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Re: massively truncated files with XFS with sudden power loss on 2.6.27

To: Russell Cattelan <cattelan@xxxxxxxxxxx>
Subject: Re: massively truncated files with XFS with sudden power loss on 2.6.27 and 2.6.28
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Mon, 29 Dec 2008 15:56:45 -0600
Cc: Chris Wedgwood <cw@xxxxxxxx>, Martin Steigerwald <Martin@xxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <49592E7D.4050208@xxxxxxx>
References: <200812291920.34123.Martin@xxxxxxxxxxxx> <4959205E.4000000@xxxxxxxxxxx> <20081229192957.GC18092@xxxxxxxxxxxxxxxxxx> <49592E7D.4050208@xxxxxxx>
User-agent: Thunderbird 2.0.0.18 (Macintosh/20081105)
Russell Cattelan wrote:
> Chris Wedgwood wrote:

>> this will bite xfs more than ext3 w/ ordered mode
>>   
> Delayed allocation is a factor (and this will be true of any fs 
> supporting delayed allocation)
> holding of data flushes helps reduce fragmentation by allowing larger 
> segments to be flushed out,
> but it increases the time data is held in cache and thus create a larger 
> window for data loss.

That's not quite accurate AFAIK; yes, xfs has delayed allocation, but it
pushes data to disk on the same schedule (by default) as any other
filesystem; when pdflush goes off (30s) or under memory pressure.

The only difference is that xfs (or any delalloc fs) allocates at flush
time not at write time.

But this does not imply that xfs is holding off flushes for longer due
to delayed allocation; I don't want it to sound like xfs is putting data
integrity at risk due to delalloc, because it's not ...

-Eric

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