linux-kernel - Re: [PATCH] writeback: Fix broken sync writeback

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Date:	Mon, 15 Feb 2010 16:05:43 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
cc:	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	jengelh@...ozas.de, stable@...nel.org, gregkh@...e.de
Subject: Re: [PATCH] writeback: Fix broken sync writeback

On Mon, 15 Feb 2010, Jan Kara wrote:
> 
> Personally, I don't see why VM shouldn't generate IO from a single inode
> larger than a few MB for data integrity syncs. There are two reasons I know
> about for MAX_WRITEBACK_PAGES:

Two issues:
 - it shouldn't matter for performance (if you have a broken disk that 
   wants multi-megabyte writes to get good performance, you need to fix 
   the driver, not the VM)
 - I generally think that _latency_ is much more important than 
   throughput, and huge writes are simply bad for latency.

But the real complaint I had about your patch was that it made no sense. 

Your statement that it speeds up sync is indicative of some _other_ 
independent problem (perhaps starting from the beginning of the file each 
time? Who knows?) and the patch _clearly_doesn't actually address the 
underlying problem, it just changes the logic in a way that makes no 
obvious sense to anybody, without actually explaining why it would matter.

So if you can explain _why_ that patch makes such a big difference for 
you, and actually write that up, I wouldn't mind it. But right now it was 
sent as a voodoo patch with no sensible explanation for why it would 
really make any difference, since the outer loop should already do what 
the patch does.

		Linus
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