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Re: Problems with filesizes on different Kernels

To: xfs@xxxxxxxxxxx
Subject: Re: Problems with filesizes on different Kernels
From: Bernhard Schrader <bernhard.schrader@xxxxxxxxxxxx>
Date: Mon, 20 Feb 2012 13:06:54 +0100
In-reply-to: <20120220110614.GA17526@xxxxxxx>
References: <4F3E3F5A.9000202@xxxxxxxxxxxx> <20120217123335.GA9671@xxxxxxx> <4F420726.6060000@xxxxxxxxxxxx> <20120220110614.GA17526@xxxxxxx>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:9.0) Gecko/20111229 Thunderbird/9.0
On 02/20/2012 12:06 PM, Matthias Schniedermeyer wrote:
On 20.02.2012 09:41, Bernhard Schrader wrote:
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
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.

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.

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.

You did "sync" before?
drop caches only drops "clean" pages, everything that is dirty isn't
dropped. Hence the need to "sync" before.

Also i persume that you didn't stop Postgres?
I don't know if the process works for files that are currently opened.

When i tested the behaviour i tested it with files copied by "cp", so
they weren't open by any program when i droped the caches.

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
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?

In /proc/mounts on my computer allocsize is there:
/dev/mapper/x1 /x1 xfs 
rw,nosuid,nodev,noatime,attr2,delaylog,allocsize=64k,noquota 0 0

I tracked down the patch. It went into 2.6.38

- snip -
commit 055388a3188f56676c21e92962fc366ac8b5cb72
Author: Dave Chinner<dchinner@xxxxxxxxxx>
Date:   Tue Jan 4 11:35:03 2011 +1100

     xfs: dynamic speculative EOF preallocation

     Currently the size of the speculative preallocation during delayed
     allocation is fixed by either the allocsize mount option of a
     default size. We are seeing a lot of cases where we need to
     recommend using the allocsize mount option to prevent fragmentation
     when buffered writes land in the same AG.

     Rather than using a fixed preallocation size by default (up to 64k),
     make it dynamic by basing it on the current inode size. That way the
     EOF preallocation will increase as the file size increases.  Hence
     for streaming writes we are much more likely to get large
     preallocations exactly when we need it to reduce fragementation.

     For default settings, the size of the initial extents is determined
     by the number of parallel writers and the amount of memory in the
     machine. For 4GB RAM and 4 concurrent 32GB file writes:

     EXT: FILE-OFFSET           BLOCK-RANGE          AG AG-OFFSET               
        0: [0..1048575]:         1048672..2097247      0 (1048672..2097247)     
        1: [1048576..2097151]:   5242976..6291551      0 (5242976..6291551)     
        2: [2097152..4194303]:   12583008..14680159    0 (12583008..14680159)   
        3: [4194304..8388607]:   25165920..29360223    0 (25165920..29360223)   
        4: [8388608..16777215]:  58720352..67108959    0 (58720352..67108959)   
        5: [16777216..33554423]: 117440584..134217791  0 (117440584..134217791) 
        6: [33554424..50331511]: 184549056..201326143  0 (184549056..201326143) 
        7: [50331512..67108599]: 251657408..268434495  0 (251657408..268434495) 

     and for 16 concurrent 16GB file writes:

      EXT: FILE-OFFSET           BLOCK-RANGE          AG AG-OFFSET              
        0: [0..262143]:          2490472..2752615      0 (2490472..2752615)     
        1: [262144..524287]:     6291560..6553703      0 (6291560..6553703)     
        2: [524288..1048575]:    13631592..14155879    0 (13631592..14155879)   
        3: [1048576..2097151]:   30408808..31457383    0 (30408808..31457383)   
        4: [2097152..4194303]:   52428904..54526055    0 (52428904..54526055)   
        5: [4194304..8388607]:   104857704..109052007  0 (104857704..109052007) 
        6: [8388608..16777215]:  209715304..218103911  0 (209715304..218103911) 
        7: [16777216..33554423]: 452984848..469762055  0 (452984848..469762055) 

     Because it is hard to take back specualtive preallocation, cases
     where there are large slow growing log files on a nearly full
     filesystem may cause premature ENOSPC. Hence as the filesystem nears
     full, the maximum dynamic prealloc size ?s reduced according to this
     table (based on 4k block size):

     freespace       max prealloc size
       >5%             full extent (8GB)
       4-5%             2GB (8GB>>  2)
       3-4%             1GB (8GB>>  3)
       2-3%           512MB (8GB>>  4)
       1-2%           256MB (8GB>>  5)
       <1%            128MB (8GB>>  6)

     This should reduce the amount of space held in speculative
     preallocation for such cases.

     The allocsize mount option turns off the dynamic behaviour and fixes
     the prealloc size to whatever the mount option specifies. i.e. the
     behaviour is unchanged.

     Signed-off-by: Dave Chinner<dchinner@xxxxxxxxxx>
- snip -

Bis denn

Yes, I did the sync, and you are right, I didn't restarted the postgres process. Well, but today i restarted the whole server. And regarding the last paragraph you wrote, the allocsize=64K should stop the dynamic preallocation... but right now it doesnt seem so, the sizes always get back to the 5.7GB, but in between it raises up. Could it be possible, because of the different mount outputs, that it didnt get loaded well?

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