linux-ext4 - Re: EXT4 nodelalloc => back to stone age.

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Date:	Tue, 02 Apr 2013 00:34:33 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Eric Sandeen <sandeen@...hat.com>
CC:	Theodore Ts'o <tytso@....edu>,
	Dmitry Monakhov <dmonakhov@...nvz.org>,
	ext4 development <linux-ext4@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, axboe@...nel.dk,
	Jan Kara <jack@...e.cz>
Subject: Re: EXT4 nodelalloc => back to stone age.

Hi Eric,

On 04/02/2013 12:00 AM, Eric Sandeen wrote:
> On 4/1/13 10:39 AM, Theodore Ts'o wrote:
>> On Mon, Apr 01, 2013 at 10:18:51AM -0500, Eric Sandeen wrote:
>>> I'd add:
>>>
>>> 3) Why do we have a "nodelalloc" mount option at all?
>>>
>>> but then I thought:
>>>
>>> Is it also this bad when using the ext4 driver to run an ext3 fs?
>>
>> Yes, and I there would be a similar performance problem if you are
>> using the ext3 file system driver, since ext3_*_writepage() also ends
>> up calling block_write_full_page() which will also result in the
>> writes happening with WRITE_SYNC.
> 
>> The main reason why we keep nodelalloc at this point is bug-for-bug
>> compatibility with ext3 file systems --- basically, for users who are
>> using this as a workaround for the O_PONIES issue instead of fixing
>> their applications to use fsync() appropriately.
> 
> Sorry for getting off the original thread here, but IMHO these are
> 2 different things:
> 
> nondelalloc behavior makes sense for ext3, but:
> -o nodelalloc mount options don't make sense for ext4.

nodelalloc makes sense to me.  In our product system, we met a latency
problem that is caused by delalloc feature.  The workload is a web app
that does some append writes (approximately 5M/s), and wait flusher to
do write out.  We obverse that on every 30 seconds the latency will
reach a high level (approximately 100-200ms or higher, but normally
10-20ms).  The reason is that when flush tries to write dirty pages out,
it will take i_data_sem lock (write lock) and allocate some blocks for
these dirty pages.  But in the mean time the app does some append
write(2)s that will try to take i_data_sem lock (read lock) too.  So the
app will be delayed.  So I think nodelalloc is still useful for us.

Regards,
						- Zheng

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