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Re: Problems with filesizes on different Kernels

To: Matthias Schniedermeyer <ms@xxxxxxx>
Subject: Re: Problems with filesizes on different Kernels
From: Bernhard Schrader <bernhard.schrader@xxxxxxxxxxxx>
Date: Mon, 20 Feb 2012 09:41:10 +0100
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20120217123335.GA9671@xxxxxxx>
References: <4F3E3F5A.9000202@xxxxxxxxxxxx> <20120217123335.GA9671@xxxxxxx>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:9.0) Gecko/20111229 Thunderbird/9.0
On 02/17/2012 01:33 PM, Matthias Schniedermeyer wrote:
On 17.02.2012 12:51, Bernhard Schrader wrote:
Hi all,

we just discovered a problem, which I think is related to XFS. Well,
I will try to explain.

The environment i am working with are around 300 Postgres databases
in separated VM's. All are running with XFS. Differences are just in
kernel versions.
- 2.6.18
- 2.6.39
- 3.1.4

Some days ago i discovered that the file nodes of my postgresql
tables have strange sizes. They are located in
/var/lib/postgresql/9.0/main/base/[databaseid]/
If I execute the following commands i get results like this:

Command: du -sh | tr "\n" " "; du --apparent-size -h
Result: 6.6G    . 5.7G  .

Since a few kernel-version XFS does speculative preallocations, which is
primarily a measure to prevent fragmentation.

The preallocations should go away when you drop the caches.

sync
echo 3>  /proc/sys/vm/drop_caches

XFS can be prevented to do that with the mount-option "allocsize".
Personally i use "allocsize=64k", since i first encountered that
behaviour, my workload primarily consists of single-thread writing which
doesn't benefit from this preallocation.
Your workload OTOH may benefit as it should prevent/lower the
fragmentation of the database files.






Bis denn


Hi Matthias,
thanks for the reply, as far as i can say the "echo 3 > /proc/sys/vm/drop_caches" didn't work. the sizes didnt shrink. Today i had the chance to test the allocsize=64k. Well, first i thought it worked, i added the mountoption, restarted the server, everything shrink to normal sizes. but right now its more or less "flapping". I have 5.7GB real data and the sizes flap between 6.9GB to 5.7GB.
But I am wondering a little about the mount output:

# mount
/dev/xvda1 on / type xfs (rw,noatime,nodiratime,logbufs=8,nobarrier,allocsize=64k)
tmpfs on /lib/init/rw type tmpfs (rw,nosuid,mode=0755)
proc on /proc type proc (rw,noexec,nosuid,nodev)
sysfs on /sys type sysfs (rw,noexec,nosuid,nodev)
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
devpts on /dev/pts type devpts (rw,noexec,nosuid,gid=5,mode=620)


# cat /proc/mounts
rootfs / rootfs rw 0 0
/dev/root / xfs rw,noatime,nodiratime,attr2,delaylog,nobarrier,noquota 0 0
tmpfs /lib/init/rw tmpfs rw,nosuid,relatime,mode=755 0 0
proc /proc proc rw,nosuid,nodev,noexec,relatime 0 0
sysfs /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0
tmpfs /dev/shm tmpfs rw,nosuid,nodev,relatime 0 0
devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0


In normal mount output i see the allocsize, but not in cat /proc/mounts?!?

Is there a way to completly disable speculative prealocations? or the behavior how it works right now?


regards
Bernhard

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