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

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Date:	Mon, 13 Jan 2014 15:39:04 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Benjamin LaHaise <bcrl@...ck.org>,
	Andreas Dilger <adilger@...ger.ca>
CC:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: high write latency bug in ext3 / jbd in 3.4

On 1/13/14, 3:16 PM, Benjamin LaHaise wrote:
> On Mon, Jan 13, 2014 at 02:01:08PM -0700, Andreas Dilger wrote:
>> Not to be flippant, but is there any reason NOT to just mount the
>> filesystem with ext4?  There are a large number of improvements in
>> the ext4 code that don't require on-disk format changes (e.g. delayed
>> allocation, multi-block allocation, etc) if there is a concern about
>> being able to downgrade to an ext3-type mount in case of problems.
> 
> I'm leaning towards doing this.  The main reason for not doing so was 
> primarily that a few of the tweaks that I had been made to ext3 would 
> have to be ported to ext4.  Thankfully, I think we're still in an early 
> enough stage of release that I should be able to do so.  The changes 
> are pretty specific, mostly allocator tweaks to improve the on-disk 
> layout for our specific use-case.
> 
>> There are further improvements in ext4 that can be used on upgraded
>> ext3 filesystems if the feature bit is enabled (in particular extent
>> mapped files).  However, extent mapped files are not accessible under
>> ext3, so it makes sense to run with ext4 w/o any new features for a
>> while until you are sure it is working for you.
> 
> 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).

I would never recommend the ext3-ext4 "tune2fs migration" - you'll end
up with a really weird hybrid filesystems containing files with different
capabilities, and missing many of the metadata layout improvements.

-Eric
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