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Date:	Mon, 29 Aug 2011 21:54:39 +1000
From:	Anton Blanchard <anton@...ba.org>
To:	Theodore Tso <tytso@....EDU>
Cc:	adilger.kernel@...ger.ca, eric.dumazet@...il.com, tj@...nel.org,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] percpu_counter: Put a reasonable upper bound on
 percpu_counter_batch


Hi Ted,

> I understand why we would want to reduce this number.
> Unfortunately, the question is what do we do if all 1024 threads try
> to do buffered writes into the file system at the same instant, when
> we have less than 4 megabytes of space left?
> 
> The problem is that we can then do more writes than we have space, and
> we will only find out about it at write back time, when the process
> may have exited already -- at which point data loss is almost
> inevitable.  (We could keep the data in cache and frantically page
> the system administrator to delete some files to make room for dirty
> data, but that's probably not going to end well….)
> 
> What we can do if we must clamp this threshold is to also increase the
> threshold at which we shift away from delayed allocation.  We'll then
> allocate each block at write time, which does mean more CPU and 
> less efficient allocation of blocks, but if we're down to our last 4
> megabytes, there's probably not much we can do that will be efficient
> as far as block layout anyway….

Thanks for the explanation, I'll go back and take another look.

Anton
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