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SubjectRe: XFS write speed drop
Hi Nathan,


>> I have noticed that after an upgrade from 2.6.16-rcX -> 2.6.17-rc4, writes
>> to one (hdc) xfs filesystem have become significantly slower (factor 6 to
>
>Buffered writes? Direct writes? Sync writes? Log writes?

How would I know? :) "The standard thing".
I shuffled a bit in the source code and the word 'order' is quite often
around 'barrier', so I thought the 'barrier' option makes the IO
scheduler strictly ordered, that is, write log first, then data. (like
ext3's data=ordered). Anyhow, just see below

>You're a bit light on details here.
>
>Can you send the benchmark results themselves please? (as in,
>the test(s) you've run that lead you to see 6-8x, and the data
>those tests produced). Also, xfs_info output, and maybe list
>the device driver(s) involved here too.

Drivers
=======
xfs.ko sis5513.ko ide_disk.ko ide_core.ko

xfs_info
========
14:20 shanghai:/root # xfs_info /
meta-data=/dev/hda3 isize=256 agcount=19, agsize=462621 blks
= sectsz=512 attr=1
data = bsize=4096 blocks=8586742, imaxpct=25
= sunit=0 swidth=0 blks, unwritten=1
naming =version 2 bsize=4096
log =internal bsize=4096 blocks=3614, version=1
= sectsz=512 sunit=0 blks
realtime =none extsz=65536 blocks=0, rtextents=0
14:20 shanghai:/root # xfs_info /D
meta-data=/dev/hdc2 isize=256 agcount=16, agsize=3821963 blks
= sectsz=512 attr=1
data = bsize=4096 blocks=61151408, imaxpct=25
= sunit=0 swidth=0 blks, unwritten=1
naming =version 2 bsize=4096
log =internal bsize=4096 blocks=29859, version=1
= sectsz=512 sunit=0 blks
realtime =none extsz=65536 blocks=0, rtextents=0
14:20 shanghai:/root # fdisk -l /dev/hda

Disk /dev/hda: 40.0 GB, 40020664320 bytes
255 heads, 63 sectors/track, 4865 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System
/dev/hda1 1 64 514048+ 82 Linux swap / Solaris
/dev/hda2 * 65 588 4209030 c W95 FAT32 (LBA)
/dev/hda3 590 4865 34346970 83 Linux
14:21 shanghai:/root # fdisk -l /dev/hdc

Disk /dev/hdc: 251.0 GB, 251000193024 bytes
255 heads, 63 sectors/track, 30515 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System
/dev/hdc1 1 63 506016 82 Linux swap / Solaris
/dev/hdc2 64 30515 244605690 83 Linux



So here are the results. I have run these tests from single-user mode (-b
bootflag) so that no other userspace programs would interfere.

Script started on Mon May 22 15:32:29 2006
15:32 (none):~ # mount /dev/hdc2 /D -o barrier
15:32 (none):~ # df -Th / /D
Filesystem Type Size Used Avail Use% Mounted on
/dev/hda3 xfs 33G 6.7G 27G 21% /
/dev/hdc2 xfs 234G 142G 92G 61% /D
15:33 (none):~ # cat /proc/mounts
rootfs / rootfs rw 0 0
udev /dev tmpfs rw 0 0
/dev/hda3 / xfs rw 0 0
devpts /dev/pts devpts rw 0 0
/dev/hdc2 /D xfs rw 0 0
proc /proc proc rw 0 0

A small nitpick BTW, the barrier flag is not listed in /proc/mounts.

hda is always in nobarrier mode.
<6>hda: cache flushes not supported
<6>hdc: cache flushes supported
<5>Filesystem "hda3": Disabling barriers, not supported by the underlying
device
hdc can do barriers if wanted.


CASE 1: Copying from one disk to another
========================================
Copying a compiled 2.6.17-rc4 tree; 306907 KB in 28566 files in 2090
directories.

15:33 (none):~ # cd /tmp
15:33 (none):/tmp # time rsync -PHSav kernel-source/ /D/kernel-0/ >/dev/null

real 7m31.776s
user 0m7.030s
sys 0m14.450s

I rebooted here (only here - no more reboots below) to get a cold slate.

15:42 (none):~ # mount /dev/hdc2 /D -o nobarrier
15:42 (none):/tmp # rsync -PHSav kernel-source/ /D/kernel-1/ >/dev/null

real 2m14.401s
user 0m7.290s
sys 0m14.900s
15:45 (none):/tmp # umount /D


CASE 2: Removing
================
Remove the copies we created in case 1.

15:45 (none):/tmp # mount /dev/hdc2 /D -o barrier
15:45 (none):/tmp # cd /D
15:45 (none):/D # time rm -Rf kernel-0

real 3m31.901s
user 0m0.050s
sys 0m3.140s
15:49 (none):/D # cd /
15:49 (none):/ # umount /D
15:49 (none):/ # mount /dev/hdc2 /D -o nobarrier
15:49 (none):/ # cd /D
15:49 (none):/D # time rm -Rf kernel-1

real 0m53.471s
user 0m0.070s
sys 0m1.990s
15:50 (none):/D # cd /
15:50 (none):/ # umount /D


CASE 3: Copying to the same disk
================================
Copy a file from hdc2 to hdc2. The result is _extremely_ interesting. So the
'barrier' thing is only relevant for large sets of files, it seems.

15:50 (none):/ # mount /dev/hdc2 /D -o barrier
15:50 (none):/ # cd /D/Video
15:50 (none):/D/Video # ls -l The*
-rw-r--r-- 1 jengelh users 6861772800 Mar 17 23:02 TLK3.iso
15:50 (none):/D/Video # time cp TLK3.iso ../tlk-0.iso

real 8m1.051s
user 0m0.670s
sys 0m41.120s
15:59 (none):/D/Video # cd /
15:59 (none):/ # umount /D
15:59 (none):/ # mount /dev/hdc2 /D -o nobarrier
15:59 (none):/ # Kcd /D/Video
15:59 (none):/D/Video # time cp TLK3.iso ../tlk-1.iso

real 7m53.275s
user 0m0.560s
sys 0m40.010s
16:07 (none):/D/Video # cd /
16:08 (none):/ # umount /D
16:08 (none):/ # exit

Script done on Mon May 22 16:08:09 2006



Jan Engelhardt
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