Re: Problems with filesizes on different Kernels

[Bernhard Schrader <bernhard.schrader@innogames.de>]

On 02/20/2012 01:06 PM, Bernhard Schrader wrote:
> 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
>>>>> /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.
>>>
>>> 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.
>>
>>> 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?
>>
>> 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@redhat.com>
>> 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 TOTAL
>> 0: [0..1048575]: 1048672..2097247 0 (1048672..2097247) 1048576
>> 1: [1048576..2097151]: 5242976..6291551 0 (5242976..6291551) 1048576
>> 2: [2097152..4194303]: 12583008..14680159 0 (12583008..14680159) 2097152
>> 3: [4194304..8388607]: 25165920..29360223 0 (25165920..29360223) 4194304
>> 4: [8388608..16777215]: 58720352..67108959 0 (58720352..67108959) 8388608
>> 5: [16777216..33554423]: 117440584..134217791 0 (117440584..134217791)
>> 16777208
>> 6: [33554424..50331511]: 184549056..201326143 0 (184549056..201326143)
>> 16777088
>> 7: [50331512..67108599]: 251657408..268434495 0 (251657408..268434495)
>> 16777088
>>
>> and for 16 concurrent 16GB file writes:
>>
>> EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL
>> 0: [0..262143]: 2490472..2752615 0 (2490472..2752615) 262144
>> 1: [262144..524287]: 6291560..6553703 0 (6291560..6553703) 262144
>> 2: [524288..1048575]: 13631592..14155879 0 (13631592..14155879) 524288
>> 3: [1048576..2097151]: 30408808..31457383 0 (30408808..31457383) 1048576
>> 4: [2097152..4194303]: 52428904..54526055 0 (52428904..54526055) 2097152
>> 5: [4194304..8388607]: 104857704..109052007 0 (104857704..109052007)
>> 4194304
>> 6: [8388608..16777215]: 209715304..218103911 0 (209715304..218103911)
>> 8388608
>> 7: [16777216..33554423]: 452984848..469762055 0 (452984848..469762055)
>> 16777208
>>
>> 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@redhat.com>
>> - 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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Just to give you the solution. Well, the allocsize setting itself was 
correct, but the mountpoint for this option was / so the flag isn't 
remountable on this point, i had to add "rootflags=allocsize=64k" to the 
extra kernel line in my *.sxp files of each VM, this way it recognized 
the option and worked as expected.

thanks all for help.

regards
Bernhard

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