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