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Re: [PATCH] xfs: fix log space reservation calculation if log stripe uni

To: Mark Tinguely <tinguely@xxxxxxx>
Subject: Re: [PATCH] xfs: fix log space reservation calculation if log stripe unit is specified
From: Jeff Liu <jeff.liu@xxxxxxxxxx>
Date: Thu, 02 May 2013 22:14:08 +0800
Cc: Dave Chinner <dchinner@xxxxxxxxxx>, Dave Chinner <dchinner@xxxxxxxxxx>, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20130502013632.GV10481@dastard>
References: <51813BA5.3070306@xxxxxxxxxx> <51814578.9060408@xxxxxxx> <20130502013632.GV10481@dastard>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:16.0) Gecko/20121028 Thunderbird/16.0.2
On 05/02/2013 09:36 AM, Dave Chinner wrote:
> On Wed, May 01, 2013 at 11:40:24AM -0500, Mark Tinguely wrote:
>> On 05/01/13 10:58, Jeff Liu wrote:
>>> Hello,
>>>
>>> About two weeks ago, Dave has found an issue by running xfstests/297.
>>> http://oss.sgi.com/archives/xfs/2013-03/msg00273.html
>>>
>>> According to our previous discussion, if the log stripe unit is configured, 
>>> we should
>>> take it into account as it will dynamically increase the log reservation 
>>> twice of it
>>> per ticket.
>>>
>>> This patch is trying to fix it by checking the given log space against the 
>>> maximum
>>> request among those transactions(this procedure is implemented similar to 
>>> xfsprogs/mkfs/maxres.c),
>>> because the fundamental limit is that no single transaction can be larger 
>>> than half of the log.
>>> Also, looks at least another two log stripe unit should be added when 
>>> calculating the minimum log
>>> space, or else I can simply trigger a DEAD LOOP via create large number of 
>>> files, I think I need
>>> some time to digest Dave's comments posted on original bug ticket, i.e.
>>>>>>>  The question is this: how much space do we need to reserve. I'm
>>>>>>>  thinking a minimum of 4*lsu - 2*lsu for the existing CIL context, and
>>>>>>>  another 2*lsu for any queued ticket waiting for space to come
>>>>>>>  available.
>>>
>>> Put simply, with this fix, mount a partition with an improper log space 
>>> setup vs log stripe
>>> unit will failed although mkfs still works. Ah, maybe we can improve the 
>>> user space xfs_mkfs
>>> with some pre-checkup similar to the implementation inside kernel?  Besides 
>>> that, it will
>>> drop a warning to syslog and the suggested log space for the given log 
>>> stripe unit is shown
>>> there, which looks like the following:
>>>
>>> # mkfs.xfs -f -b size=512 -d agcount=16,su=256k,sw=12 -l su=256k,size=2560b 
>>> /dev/sdb1
>>> meta-data=/dev/sdb1              isize=256    agcount=16, agsize=524288 blks
>>>          =                       sectsz=512   attr=2, projid32bit=0
>>> data     =                       bsize=512    blocks=8388608, imaxpct=25
>>>          =                       sunit=512    swidth=6144 blks
>>> naming   =version 2              bsize=4096   ascii-ci=0
>>> log      =internal log           bsize=512    blocks=2560, version=2
>>>          =                       sectsz=512   sunit=512 blks, lazy-count=1
>>> realtime =none                   extsz=4096   blocks=0, rtextents=0
>>>
>>
>> Shouldn't mkfs.xfs also know it is building a filesystem that cannot
>> be mounted?
> 
> Yes, it should - this is what mkfs/maxtrres.c does. It looks like some
> of this patch came from that code.....
Yes, especially for figuring out the transaction with maximum log space
reservation.  I'm going to sync up kernel changes to user space from the
next round post.
> 
>> When mkfs.xfs is given a log stripe unit is greater than 256KB,
>> should we divide the specified log stripe unit by 2 until it is
>> under 256KB rather than reset to 32KB?
> 
> I think if it is specified on the command line, it shoul dbe
> rejected. If it's automatically determined from the data device
> sunit, then the divide-by-2-until-in-range algorithm seems fine to
> me.
> 
>>
>>> # mount /dev/sdb1 /xfs1
>>> mount: wrong fs type, bad option, bad superblock on /dev/sdb1,
>>>        missing codepage or helper program, or other error
>>>        In some cases useful info is found in syslog - try
>>>        dmesg | tail  or so
>>>
>>> # dmesg:
>>> .......
>>> XFS (sdb1): log space of 2560 blocks too small, minimum request 6656
>>> XFS (sdb1): log space validation failed
>>> XFS (sdb1): log mount failed
>>>
>>>
>>> Tests:
>>> Ran some cases in xfstests as well as a few self-defined Bonnie++/FIO tests 
>>> with above
>>> configuration(6656 log blocks), looks the current fix works, at least no 
>>> crash to me.:)
>>>
>>> But I have not yet dig into the detailed of how the suggested minimum log 
>>> space would
>>> affect the performance, given that the AIL push thresholds is defined to 
>>> 25% of the log
>>> space, a small logs might introduce IO overheads for pushing AIL too 
>>> frequently.
> 
> That's already a problem of using small logs. That's not something
> we need to worry about when trying to determine the minimum valid
> log size....
> 
>>> In addition, considering the backgroup CIL commit threshold is 1/8 of the 
>>> log, this would
>>> also impact the log IO throughput IMHO.  Maybe we can figure out an 
>>> optimized log space
>>> combine those two cases and drop it to syslog along with the minimum size?
> 
> Anyone who cares about metadata performance on their small
> filesystems is not going to use a minimum sized log. As it is, on
> any filesystem larger than about 50GB using a default log size
> (about 25MB for a default mkfs), the log stripe unit simply isn't an
> issue...
Thanks for the clarification.
> 
>> I think 1 MB is the smallest log size before we soft hang even
>> without stripe units define.
> 
> $ sudo mkfs.xfs -f -l size=256b -dsize=1g /dev/vdc
> log size 256 blocks too small, minimum size is 512 blocks
> $
> 
> mkfs won't allow a log size smaller than 2MB for default
> configurations.
> 
>>> To Dave,
>>>
>>> Sorry for the delay in drop this patch since I have mentioned that I'll 
>>> post a fix
>>> last night.  However, I have ran into an issue when testing it by 
>>> creating/removing a
>>> tons of files in parallel at that time:(
>>
>> The iclog buffers have to be a multiple of the log stripe unit or we
>> start punching the lsn in places that it should not. I think the
>> idea that was mentioned is to remove the power of two on the iclog
>> buffer size and replace with multiple of log stripe unit.
>>
>> http://oss.sgi.com/archives/xfs/2013-03/msg00039.html
> 
> Right:
> 
> | Personally, I'd prefer that logbsize be limited to power-of-2
> | multiples of the lsunit or XLOG_MIN_RECORD_BSIZE (if lsunit = 0) as
> | allowing arbitrary values to be specified by users leads to a
> | testing and bug triage nightmare.
> 
> ie I think that the valid values for logbsize are:
> 
> 32k <= logbsize <= 256k
> logbsize = logsunit * 2^N for N that does not violate the first rule.
So I'll take this into account for validating log stripe unit.

Thanks,
-Jeff

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