Re: very poor ext3 write performance on big filesystems?

From: Tomasz Chmielewski
Date: Wed Feb 27 2008 - 06:21:17 EST

Theodore Tso schrieb:
On Mon, Feb 18, 2008 at 03:03:44PM +0100, Andi Kleen wrote:
Tomasz Chmielewski <mangoo@xxxxxxxx> writes:
Is it normal to expect the write speed go down to only few dozens of
kilobytes/s? Is it because of that many seeks? Can it be somehow
optimized?
I have similar problems on my linux source partition which also
has a lot of hard linked files (although probably not quite
as many as you do). It seems like hard linking prevents
some of the heuristics ext* uses to generate non fragmented
disk layouts and the resulting seeking makes things slow.

A follow-up to this thread.
Using small optimizations like playing with /proc/sys/vm/* didn't help much, increasing "commit=" ext3 mount option helped only a tiny bit.

What *did* help a lot was... disabling the internal bitmap of the RAID-5 array. "rm -rf" doesn't "pause" for several seconds any more.

If md and dm supported barriers, it would be even better I guess (I could enable write cache with some degree of confidence).


This is "iostat sda -d 10" output without the internal bitmap.
The system mostly tries to read (Blk_read/s), and once in a while it
does a big commit (Blk_wrtn/s):

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 164,67 2088,62 0,00 20928 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 180,12 1999,60 0,00 20016 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 172,63 2587,01 0,00 25896 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 156,53 2054,64 0,00 20608 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 170,20 3013,60 0,00 30136 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 119,46 1377,25 5264,67 13800 52752

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 154,05 1897,10 0,00 18952 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 197,70 2177,02 0,00 21792 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 166,47 1805,19 0,00 18088 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 150,95 1552,05 0,00 15536 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 158,44 1792,61 0,00 17944 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 132,47 1399,40 3781,82 14008 37856



With the bitmap enabled, it sometimes behave similarly, but mostly, I
can see as reads compete with writes, and both have very low numbers then:

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 112,57 946,11 5837,13 9480 58488

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 157,24 1858,94 0,00 18608 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 116,90 1173,60 44,00 11736 440

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 24,05 85,43 172,46 856 1728

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 25,60 90,40 165,60 904 1656

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 25,05 276,25 180,44 2768 1808

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 22,70 65,60 229,60 656 2296

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 21,66 202,79 786,43 2032 7880

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 20,90 83,20 1800,00 832 18000

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 51,75 237,36 479,52 2376 4800

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 35,43 129,34 245,91 1296 2464

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 34,50 88,00 270,40 880 2704


Now, let's disable the bitmap in the RAID-5 array:

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 110,59 536,26 973,43 5368 9744

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 119,68 533,07 1574,43 5336 15760

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 123,78 368,43 2335,26 3688 23376

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 122,48 315,68 1990,01 3160 19920

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 117,08 580,22 1009,39 5808 10104

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 119,50 324,00 1080,80 3240 10808

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 118,36 353,69 1926,55 3544 19304


And let's enable it again - after a while, it degrades again, and I can
see "rm -rf" stops for longer periods:

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 162,70 2213,60 0,00 22136 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 165,73 1639,16 0,00 16408 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 119,76 1192,81 3722,16 11952 37296

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 178,70 1855,20 0,00 18552 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 162,64 1528,07 0,80 15296 8

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 182,87 2082,07 0,00 20904 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 168,93 1692,71 0,00 16944 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 177,45 1572,06 0,00 15752 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 123,10 1436,00 4941,60 14360 49416

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 201,30 1984,03 0,00 19880 0

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 165,50 1555,20 22,40 15552 224

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 25,35 273,05 189,22 2736 1896

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 22,58 63,94 165,43 640 1656

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 69,40 435,20 262,40 4352 2624



There is a related thread (although not much kernel-related) on a BackupPC mailing list:

http://thread.gmane.org/gmane.comp.sysutils.backup.backuppc.general/14009

As it's BackupPC software which makes this amount of hardlinks (but hey, I can keep ~14 TB of data on a 1.2 TB filesystem which is not even 65% full).


--
Tomasz Chmielewski
http://wpkg.org
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