After the IO/FS workshop last week, I posted some details on the slow
down we see with ext3 when we have a low latency back end instead of a
normal local disk (SCSI/S-ATA/etc).
As a follow up to that thread, I wanted to post some real numbers that
Andy from our performance team pulled together. Andy tested various
patches using three classes of storage (S-ATA, RAM disk and Clariion array).
Note that this testing was done on a SLES10/SP1 kernel, but the code in
question has not changed in mainline but we should probably retest on
something newer just to clear up any doubts.
The work load is generated using fs_mark
(http://sourceforge.net/projects/fsmark/) which is basically a write
workload with small files, each file gets fsync'ed before close. The
metric is "files/sec".
The clearest result used a ramdisk to store 4k files.
We modified ext3 and jbd to accept a new mount option: bdelay Use it like:
mount -o bdelay=n dev mountpoint
n is passed to schedule_timeout_interruptible() in the jbd code. if n ==
0, it skips the whole loop. if n is "yield", then substitute the
schedule...(n) with yield().
Note that the first row is the value of the delay with a 250HZ build
followed by the number of concurrent threads writing 4KB files.
Ramdisk test:
bdelay 1 2 4 8 10 20
0 4640 4498 3226 1721 1436 664
yield 4640 4078 2977 1611 1136 551
1 4647 250 482 588 629 483
2 4522 149 233 422 450 389
3 4504 86 165 271 308 334
4 4425 84 128 222 253 293
Midrange clariion:
bdelay 1 2 4 8 10 20
0 778 923 1567 1424 1276 785
yield 791 931 1551 1473 1328 806
1 793 304 499 714 751 760
2 789 132 201 382 441 589
3 792 124 168 298 342 471
4 786 71 116 237 277 393
Local disk:
bdelay 1 2 4 8 10 20
0 47 51 81 135 160 234
yield 36 45 74 117 138 214
1 44 52 86 148 183 258
2 40 60 109 163 184 265
3 40 52 97 148 171 264
4 35 42 83 149 169 246
Apologies for mangling the nicely formatted tables.
Note that the justification for the batching as we have it today is
basically this last local drive test case.
It would be really interesting to rerun some of these tests on xfs which
Dave explained in the thread last week has a more self tuning way to
batch up transactions....
Note that all of those poor users who have a synchronous write workload
today are in the "1" row for each of the above tables.
ric
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