mirror of
https://git.FreeBSD.org/src.git
synced 2024-12-21 11:13:30 +00:00
321 lines
15 KiB
Plaintext
321 lines
15 KiB
Plaintext
Introduction
|
|
|
|
This package constitutes the alpha distribution of the soft update
|
|
code updates for the fast filesystem.
|
|
|
|
For More information on what Soft Updates is, see:
|
|
http://www.ece.cmu.edu/~ganger/papers/CSE-TR-254-95/
|
|
|
|
Status
|
|
|
|
My `filesystem torture tests' (described below) run for days without
|
|
a hitch (no panic's, hangs, filesystem corruption, or memory leaks).
|
|
However, I have had several panic's reported to me by folks that
|
|
are field testing the code which I have not yet been able to
|
|
reproduce or fix. Although these panic's are rare and do not cause
|
|
filesystem corruption, the code should only be put into production
|
|
on systems where the system administrator is aware that it is being
|
|
run, and knows how to turn it off if problems arise. Thus, you may
|
|
hand out this code to others, but please ensure that this status
|
|
message is included with any distributions. Please also include
|
|
the file ffs_softdep.stub.c in any distributions so that folks that
|
|
cannot abide by the need to redistribute source will not be left
|
|
with a kernel that will not link. It will resolve all the calls
|
|
into the soft update code and simply ignores the request to enable
|
|
them. Thus you will be able to ensure that your other hooks have
|
|
not broken anything and that your kernel is softdep-ready for those
|
|
that wish to use them. Please report problems back to me with
|
|
kernel backtraces of panics if possible. This is massively complex
|
|
code, and people only have to have their filesystems hosed once or
|
|
twice to avoid future changes like the plague. I want to find and
|
|
fix as many bugs as soon as possible so as to get the code rock
|
|
solid before it gets widely released. Please report any bugs that
|
|
you uncover to mckusick@mckusick.com.
|
|
|
|
Performance
|
|
|
|
Running the Andrew Benchmarks yields the following raw data:
|
|
|
|
Phase Normal Softdep What it does
|
|
1 3s <1s Creating directories
|
|
2 8s 4s Copying files
|
|
3 6s 6s Recursive directory stats
|
|
4 8s 9s Scanning each file
|
|
5 25s 25s Compilation
|
|
|
|
Normal: 19.9u 29.2s 0:52.8 135+630io
|
|
Softdep: 20.3u 28.5s 0:47.8 103+363io
|
|
|
|
Another interesting datapoint are my `filesystem torture tests'.
|
|
They consist of 1000 runs of the andrew benchmarks, 1000 copy and
|
|
removes of /etc with randomly selected pauses of 0-60 seconds
|
|
between each copy and remove, and 500 find from / with randomly
|
|
selected pauses of 100 seconds between each run). The run of the
|
|
torture test compares as follows:
|
|
|
|
With soft updates: writes: 6 sync, 1,113,686 async; run time 19hr, 50min
|
|
Normal filesystem: writes: 1,459,147 sync, 487,031 async; run time 27hr, 15min
|
|
|
|
The upshot is 42% less I/O and 28% shorter running time.
|
|
|
|
Another interesting test point is a full MAKEDEV. Because it runs
|
|
as a shell script, it becomes mostly limited by the execution speed
|
|
of the machine on which it runs. Here are the numbers:
|
|
|
|
With soft updates:
|
|
|
|
labrat# time ./MAKEDEV std
|
|
2.2u 32.6s 0:34.82 100.0% 0+0k 11+36io 0pf+0w
|
|
|
|
labrat# ls | wc
|
|
522 522 3317
|
|
|
|
Without soft updates:
|
|
|
|
labrat# time ./MAKEDEV std
|
|
2.0u 40.5s 0:42.53 100.0% 0+0k 11+1221io 0pf+0w
|
|
|
|
labrat# ls | wc
|
|
522 522 3317
|
|
|
|
Of course, some of the system time is being pushed
|
|
to the syncer process, but that is a different story.
