Unionfs/aufs instead of LVM?

[Rick Stevens]

On Wed, 2007-08-01 at 23:38 -0400, Omri Schwarz wrote:
> I thought I'd ask if 
> anyone else is thinking of this: merging disks and partitions at the
> filesystem level rather than the block device level.
> 
> The advantages that come to mind:
> 
> - If a disk is lost, only the files belonging to it are lost,
> instead of the whole logical volume.
> 
> - Copying a component volume to another one is easier, and can
> be done without any outage to the filesystem at large.
> 
> - Portions of the filesystem can be backed up instantly by
> mandating for example that /home belong to two component
> filesystems that are simple copies of each other.
> 
> - Facile interface for snapshots.
> 
> - Component filesystems that can be set read-only.
> 
> Disadvantages:
> 
> Getting unionfs/aufs to DTRT in this context.

There is a commercial filesystem, Ibrix Fusion, which sort of does
that.  LVM volumes become "segments", owned by a "segment server".
If the segment server dies or if the LVM holding a segment goes off-
line, then only the files on that LVM disappear.  The filesystem
keeps going.  When the LVM or segment server comes back up, the files
magically reappear.  It's sort of like GFS, but on steroids and there's
no single lock manager machine that can bottleneck it.

(yes, we use Ibrix.  It manages five 70TB filesystems for us).

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- Rick Stevens, Principal Engineer             rstevens at internap.com -
- VitalStream, Inc.                       http://www.vitalstream.com -
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-  Perseverance:  When you're too damned stubborn to say "I quit!"   -
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