linux-kernel - Re: [PATCH 0/8][RFC] IO latency/throughput fixes

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Date:	Tue, 07 Apr 2009 15:45:31 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Mark Lord <lkml@....ca>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ray Lee <ray-lk@...rabbit.org>, Hua Zhong <hzhong@...il.com>,
	Theodore Tso <tytso@....edu>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/8][RFC] IO latency/throughput fixes

Mark Lord wrote:
> Linus Torvalds wrote:
>>
>> On Tue, 7 Apr 2009, Mark Lord wrote:
>>> What happens with ext3 "writeback", and ext4 "whatever",
>>> when one does the quickie reboot method:
>>>
>>>   ALT-SYSRQ-S ALT-SYSRQ-U ALT-SYSRQ-S ALT-SYSRQ-B
>>>
>>> ???
>>
>> Since 's' syncs (I think 'u' does too, as part of making things 
>> read-only), the data blocks will be on disk after the boot regardless 
>> of any other ordering.
> ..
> 
> I was thinking more about delayed allocation in ext4, though.
> If it hasn't allocated the blocks, then sync() has nothing to write out.
> Or do they have hooks into the block layer to force alloc/commit when
> somebody does a sync() ??

sync(2) doesn't just sync dirty buffers...  it sync's inodes, which 
pokes the filesystem to do something intelligent, perhaps triggering (a) 
write-out of data, (b) write-out of zeroed blocks, or (c) annotation in 
filesystem metadata that certain blocks are allocated, but not initialized.

	Jeff



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