|
|
|
|
To show a benchmark designed to highlight the soft update code
|
|
consider a tar of zero-sized files and an rm -rf of a directory tree
|
|
that has at least 50 files or so at each level. Running a test with
|
|
a directory tree containing 28 directories holding 202 empty files
|
|
produces the following numbers:
|
|
|
|
With soft updates:
|
|
tar: 0.0u 0.5s 0:00.65 76.9% 0+0k 0+44io 0pf+0w (0 sync, 33 async writes)
|
|
rm: 0.0u 0.2s 0:00.20 100.0% 0+0k 0+37io 0pf+0w (0 sync, 72 async writes)
|
|
|
|
Normal filesystem:
|
|
tar: 0.0u 1.1s 0:07.27 16.5% 0+0k 60+586io 0pf+0w (523 sync, 0 async writes)
|
|
rm: 0.0u 0.5s 0:01.84 29.3% 0+0k 0+318io 0pf+0w (258 sync, 65 async writes)
|
|
|
|
The large reduction in writes is because inodes are clustered, so
|
|
most of a block gets allocated, then the whole block is written
|
|
out once rather than having the same block written once for each
|
|
inode allocated from it. Similarly each directory block is written
|
|
once rather than once for each new directory entry. Effectively
|
|
what the update code is doing is allocating a bunch of inodes
|
|
and directory entries without writing anything, then ensuring that
|
|
the block containing the inodes is written first followed by the
|
|
directory block that references them. If there were data in the
|
|
files it would further ensure that the data blocks were written
|
|
before their inodes claimed them.
|
|
|
|
Copyright Restrictions
|
|
|
|
Please familiarize yourself with the copyright restrictions
|
|
contained at the top of either the sys/ufs/ffs/softdep.h or
|
|
sys/ufs/ffs/ffs_softdep.c file. The key provision is similar
|
|
to the one used by the DB 2.0 package and goes as follows:
|
|
|
|
Redistributions in any form must be accompanied by information
|
|
on how to obtain complete source code for any accompanying
|
|
software that uses the this software. This source code must
|
|
either be included in the distribution or be available for
|
|
no more than the cost of distribution plus a nominal fee,
|
|
and must be freely redistributable under reasonable
|
|
conditions. For an executable file, complete source code
|
|
means the source code for all modules it contains. It does
|
|
not mean source code for modules or files that typically
|
|
accompany the operating system on which the executable file
|
|
runs, e.g., standard library modules or system header files.
|
|
|
|
The idea is to allow those of you freely redistributing your source
|
|
to use it while retaining for myself the right to peddle it for
|
|
money to the commercial UNIX vendors. Note that I have included a
|
|
stub file ffs_softdep.c.stub that is freely redistributable so that
|
|
you can put in all the necessary hooks to run the full soft updates
|
|
code, but still allow vendors that want to maintain proprietary
|
|
source to have a working system. I do plan to release the code with
|
|
a `Berkeley style' copyright once I have peddled it around to the
|
|
commercial vendors. If you have concerns about this copyright,
|
|
feel free to contact me with them and we can try to resolve any
|
|
difficulties.
|
|
|
|
Soft Dependency Operation
|
|
|
|
The soft update implementation does NOT require ANY changes
|
|
to the on-disk format of your filesystems. Furthermore it is
|
|
not used by default for any filesystems. It must be enabled on
|
|
a filesystem by filesystem basis by running tunefs to set a
|
|
bit in the superblock indicating that the filesystem should be
|
|
managed using soft updates. If you wish to stop using
|
|
soft updates due to performance or reliability reasons,
|
|
you can simply run tunefs on it again to turn off the bit and
|
|
revert to normal operation. The additional dynamic memory load
|
|
placed on the kernel malloc arena is approximately equal to
|
|
the amount of memory used by vnodes plus inodes (for a system
|
|
with 1000 vnodes, the additional peak memory load is about 300K).
