linux-kernel - Re: [RFC PATCH v2 15/21] loop: use aio to perform io on the underlying file

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Message-ID: <4F918758.8020001@oracle.com>
Date:	Fri, 20 Apr 2012 10:57:12 -0500
From:	Dave Kleikamp <dave.kleikamp@...cle.com>
To:	Zach Brown <zab@...bo.net>
CC:	Jeff Moyer <jmoyer@...hat.com>,
	"Maxim V. Patlasov" <mpatlasov@...allels.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2 15/21] loop: use aio to perform io on the underlying
 file

On 04/20/2012 10:52 AM, Zach Brown wrote:
> On 04/20/2012 11:20 AM, Jeff Moyer wrote:
>> Dave Kleikamp<dave.kleikamp@...cle.com>  writes:
>>
>>> On 04/20/2012 09:48 AM, Maxim V. Patlasov wrote:
>>>> On 03/30/2012 07:43 PM, Dave Kleikamp wrote:
>>>>> From: Zach Brown<zab@...bo.net>
>>>>>
>>>>> This uses the new kernel aio interface to process loopback IO by
>>>>> submitting concurrent direct aio.  Previously loop's IO was serialized
>>>>> by synchronous processing in a thread.
>>>>>
>>>>
>>>> The patch ignores REQ_FLUSH bit of bi_rw. Is it simply overlook?
>>>
>>> Good question. Since the loop device is sending only direct IO requests,
>>> it shouldn't be necessary to explicitly flush page cache, but REQ_FLUSH
>>
>> REQ_FLUSH isn't about the page cache, it's about flushing the volatile
>> disk write cache.  You need to handle that.
> 
> I guess O_DIRECT doesn't routinely issue flushes simply because it's too
> expensive?  Apps that care about consistent IO and O_DIRECT are expected
> to not have writeback caching enabled?  'cause there's no way they're
> issuing syncs themselves.

If we weren't using aio, we might be okay, but we don't know that any
prior asynchronous request has completed.
> 
> So yeah, I'd agree that the loop code should be reworked a bit so that
> both the filebacked and aio methods call vfs_sync() when they see
> REQ_FLUSH.

It's an easy fix. I don't anticipate that it will hurt performance too
badly.

> 
> Bleh.
> 
> - z
> (Sorry, no real time to dig into this now. Lots more time in two months!)
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