linux-kernel - Re: Poor interactive performance with I/O loads with fsync()ing

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Date:	Sun, 11 Apr 2010 19:20:07 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Ben Gamari <bgamari.foss@...il.com>
Cc:	Avi Kivity <avi@...hat.com>, linux-btrfs@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, tytso@....edu, npiggin@...e.de,
	mingo@...e.hu, Ruald Andreae <ruald.a@...il.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Olly Betts <olly@...vex.com>,
	martin f krafft <madduck@...duck.net>
Subject: Re: Poor interactive performance with I/O loads with fsync()ing

> Has the reason for this been identified? Judging from the nature of metadata
> loads, it would seem that it should be substantially easier to implement
> fsync() efficiently.

By design a copy on write tree fs would need to flush a whole
tree hierarchy on a sync. btrfs avoids this by using a special
log for fsync, but that causes more overhead if you have that
log on the same disk.  So IO subsystem will do more work.

It's a bit like JBD data journaling.

However it should not have the stalls inherent in ext3's journaling.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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