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Re: mkfs.xfs states log stripe unit is too large

To: Ingo Jürgensmann <ij@xxxxxxxxxxxxxxxxxx>
Subject: Re: mkfs.xfs states log stripe unit is too large
From: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Date: Sun, 24 Jun 2012 14:28:16 -0500
Cc: xfs@xxxxxxxxxxx
In-reply-to: <4954261F-D4FB-4589-BE18-2BFCEC05E9B4@xxxxxxxxxxxxxxxxxx>
References: <D3F781FA-CEB0-4896-9441-772A9E533354@xxxxxxxxxxxxxxxxxx> <20120623234445.GZ19223@dastard> <4FE67970.2030008@xxxxxxxxxxx> <4FE710B7.5010704@xxxxxxxxxxxxxxxxx> <4954261F-D4FB-4589-BE18-2BFCEC05E9B4@xxxxxxxxxxxxxxxxxx>
Reply-to: stan@xxxxxxxxxxxxxxxxx
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On 6/24/2012 8:17 AM, Ingo Jürgensmann wrote:
> Am 24.06.2012 um 15:05 schrieb Stan Hoeppner:
> 
>> On 6/23/2012 9:20 PM, Eric Sandeen wrote:
>>> On 6/23/12 6:44 PM, Dave Chinner wrote:
>>>> On Sat, Jun 23, 2012 at 02:50:49PM +0200, Ingo Jürgensmann wrote:
>>>>> muaddib:~# cat /proc/mdstat 
>>>>> Personalities : [raid1] [raid6] [raid5] [raid4] 
>>>>> md7 : active raid5 sdf4[3] sdd4[1] sde4[0]
>>>>>      7811261440 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] 
>>>>> [UUU]
>>                                               ^^^^^^^^^^
>>
>> The the log stripe unit mismatch error is a direct result of Ingo
>> manually choosing a rather large chunk size for his two stripe spindle
>> md array, yielding a 1MB stripe, and using an internal log with it.
>> Maybe there is a good reason for this, but I'm going to challenge it.
> 
> Correction: I did not manually choose that chunk size, but it was 
> automatically chosen by mdadm when creating the RAID5. 
> 
>> The default md chunk size IIRC is 64KB, 8x smaller than Ingo's chunk.
> 
> 64k is the default for creating RAIDs with 0.90 format superblock. My RAID5 
> has a 1.2 format superblock. 
> 
>> Does backup PC perform better writing 2048 sectors per stripe write,
>> 1024 per spindle, with two spindles, than 256 sectors per stripe write,
>> 128 per spindle, using two spindles?
> 
> Don't know how BackupPC actually writes the data, but it does make extensive 
> use of hardlinks to save some diskspace. Some sort of deduplicating, if you 
> like to say it that way. 
> 
>>> Rule of Silence: When a program has nothing surprising to say, it should 
>>> say nothing.
>> I think this goes to the heart of the matter.  Ingo chose an arbitrarily
>> large chunk size apparently without understanding the ramifications.
> 
> That's wrong! I've just worked with the defaults.  

At this point I get the feeling you're sandbagging us Ingo.  AFAIK you
have the distinction of being the very first person on earth to report
this problem.  This would suggest you're the first XFS user with an
internal log to use the mdadm defaults.  Do you think that's likely?

Thus, I'd guess that the metadata format changed from 0.90 to 1.2 with a
very recent release of mdadm.  Are you using distro supplied mdadm, a
backported more recent mdadm, or did you build mdadm from the most
recent source?

If either of the latter two, don't you think it would have been wise to
inform us that "hay, I'm using the bleeding edge mdadm just released"?
Or if you're using a brand new distro release?

-- 
Stan

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