Re: Proposal to improve filesystem/block snapshot interaction

[David Chinner <dgc@sgi.com>]

On Tue, Oct 30, 2007 at 03:16:06PM +1100, Neil Brown wrote:
> On Tuesday October 30, gnb@sgi.com wrote:
> > BIO_HINT_RELEASE
> >     The bio's block extent is no longer in use by the filesystem
> >     and will not be read in the future.  Any storage used to back
> >     the extent may be released without any threat to filesystem
> >     or data integrity.
> 
> 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.

I figured that the easiest way around this is reporting free space
extents, not the amoutn actually freed. e.g.

	4k in file A @ block 10
	4k in file B @ block 11
	4k free space @ block 12
	4k in file C @ block 13
	1008k in free space at block 14.

If we free file A, we report that we've released an extent of 4k @ block 10.
if we then free file B, we report we've released an extent of 12k @ block 10.
If we then free file C, we report a release of 1024k @ block 10.

Then the underlying device knows what the aggregated free space regions
are and can easily release large regions without needing to track tiny
allocations and frees done by the filesystem.

> I guess that is equally domain specific, but the difference is that if
> you try to read from the DONTCOW part of the snapshot, you get bad
> old data, where as if you try to access the subordinate device of a
> snapshot, you get an IO error - which is probably safer.

If you read from a DONTCOW region you should get zeros back - it's
a hole in the snapshot.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group