linux-ext4 - Re: high write latency bug in ext3 / jbd in 3.4

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Date:	Mon, 13 Jan 2014 19:01:50 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Andreas Dilger <adilger@...ger.ca>, "Theodore Ts'o" <tytso@....edu>
CC:	Benjamin LaHaise <bcrl@...ck.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: high write latency bug in ext3 / jbd in 3.4

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On 1/13/14, 6:55 PM, Andreas Dilger wrote:
> On Jan 13, 2014, at 3:52 PM, Theodore Ts'o <tytso@....edu> wrote:
>> On Mon, Jan 13, 2014 at 04:16:10PM -0500, Benjamin LaHaise wrote:
>>> I had hoped to use ext4, but the recommended fsck after changing the 
>>> various feature bits is a non-starter during our upgrade process (a 22 
>>> minute outage isn't acceptable).
>>
>> You can move to ext4 without necessarily using those features which
>> require an fsck after the upgrade process.  That's hwo we handled the
>> upgrade to ext4 at Google.  New disks were formatted using ext4, but
>> for legacy file systems, we enabled extents feature (maybe one or two
>> other ones, but that was the main one) and then remounted those file
>> systems using ext4.
> 
> We also did this for upgraded Lustre ext3 filesystems in the past
> (just enabling the extents feature) without any problems.  So long
> as you don't need things like fallocate() (which presumably you don't
> since that doesn't work for ext3) then the application can't tell the
> difference between new extent-mapped and old block-mapped files.
> 
> This only affects new files, so old files are not changed.

Which was my point about ending up with a mishmash.  Maybe it's
ok, depends on your usecase.

But you wind up with different files on the same fs having different
capabilities depending on whether they are extent format or not.
(for example you can't do preallocation on the old format files,
they have different maximum offset limits, different direct IO
behavior...)

Just something to be aware of.

- -Eric

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