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Poor performance using discard

To: xfs@xxxxxxxxxxx
Subject: Poor performance using discard
From: Thomas Lynema <lyz27@xxxxxxxxx>
Date: Tue, 28 Feb 2012 17:56:18 -0500
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I have a PP120GS25SSDR it does support trim 

cat /sys/block/sdc/queue/discard_max_bytes 
2147450880

The entire drive is one partition that is totally used by LVM.

I made a test vg and formatted it with mkfs.xfs.  Then mounted it with
discard and got the following result when deleting a kernel source:

/dev/mapper/ssdvg0-testLV on /media/temp type xfs
(rw,noatime,nodiratime,discard)

time rm -rf linux-3.2.6-gentoo/
real   5m7.139s
user   0m0.080s
sys   0m1.580s 

There where lockups where the system would pause for about a minute
during the process.

ext4 handles this scenerio fine:

/dev/mapper/ssdvg0-testLV on /media/temp type ext4
(rw,noatime,nodiratime,discard)

time rm -rf linux-3.2.6-gentoo/

real   0m0.943s
user   0m0.050s
sys   0m0.830s 

xfs mounted without discard seems to handle this fine:

/dev/mapper/ssdvg0-testLV on /media/temp type xfs
(rw,noatime,nodiratime)

time rm -rf linux-3.2.6-gentoo/
real    0m1.634s
user    0m0.040s
sys     0m1.420s

uname -a
Linux core24 3.2.5-gentoo #11 SMP PREEMPT Sat Feb 11 15:46:22 EST 2012
x86_64 Intel(R) Core(TM)2 Quad CPU Q6700 @ 2.66GHz GenuineIntel
GNU/Linux


Any suggestions?

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