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SubjectRe: very poor ext3 write performance on big filesystems?
I'm CCing the linux-raid mailing list, since I suspect they will be
interested in this result.

I would suspect that the "journal guided RAID recovery" mechanism
developed by U.Wisconsin may significantly benefit this workload
because the filesystem journal is already recording all of these
block numbers and the MD bitmap mechanism is pure overhead.

On Feb 27, 2008 12:20 +0100, Tomasz Chmielewski wrote:
> Theodore Tso schrieb:
>> On Mon, Feb 18, 2008 at 03:03:44PM +0100, Andi Kleen wrote:
>>> Tomasz Chmielewski <mangoo@wpkg.org> 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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Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.



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