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On 12/29/2013 08:12 AM, Day, Phil wrote:
> Hi Folks,
> 
>  
> 
> As highlighted in the thread “minimal review period for functional changes” I’d like to propose that change is https://review.openstack.org/#/c/63209/ is reverted because:
> 
>  
> 
> -          It causes inconsistent behaviour in the system, as any existing "default" backing files will have ext3 in them, so a VM will now get either ext3 or 3ext4 depending on whether the host they get created on already has a backing file of the required size or not.   I don't think the existing design ever considered the default FS changing - maybe we shouldn’t have files that include "default" as the file system type if it can change over time, and the name should always reflect the FS type.

I'm not sure this is a practical issue since ephemeral storage is built up from blank by each instance

> -          It introduces a new a new requirement for GuestOS's to have to support ext4 in order to work with the default configuration.   I think that's a significant enough change that it needs to be flagged, discussed, and planned.

This is a genuine concern, though given how long ext4 has been available,
ext4 does seem to be the right default.  I.E. the significant performance benefits
of ext4 creation over ext3, the onus should be on systems requiring support for ext3
only guests, to config appropriately through the existing config vars.
This info would be appropriate for Icehouse release notes.

In any case I agree that this should have been discussed more, and
it would be useful for people to mention on this thread,
any guests that would have issues with ext4 support.

Note I'm assuming that mke2fs creates an ext4 file system compat with
the earliest ext4 implementations, though have not confirmed that?

BTW, if you disable ext4 options so that the FS is mountable by ext3,
you essentially are creating an ext3 file system and so lose the benefits.

$ truncate -s 1T t.fs

$ time mke2fs -F -t ext3 t.fs
real    1m6.223s
$ du -hs t.fs
17G     t.fs

$ time mke2fs -F -t ext4 -O ^extent,^flex_bg,^huge_file,^uninit_bg,^dir_nlink,^extra_isize t.fs
real    1m16.809s
$ du -hs t.fs
17G     t.fs

$ time mke2fs -F -t ext4 t.fs
real    0m1.925s
$ du -hs t.fs
139M    .fs