Re: [Ceph-maintainers] Deprecating ext4 support

[Loic Dachary <loic@dachary.org>]

Hi Sage,

I suspect most people nowadays run tests and develop on ext4. Not supporting ext4 in the future means we'll need to find a convenient way for developers to run tests against the supported file systems.

My 2cts :-)

On 11/04/2016 23:39, Sage Weil wrote:
> Hi,
> 
> ext4 has never been recommended, but we did test it.  After Jewel is out, 
> we would like explicitly recommend *against* ext4 and stop testing it.
> 
> Why:
> 
> Recently we discovered an issue with the long object name handling that is 
> not fixable without rewriting a significant chunk of FileStores filename 
> handling.  (There is a limit in the amount of xattr data ext4 can store in 
> the inode, which causes problems in LFNIndex.)
> 
> We *could* invest a ton of time rewriting this to fix, but it only affects 
> ext4, which we never recommended, and we plan to deprecate FileStore once 
> BlueStore is stable anyway, so it seems like a waste of time that would be 
> better spent elsewhere.
> 
> Also, by dropping ext4 test coverage in ceph-qa-suite, we can 
> significantly improve time/coverage for FileStore on XFS and on BlueStore.
> 
> The long file name handling is problematic anytime someone is storing 
> rados objects with long names.  The primary user that does this is RGW, 
> which means any RGW cluster using ext4 should recreate their OSDs to use 
> XFS.  Other librados users could be affected too, though, like users 
> with very long rbd image names (e.g., > 100 characters), or custom 
> librados users.
> 
> How:
> 
> To make this change as visible as possible, the plan is to make ceph-osd 
> refuse to start if the backend is unable to support the configured max 
> object name (osd_max_object_name_len).  The OSD will complain that ext4 
> cannot store such an object and refuse to start.  A user who is only using 
> RBD might decide they don't need long file names to work and can adjust 
> the osd_max_object_name_len setting to something small (say, 64) and run 
> successfully.  They would be taking a risk, though, because we would like 
> to stop testing on ext4.
> 
> Is this reasonable?  If there significant ext4 users that are unwilling to 
> recreate their OSDs, now would be the time to speak up.
> 
> Thanks!
> sage
> 
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-- 
Loïc Dachary, Artisan Logiciel Libre
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