modifyrev rev;
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nqnfs_fattr attributes;
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default:
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void;
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};
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getleaseres
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NQNFSPROC_GETLEASE(getleaseargs) = 19;
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.)l
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Gets a lease for "file" valid for "duration" seconds from when the lease
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was issued on the server\**.
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.(f
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\** To be safe, the client may only assume that the lease is valid
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for ``duration'' seconds from when the RPC request was sent to the server.
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.)f
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The lease permits client caching if "cachable" is true.
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The modify revision level and attributes for the file are also returned.
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.ip \(bu
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Eviction Message
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.(l
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void
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NQNFSPROC_EVICTED (fhandle) = 21;
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.)l
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This message is sent from the server to the client. When the client receives
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the message, it should flush data associated with the file represented by
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"fhandle" from its caches and then send the \fBVacated Message\fR back to
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the server. Flushing includes pushing any dirty writes via. write RPCs.
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.ip \(bu
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Vacated Message
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.(l
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void
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NQNFSPROC_VACATED (fhandle) = 20;
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.)l
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This message is sent from the client to the server in response to the
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\fBEviction Message\fR. See above.
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.ip \(bu
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Access RPC
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.(l
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struct accessargs {
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fhandle file;
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bool read_access;
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bool write_access;
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bool exec_access;
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};
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stat
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NQNFSPROC_ACCESS(accessargs) = 22;
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.)l
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The access RPC does permission checking on the server for the given type
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of access required by the client for the file.
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Use of this RPC avoids accessibility problems caused by client->server uid
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mapping.
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.ip \(bu
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Piggybacked Get Lease Request
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.pp
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The piggybacked get lease request is functionally equivalent to the Get Lease
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RPC except that is attached to one of the other NQNFS RPC requests as follows.
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A getleaserequest is prepended to all of the request arguments for NQNFS
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and a getleaserequestres is inserted in all NFS result structures just after
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the "stat" field only if "stat == NFS_OK".
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.(l
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union getleaserequest switch (cachetype type) {
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case NQLREAD:
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case NQLWRITE:
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unsigned duration;
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default:
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void;
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};
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union getleaserequestres switch (cachetype type) {
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case NQLREAD:
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case NQLWRITE:
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bool cachable;
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unsigned duration;
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modifyrev rev;
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default:
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void;
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};
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.)l
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The get lease request applies to the file that the attached RPC operates on
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and the file attributes remain in the same location as for the NFS RPC reply
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structure.
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.ip \(bu
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Three additional "stat" values
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.pp
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Three additional values have been added to the enumerated type "stat".
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.(l
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NQNFS_EXPIRED=500
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NQNFS_TRYLATER=501
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NQNFS_AUTHERR=502
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.)l
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The "expired" value indicates that a lease has expired.
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The "try later"
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value is returned by the server when it wishes the client to retry the
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RPC request after a short delay. It is used during crash recovery (Section 2)
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and may also be useful for server congestion control.
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The "authetication error" value is returned for kerberized mount points to
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indicate that there is no cached authentication mapping and a Kerberos ticket
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for the principal is required.
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.sh 2 "Data Types"
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.ip \(bu
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cachetype
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.(l
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enum cachetype {
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NQLNONE = 0,
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NQLREAD = 1,
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NQLWRITE = 2
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};
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.)l
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Type of lease requested. NQLNONE is used to indicate no piggybacked lease
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request.
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.ip \(bu
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modifyrev
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.(l
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typedef unsigned hyper modifyrev;
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.)l
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The "modifyrev" is a unsigned quadword integer value that is never zero
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and increases every time the corresponding file is modified on the server.
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.ip \(bu
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nqnfs_time
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.(l
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struct nqnfs_time {
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unsigned seconds;
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unsigned nano_seconds;
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};
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.)l
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For NQNFS times are handled at nano second resolution instead of micro second
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resolution for NFS.
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.ip \(bu
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nqnfs_fattr
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.(l
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struct nqnfs_fattr {
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ftype type;
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unsigned mode;
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unsigned nlink;
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unsigned uid;
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unsigned gid;
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unsigned hyper size;
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unsigned blocksize;
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unsigned rdev;
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unsigned hyper bytes;
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unsigned fsid;
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unsigned fileid;
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nqnfs_time atime;
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nqnfs_time mtime;
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nqnfs_time ctime;
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unsigned flags;
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unsigned generation;
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modifyrev rev;
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};
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.)l
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The nqnfs_fattr structure is modified from the NFS fattr so that it stores
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the file size as a 64bit quantity and the storage occupied as a 64bit number
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of bytes. It also has fields added for the 4.4BSD va_flags and va_gen fields
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as well as the file's modify rev level.
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.ip \(bu
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nqnfs_sattr
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.(l
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struct nqnfs_sattr {
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unsigned mode;
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unsigned uid;
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unsigned gid;
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unsigned hyper size;
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nqnfs_time atime;
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nqnfs_time mtime;
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unsigned flags;
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unsigned rdev;
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};
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.)l
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The nqnfs_sattr structure is modified from the NFS sattr structure in the
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same manner as fattr.
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.lp
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The arguments to several of the NFS RPCs have been modified as well. Mostly,
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these are minor changes to use 64bit file offsets or similar. The modified
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argument structures follow.
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.ip \(bu
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Lookup RPC
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.(l
|
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struct lookup_diropargs {
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unsigned duration;
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fhandle dir;
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filename name;
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};
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union lookup_diropres switch (stat status) {
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case NFS_OK:
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struct {
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union getleaserequestres lookup_lease;
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fhandle file;
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nqnfs_fattr attributes;
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} lookup_diropok;
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default:
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void;
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};
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.)l
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The additional "duration" argument tells the server to get a lease for the
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name being looked up if it is non-zero and the lease is specified
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in "lookup_lease".
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.ip \(bu
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Read RPC
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.(l
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struct nqnfs_readargs {
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fhandle file;
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unsigned hyper offset;
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unsigned count;
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};
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.)l
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.ip \(bu
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Write RPC
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.(l
|
|
struct nqnfs_writeargs {
|
|
fhandle file;
|
|
unsigned hyper offset;
|
|
bool append;
|
|
nfsdata data;
|
|
};
|
|
.)l
|
|
The "append" argument is true for apeend only write operations.
|
|
.ip \(bu
|
|
Get Filesystem Attributes RPC
|
|
.(l
|
|
union nqnfs_statfsres (stat status) {
|
|
case NFS_OK:
|
|
struct {
|
|
unsigned tsize;
|
|
unsigned bsize;
|
|
unsigned blocks;
|
|
unsigned bfree;
|
|
unsigned bavail;
|
|
unsigned files;
|
|
unsigned files_free;
|
|
} info;
|
|
default:
|
|
void;
|
|
};
|
|
.)l
|
|
The "files" field is the number of files in the file system and the "files_free"
|
|
is the number of additional files that can be created.
|
|
.sh 1 "Summary"
|
|
.pp
|
|
The configuration and tuning of an NFS environment tends to be a bit of a
|
|
mystic art, but hopefully this paper along with the man pages and other
|
|
reading will be helpful. Good Luck.
|