[PATCH,RFC] Add support for external log devices for XFS and ext3/4

[Michael Tautschnig]

And another one ...

> Hi,
> 
> The attached patch (code only) adds support for external log devices for
> XFS and ext3/4 to setup-storage.
> 
> External log devices can improve performance a lot when they reside on a
> different physical device than the filesystem itself.
> 

Ok.

> Due to the way mkfs commands are ordered in setup-storage, the commands
> fire as soon as possible, so the mkfs command can run before all the
> physical devices have been partitioned, make it impossible to just use
> createargs to enable an external log device.
> 
> There are two approaches to this issue:
>  - delay all mkfs commands until all the devices have been set up
>    (physical devices, RAID, LVM, cryptsetup ...)
>  - integrate external journals into setup-storage
> 
> I went with the latter because the former requires more profound changes
> in setup-storage and I think it's not worth it/better this way.
> 

I fully concur - I believe the first approach would be a really really ugly
hack. Parsing createopts is really something I'd like to avoid...

> If the patch is accepted, I'll resend a patch including documentation.
> 
> Config example:
> 
> disk_config /dev/sda fstabkey:uuid
> primary    -         256    ext3_journal              -
> primary    -         256    ext4_journal              -
> primary    -         256    xfs_journal               -
> logical    /        2000    ext4:journal=/dev/sda2    defaults
> logical    /home    2000    xfs:journal=/dev/sda3     defaults
> 
> disk_config /dev/sdb fstabkey:uuid
> primary    /foo      10-    ext3:journal=/dev/sda1    defaults
> 
> 
> Thoughts?

[...]

Hmm, the _journal stuff feels a bit like a hack. Couldn't one just have "-" or
ext4 in there and say "-O journal_dev" in createopts? Arguably, that requires a
bit more typing, but the code to process the _journal entries feels hackish. It
requires a lot of special-casing...

Best,
Michael

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