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

To: xfs@xxxxxxxxxxx
Subject: Re: mkfs.xfs states log stripe unit is too large
From: Ingo Jürgensmann <ij@xxxxxxxxxxxxxxxxxx>
Date: Sun, 24 Jun 2012 15:17:59 +0200
Cc: stan@xxxxxxxxxxxxxxxxx
In-reply-to: <4FE710B7.5010704@xxxxxxxxxxxxxxxxx>
References: <D3F781FA-CEB0-4896-9441-772A9E533354@xxxxxxxxxxxxxxxxxx> <20120623234445.GZ19223@dastard> <4FE67970.2030008@xxxxxxxxxxx> <4FE710B7.5010704@xxxxxxxxxxxxxxxxx>
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.  

-- 
Ciao...            //      Fon: 0381-2744150
      Ingo       \X/       http://blog.windfluechter.net


gpg pubkey:  http://www.juergensmann.de/ij_public_key.asc

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