Re: Proposal to improve filesystem/block snapshot interaction

[Arnd Bergmann <arnd@arndb.de>]

On Tuesday 30 October 2007, Greg Banks wrote:
> 
> > If the allocation unit of the storage device (e.g. a few MB) does not
> > match the allocation unit of the filesystem (e.g. a few KB) then for
> > this to be useful either the storage device must start recording tiny
> > allocations, or the filesystem should re-release areas as they grow.
> > i.e. when releasing a range of a device, look in the filesystem's usage
> > records for the largest surrounding free space, and release all of that.
> 
> Good point.  I was planning on ignoring this problem :-/ Given that
> current snapshot implementations waste *all* the blocks in deleted
> files, it would be an improvement to scavenge the blocks in large
> extents.  This is especially true for XFS which goes to some effort
> to achieve large linear extents.
> 

Ah, this is an important difference to my idea about an erase operation
on the block device. For erase to be meaningful, you need to know the
erase block size at the file system or user space, so it would be
encoded in the struct block_device, and the user has to issue erase
requests at erase block grenularity.

	Arnd <><
-
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html