|
|
|
|
Kernel Changes
|
|
|
|
There are two new changes to the kernel functionality that are not
|
|
contained in in the soft update files. The first is a `trickle
|
|
sync' facility running in the kernel as process 3. This trickle
|
|
sync process replaces the traditional `update' program (which should
|
|
be commented out of the /etc/rc startup script). When a vnode is
|
|
first written it is placed 30 seconds down on the trickle sync
|
|
queue. If it still exists and has dirty data when it reaches the
|
|
top of the queue, it is sync'ed. This approach evens out the load
|
|
on the underlying I/O system and avoids writing short-lived files.
|
|
The papers on trickle-sync tend to favor aging based on buffers
|
|
rather than files. However, I sync on file age rather than buffer
|
|
age because the data structures are much smaller as there are
|
|
typically far fewer files than buffers. Although this can make the
|
|
I/O spikey when a big file times out, it is still much better than
|
|
the wholesale sync's that were happening before. It also adapts
|
|
much better to the soft update code where I want to control
|
|
aging to improve performance (inodes age in 10 seconds, directories
|
|
in 15 seconds, files in 30 seconds). This ensures that most
|
|
dependencies are gone (e.g., inodes are written when directory
|
|
entries want to go to disk) reducing the amount of rollback that
|
|
is needed.
|
|
|
|
The other main kernel change is to split the vnode freelist into
|
|
two separate lists. One for vnodes that are still being used to
|
|
identify buffers and the other for those vnodes no longer identifying
|
|
any buffers. The latter list is used by getnewvnode in preference
|
|
to the former.
|
|
|
|
Packaging of Kernel Changes
|
|
|
|
The sys subdirectory contains the changes and additions to the
|
|
kernel. My goal in writing this code was to minimize the changes
|
|
that need to be made to the kernel. Thus, most of the new code
|
|
is contained in the two new files softdep.h and ffs_softdep.c.
|
|
The rest of the kernel changes are simply inserting hooks to
|
|
call into these two new files. Although there has been some
|
|
structural reorganization of the filesystem code to accommodate
|
|
gathering the information required by the soft update code,
|
|
the actual ordering of filesystem operations when soft updates
|
|
are disabled is unchanged.
|
|
|
|
The kernel changes are packaged as a set of diffs. As I am
|
|
doing my development in BSD/OS, the diffs are relative to the
|
|
BSD/OS versions of the files. Because BSD/OS recently had
|
|
4.4BSD-Lite2 merged into it, the Lite2 files are a good starting
|
|
point for figuring out the changes. There are 40 files that
|
|
require change plus the two new files. Most of these files have
|
|
only a few lines of changes in them. However, four files have
|
|
fairly extensive changes: kern/vfs_subr.c, ufs/ufs/ufs_lookup.c,
|
|
ufs/ufs/ufs_vnops.c, and ufs/ffs/ffs_alloc.c. For these four
|
|
files, I have provided the original Lite2 version, the Lite2
|
|
version with the diffs merged in, and the diffs between the
|
|
BSD/OS and merged version. Even so, I expect that there will
|
|
be some difficulty in doing the merge; I am certainly willing
|
|
to assist in helping get the code merged into your system.
|
|
|
|
Packaging of Utility Changes
|
|
|
|
The utilities subdirectory contains the changes and additions
|
|
to the utilities. There are diffs to three utilities enclosed:
|
|
|
|
tunefs - add a flag to enable and disable soft updates
|
|
|
|
mount - print out whether soft updates are enabled and
|
|
also statistics on number of sync and async writes
|
|
|
|
fsck - tighter checks on acceptable errors and a slightly
|
|
different policy for what to put in lost+found on
|
|
filesystems using soft updates
|
|
|
|
In addition you should recompile vmstat so as to get reports
|
|
on the 13 new memory types used by the soft update code.
|
|
It is not necessary to use the new version of fsck, however it
|
|
would aid in my debugging if you do. Also, because of the time
|
|
lag between deleting a directory entry and the inode it
|
|
references, you will find a lot more files showing up in your
|
|
lost+found if you do not use the new version. Note that the
|
|
new version checks for the soft update flag in the superblock
|
|
and only uses the new algorithms if it is set. So, it will run
|
|
unchanged on the filesystems that are not using soft updates.
