Kernels and tools understand both v1 and v2 filesystems; newfs_lfs
generates v2 by default. Changes for the v2 layout include:
- Segments of non-PO2 size and arbitrary block offset, so these can be
matched to convenient physical characteristics of the partition (e.g.,
stripe or track size and offset).
- Address by fragment instead of by disk sector, paving the way for
non-512-byte-sector devices. In theory fragments can be as large
as you like, though in reality they must be smaller than MAXBSIZE in size.
- Use serial number and filesystem identifier to ensure that roll-forward
doesn't get old data and think it's new. Roll-forward is enabled for
v2 filesystems, though not for v1 filesystems by default.
- The inode free list is now a tailq, paving the way for undelete (undelete
is not yet implemented, but can be without further non-backwards-compatible
changes to disk structures).
- Inode atime information is kept in the Ifile, instead of on the inode;
that is, the inode is never written *just* because atime was changed.
Because of this the inodes remain near the file data on the disk, rather
than wandering all over as the disk is read repeatedly. This speeds up
repeated reads by a small but noticeable amount.
Other changes of note include:
- The ifile written by newfs_lfs can now be of arbitrary length, it is no
longer restricted to a single indirect block.
- Fixed an old bug where ctime was changed every time a vnode was created.
I need to look more closely to make sure that the times are only updated
during write(2) and friends, not after-the-fact during a segment write,
and certainly not by the cleaner.
bandwidth and seek time of the disk, using the "4 * bandwidth * seek
time" formula from Neefe-Matthews' 1997 paper. An RZ25 disk with this
option gets 200K segments. Reference the paper in the manual page.
Also, change the segment size report to include the total size of the disk,
similar to newfs, e.g.
newfs_lfs -N -F -B 65536 /dev/rsd0b
272.7MB in 4363 segments of size 65536
super-block backups (for fsck -b #) at:
16, 55824, 111632, 167440, 223248, 279056, 334864, 390672, 446480, 502288
Kernel:
* Add runtime quantity lfs_ravail, the number of disk-blocks reserved
for writing. Writes to the filesystem first reserve a maximum amount
of blocks before their write is allowed to proceed; after the blocks
are allocated the reserved total is reduced by a corresponding amount.
If the lfs_reserve function cannot immediately reserve the requested
number of blocks, the inode is unlocked, and the thread sleeps until
the cleaner has made enough space available for the blocks to be
reserved. In this way large files can be written to the filesystem
(or, smaller files can be written to a nearly-full but thoroughly
clean filesystem) and the cleaner can still function properly.
* Remove explicit switching on dlfs_minfreeseg from the kernel code; it
is now merely a fs-creation parameter used to compute dlfs_avail and
dlfs_bfree (and used by fsck_lfs(8) to check their accuracy). Its
former role is better assumed by a properly computed dlfs_avail.
* Bounds-check inode numbers submitted through lfs_bmapv and lfs_markv.
This prevents a panic, but, if the cleaner is feeding the filesystem
the wrong data, you are still in a world of hurt.
* Cleanup: remove explicit references of DEV_BSIZE in favor of
btodb()/dbtob().
lfs_cleanerd:
* Make -n mean "send N segments' blocks through a single call to
lfs_markv". Previously it had meant "clean N segments though N calls
to lfs_markv, before looking again to see if more need to be cleaned".
The new behavior gives better packing of direct data on disk with as
little metadata as possible, largely alleviating the problem that the
cleaner can consume more disk through inefficient use of metadata than
it frees by moving dirty data away from clean "holes" to produce
entirely clean segments.
* Make -b mean "read as many segments as necessary to write N segments
of dirty data back to disk", rather than its former meaning of "read
as many segments as necessary to free N segments worth of space". The
new meaning, combined with the new -n behavior described above,
further aids in cleaning storage efficiency as entire segments can be
written at once, using as few blocks as possible for segment summaries
and inode blocks.
* Make the cleaner take note of segments which could not be cleaned due
to error, and not attempt to clean them until they are entirely free
of dirty blocks. This prevents the case in which a cleanerd running
with -n 1 and without -b (formerly the default) would spin trying
repeatedly to clean a corrupt segment, while the remaining space
filled and deadlocked the filesystem.
* Update the lfs_cleanerd manual page to describe all the options,
including the changes mentioned here (in particular, the -b and -n
flags were previously undocumented).
fsck_lfs:
* Check, and optionally fix, lfs_avail (to an exact figure) and
lfs_bfree (within a margin of error) in pass 5.
newfs_lfs:
* Reduce the default dlfs_minfreeseg to 1/20 of the total segments.
* Add a warning if the sgs disklabel field is 16 (the default for FFS'
cpg, but not usually desirable for LFS' sgs: 5--8 is a better range).
* Change the calculation of lfs_avail and lfs_bfree, corresponding to
the kernel changes mentioned above.
mount_lfs:
* Add -N and -b options to pass corresponding -n and -b options to
lfs_cleanerd.
* Default to calling lfs_cleanerd with "-b -n 4".
[All of these changes were largely tested in the 1.5 branch, with the
idea that they (along with previous un-pulled-up work) could be applied
to the branch while it was still in ALPHA2; however my test system has
experienced corruption on another filesystem (/dev/console has gone
missing :^), and, while I believe this unrelated to the LFS changes, I
cannot with good conscience request that the changes be pulled up.]
kernel.
Don't make more superblock segments than we have a record of in
the superblock; and print these out as we go, like newfs.
Add am "-M" flag to specify the number of reserved segments, with a note
in the man page not to use it.
newfs_lfs gives lfs_minfreeseg a value of 1/8 of the total segments on
the disk, based on rough empirical data, but this should be refined in
the future.
Set MINFREE to 80, since that's a more reasonable value according to the
literature than FFS' 90. Remove a bunch of other unused FFS cruft from
config.h.
Initialize lfs_bfree correctly vis-a-vis MIN_FREE_SEGS, so the
filesystem doesn't over-represent the amount of free space it has.
Initialize lfs_dmeta so the kernel can estimate starting from a
reasonable value.
size" (for consistency with bsize/fsize, and since segment size == block
size is never a valid combination).
Updated the man page to include explicit reasonable values for fsize, bsize,
and sgs, at suggestion from Hubert Feyrer.
segment size from block size).
newfs_lfs now reads the disklabel to find segment, block, and fragment
sizes. Because reading this info from the wrong fs type could result in
very poor fs layout (e.g. ffs has "16" where the segshift would go,
resulting in 512-*megabyte* segments for 8K blocks), newfs_lfs refuses
to create a filesystem on a partition not labeled "4.4LFS".
Man pages for newfs_lfs updated to reflect this change.
that not enough segments are available for the second superblock, or to have
MIN_FREE_SEGS free for work room for the cleaner, newfs_lfs will now exit
with an error.
also fix newfs_lfs to get rid of all sorts of useless options that applied
only to newfs_ffs. Corrected reference to the FFS paper to the reference
to the BSD-LFS paper.
scw