Re: Proposal to improve filesystem/block snapshot interaction

[Greg Banks <gnb@sgi.com>]

On Wed, Oct 31, 2007 at 10:56:52AM +1100, David Chinner wrote:
> 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.

If you could do that in the filesystem, it certainly solve the problem.
In which case I'll explicitly allow for the hint's extent to overlap
extents previous extents thus hinted, and define the semantics
for overlaps.  I think I'll rename the hint to BIO_HINT_RELEASED,
I think that will make the semantics a little clearer.

Greg.
-- 
Greg Banks, R&D Software Engineer, SGI Australian Software Group.
Apparently, I'm Bedevere.  Which MPHG character are you?
I don't speak for SGI.