|
|
|
|
Operation
|
|
|
|
Once you have booted a kernel that incorporates the soft update
|
|
code and installed the updated utilities, do the following:
|
|
|
|
1) Comment out the update program in /etc/rc.
|
|
|
|
2) Run `tunefs -n enable' on one or more test filesystems.
|
|
|
|
3) Mount these filesystems and then type `mount' to ensure that
|
|
they have been enabled for soft updates.
|
|
|
|
4) Copy the test directory to a softdep filesystem, chdir into
|
|
it and run `./doit'. You may want to check out each of the
|
|
three subtests individually first: doit1 - andrew benchmarks,
|
|
doit2 - copy and removal of /etc, doit3 - find from /.
|
|
|
|
====
|
|
Additional notes from Feb 13
|
|
|
|
hen removing huge directories of files, it is possible to get
|
|
the incore state arbitrarily far ahead of the disk. Maintaining
|
|
all the associated depedency information can exhaust the kernel
|
|
malloc arena. To avoid this senario, I have put some limits on
|
|
the soft update code so that it will not be allowed to rampage
|
|
through all of the kernel memory. I enclose below the relevant
|
|
patches to vnode.h and vfs_subr.c (which allow the soft update
|
|
code to speed up the filesystem syncer process). I have also
|
|
included the diffs for ffs_softdep.c. I hope to make a pass over
|
|
ffs_softdep.c to isolate the differences with my standard version
|
|
so that these diffs are less painful to incorporate.
|
|
|
|
Since I know you like to play with tuning, I have put the relevant
|
|
knobs on sysctl debug variables. The tuning knobs can be viewed
|
|
with `sysctl debug' and set with `sysctl -w debug.<name>=value'.
|
|
The knobs are as follows:
|
|
|
|
debug.max_softdeps - limit on any given resource
|
|
debug.tickdelay - ticks to delay before allocating
|
|
debug.max_limit_hit - number of times tickdelay imposed
|
|
debug.rush_requests - number of rush requests to filesystem syncer
|
|
|
|
The max_softdeps limit is derived from vnodesdesired which in
|
|
turn is sized based on the amount of memory on the machine.
|
|
When the limit is hit, a process requesting a resource first
|
|
tries to speed up the filesystem syncer process. Such a
|
|
request is recorded as a rush_request. After syncdelay / 2
|
|
unserviced rush requests (typically 15) are in the filesystem
|
|
syncers queue (i.e., it is more than 15 seconds behind in its
|
|
work), the process requesting the memory is put to sleep for
|
|
tickdelay seconds. Such a delay is recorded in max_limit_hit.
|
|
Following this delay it is granted its memory without further
|
|
delay. I have tried the following experiments in which I
|
|
delete an MH directory containing 16,703 files:
|
|
|
|
Run # 1 2 3
|
|
|
|
max_softdeps 4496 4496 4496
|
|
tickdelay 100 == 1 sec 20 == 0.2 sec 2 == 0.02 sec
|
|
max_limit_hit 16 == 16 sec 27 == 5.4 sec 203 == 4.1 sec
|
|
rush_requests 147 102 93
|
|
run time 57 sec 46 sec 45 sec
|
|
I/O's 781 859 936
|
|
|
|
When run with no limits, it completes in 40 seconds. So, the
|
|
time spent in delay is directly added to the bottom line.
|
|
Shortening the tick delay does cut down the total running time,
|
|
but at the expense of generating more total I/O operations
|
|
due to the rush orders being sent to the filesystem syncer.
|
|
Although the number of rush orders decreases with a shorter
|
|
tick delay, there are more requests in each order, hence the
|
|
increase in I/O count. Also, although the I/O count does rise
|
|
with a shorter delay, it is still at least an order of magnitude
|
|
less than without soft updates. Anyway, you may want to play
|
|
around with these value to see what works best and to see if
|
|
you can get an insight into how best to tune them. If you get
|
|
out of memory panic's, then you have max_softdeps set too high.
|
|
The max_limit_hit and rush_requests show be reset to zero
|
|
before each run. The minimum legal value for tickdelay is 2
|
|
(if you set it below that, the code will use 2).
|
|
|
|
|