264 lines
10 KiB
Perl
264 lines
10 KiB
Perl
.\" $NetBSD: 2.t,v 1.3 2003/08/07 10:04:22 agc Exp $
|
|
.\"
|
|
.\" Copyright (c) 1982, 1993
|
|
.\" The Regents of the University of California. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\" 3. Neither the name of the University nor the names of its contributors
|
|
.\" may be used to endorse or promote products derived from this software
|
|
.\" without specific prior written permission.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
.\" SUCH DAMAGE.
|
|
.\"
|
|
.\" @(#)2.t 8.1 (Berkeley) 6/5/93
|
|
.\"
|
|
.ds RH Overview of the file system
|
|
.NH
|
|
Overview of the file system
|
|
.PP
|
|
The file system is discussed in detail in [Mckusick84];
|
|
this section gives a brief overview.
|
|
.NH 2
|
|
Superblock
|
|
.PP
|
|
A file system is described by its
|
|
.I "super-block" .
|
|
The super-block is built when the file system is created (\c
|
|
.I newfs (8))
|
|
and never changes.
|
|
The super-block
|
|
contains the basic parameters of the file system,
|
|
such as the number of data blocks it contains
|
|
and a count of the maximum number of files.
|
|
Because the super-block contains critical data,
|
|
.I newfs
|
|
replicates it to protect against catastrophic loss.
|
|
The
|
|
.I "default super block"
|
|
always resides at a fixed offset from the beginning
|
|
of the file system's disk partition.
|
|
The
|
|
.I "redundant super blocks"
|
|
are not referenced unless a head crash
|
|
or other hard disk error causes the default super-block
|
|
to be unusable.
|
|
The redundant blocks are sprinkled throughout the disk partition.
|
|
.PP
|
|
Within the file system are files.
|
|
Certain files are distinguished as directories and contain collections
|
|
of pointers to files that may themselves be directories.
|
|
Every file has a descriptor associated with it called an
|
|
.I "inode".
|
|
The inode contains information describing ownership of the file,
|
|
time stamps indicating modification and access times for the file,
|
|
and an array of indices pointing to the data blocks for the file.
|
|
In this section,
|
|
we assume that the first 12 blocks
|
|
of the file are directly referenced by values stored
|
|
in the inode structure itself\(dg.
|
|
.FS
|
|
\(dgThe actual number may vary from system to system, but is usually in
|
|
the range 5-13.
|
|
.FE
|
|
The inode structure may also contain references to indirect blocks
|
|
containing further data block indices.
|
|
In a file system with a 4096 byte block size, a singly indirect
|
|
block contains 1024 further block addresses,
|
|
a doubly indirect block contains 1024 addresses of further single indirect
|
|
blocks,
|
|
and a triply indirect block contains 1024 addresses of further doubly indirect
|
|
blocks (the triple indirect block is never needed in practice).
|
|
.PP
|
|
In order to create files with up to
|
|
2\(ua32 bytes,
|
|
using only two levels of indirection,
|
|
the minimum size of a file system block is 4096 bytes.
|
|
The size of file system blocks can be any power of two
|
|
greater than or equal to 4096.
|
|
The block size of the file system is maintained in the super-block,
|
|
so it is possible for file systems of different block sizes
|
|
to be accessible simultaneously on the same system.
|
|
The block size must be decided when
|
|
.I newfs
|
|
creates the file system;
|
|
the block size cannot be subsequently
|
|
changed without rebuilding the file system.
|
|
.NH 2
|
|
Summary information
|
|
.PP
|
|
Associated with the super block is non replicated
|
|
.I "summary information" .
|
|
The summary information changes
|
|
as the file system is modified.
|
|
The summary information contains
|
|
the number of blocks, fragments, inodes and directories in the file system.
|
|
.NH 2
|
|
Cylinder groups
|
|
.PP
|
|
The file system partitions the disk into one or more areas called
|
|
.I "cylinder groups".
|
|
A cylinder group is comprised of one or more consecutive
|
|
cylinders on a disk.
|
|
Each cylinder group includes inode slots for files, a
|
|
.I "block map"
|
|
describing available blocks in the cylinder group,
|
|
and summary information describing the usage of data blocks
|
|
within the cylinder group.
|
|
A fixed number of inodes is allocated for each cylinder group
|
|
when the file system is created.
|
|
The current policy is to allocate one inode for each 2048
|
|
bytes of disk space;
|
|
this is expected to be far more inodes than will ever be needed.
|
|
.PP
|
|
All the cylinder group bookkeeping information could be
|
|
placed at the beginning of each cylinder group.
|
|
However if this approach were used,
|
|
all the redundant information would be on the top platter.
|
|
A single hardware failure that destroyed the top platter
|
|
could cause the loss of all copies of the redundant super-blocks.
|
|
Thus the cylinder group bookkeeping information
|
|
begins at a floating offset from the beginning of the cylinder group.
