linux-kernel - Re: Read starvation by sync writes

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Message-ID: <x49y5fjx0nk.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 23 Jan 2013 12:35:43 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Jan Kara <jack@...e.cz>, Shaohua Li <shli@...nel.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Read starvation by sync writes

Jens Axboe <axboe@...nel.dk> writes:

>>> The problem is really that the WRITE_SYNC is (for Jan's case) behaving
>>> like buffered writes, so it eats up a queue of requests very easily. On
>>> the allocation side, the assumption is that WRITE_SYNC behaves like
>>> dependent reads. Similar to a dd with oflag=direct, not like a flood of
>>> requests. For dependent sync writes, our current behaviour is fine, we
>>> treat them like reads. For commits of WRITE_SYNC, they should be treated
>>> like async WRITE instead.
>>   Yeah. But it's similar to what happens when you run fsync() on a large
>> dirty file. That will also submit a lot of WRITE_SYNC requests... kjournald
>> could probably use WRITE instead of WRITE_SYNC for large commits. It's just
>> that we don't really want to give e.g. DIO a preference over kjournald
>> because transaction commit can effectively block any metadata changes on
>> the filesystem.
>
> Sure, I'm not advocating against changing WRITE_SYNC, we just need to be
> able to handle it a bit better. I've got a test patch, will post it
> later.

Jens, did you ever post your test patch?

-Jeff
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