|
|
The offset for
|
|
the
|
|
.I "i+1" st
|
|
cylinder group is about one track further
|
|
from the beginning of the cylinder group
|
|
than it was for the
|
|
.I "i" th
|
|
cylinder group.
|
|
In this way,
|
|
the redundant
|
|
information spirals down into the pack;
|
|
any single track, cylinder,
|
|
or platter can be lost without losing all copies of the super-blocks.
|
|
Except for the first cylinder group,
|
|
the space between the beginning of the cylinder group
|
|
and the beginning of the cylinder group information stores data.
|
|
.NH 2
|
|
Fragments
|
|
.PP
|
|
To avoid waste in storing small files,
|
|
the file system space allocator divides a single
|
|
file system block into one or more
|
|
.I "fragments".
|
|
The fragmentation of the file system is specified
|
|
when the file system is created;
|
|
each file system block can be optionally broken into
|
|
2, 4, or 8 addressable fragments.
|
|
The lower bound on the size of these fragments is constrained
|
|
by the disk sector size;
|
|
typically 512 bytes is the lower bound on fragment size.
|
|
The block map associated with each cylinder group
|
|
records the space availability at the fragment level.
|
|
Aligned fragments are examined
|
|
to determine block availability.
|
|
.PP
|
|
On a file system with a block size of 4096 bytes
|
|
and a fragment size of 1024 bytes,
|
|
a file is represented by zero or more 4096 byte blocks of data,
|
|
and possibly a single fragmented block.
|
|
If a file system block must be fragmented to obtain
|
|
space for a small amount of data,
|
|
the remainder of the block is made available for allocation
|
|
to other files.
|
|
For example,
|
|
consider an 11000 byte file stored on
|
|
a 4096/1024 byte file system.
|
|
This file uses two full size blocks and a 3072 byte fragment.
|
|
If no fragments with at least 3072 bytes
|
|
are available when the file is created,
|
|
a full size block is split yielding the necessary 3072 byte
|
|
fragment and an unused 1024 byte fragment.
|
|
This remaining fragment can be allocated to another file, as needed.
|
|
.NH 2
|
|
Updates to the file system
|
|
.PP
|
|
Every working day hundreds of files
|
|
are created, modified, and removed.
|
|
Every time a file is modified,
|
|
the operating system performs a
|
|
series of file system updates.
|
|
These updates, when written on disk, yield a consistent file system.
|
|
The file system stages
|
|
all modifications of critical information;
|
|
modification can
|
|
either be completed or cleanly backed out after a crash.
|
|
Knowing the information that is first written to the file system,
|
|
deterministic procedures can be developed to
|
|
repair a corrupted file system.
|
|
To understand this process,
|
|
the order that the update
|
|
requests were being honored must first be understood.
|
|
.PP
|
|
When a user program does an operation to change the file system,
|
|
such as a
|
|
.I write ,
|
|
the data to be written is copied into an internal
|
|
.I "in-core"
|
|
buffer in the kernel.
|
|
Normally, the disk update is handled asynchronously;
|
|
the user process is allowed to proceed even though
|
|
the data has not yet been written to the disk.
|
|
The data,
|
|
along with the inode information reflecting the change,
|
|
is eventually written out to disk.
|
|
The real disk write may not happen until long after the
|
|
.I write
|
|
system call has returned.
|
|
Thus at any given time, the file system,
|
|
as it resides on the disk,
|
|
lags the state of the file system represented by the in-core information.
|
|
.PP
|
|
The disk information is updated to reflect the in-core information
|
|
when the buffer is required for another use,
|
|
when a
|
|
.I sync (2)
|
|
is done (at 30 second intervals) by
|
|
.I "/etc/update" "(8),"
|
|
or by manual operator intervention with the
|
|
.I sync (8)
|
|
command.
|
|
If the system is halted without writing out the in-core information,
|
|
the file system on the disk will be in an inconsistent state.
|
|
.PP
|
|
If all updates are done asynchronously, several serious
|
|
inconsistencies can arise.
|
|
One inconsistency is that a block may be claimed by two inodes.
|
|
Such an inconsistency can occur when the system is halted before
|
|
the pointer to the block in the old inode has been cleared
|
|
in the copy of the old inode on the disk,
|
|
and after the pointer to the block in the new inode has been written out
|
|
to the copy of the new inode on the disk.
|
|
Here,
|
|
there is no deterministic method for deciding
|
|
which inode should really claim the block.
|
|
A similar problem can arise with a multiply claimed inode.
|
|
.PP
|
|
The problem with asynchronous inode updates
|
|
can be avoided by doing all inode deallocations synchronously.
|
|
Consequently,
|
|
inodes and indirect blocks are written to the disk synchronously
|
|
(\fIi.e.\fP the process blocks until the information is
|
|
really written to disk)
|
|
when they are being deallocated.
|
|
Similarly inodes are kept consistent by synchronously
|
|
deleting, adding, or changing directory entries.
|
|
.ds RH Fixing corrupted file systems
|