Re: [PATCH 1/6] fs: add hole punching to fallocate

Jan Kara
2010-11-09T21:52:33Z

On Tue 09-11-10 16:41:47, Ted Ts'o wrote:
> On Tue, Nov 09, 2010 at 03:42:42PM +1100, Dave Chinner wrote:
> > Implementation is up to the filesystem. However, XFS does (b)
> > because:
> >
> > 1) it was extremely simple to implement (one of the
> > advantages of having an exceedingly complex allocation
> > interface to begin with :P)
> > 2) conversion is atomic, fast and reliable
> > 3) it is independent of the underlying storage; and
> > 4) reads of unwritten extents operate at memory speed,
> > not disk speed.
>
> Yeah, I was thinking that using a device-style TRIM might be better
> since future attempts to write to it won't require a separate seek to
> modify the extent tree. But yeah, there are a bunch of advantages of
> simply mutating the extent tree.
>
> While we're on the subject of changes to fallocate, what do people
> think of FALLOC_FL_EXPOSE_OLD_DATA, which requires either root
> privileges or (if capabilities are in use) CAP_DAC_OVERRIDE &&
> CAP_MAC_OVERRIDE && CAP_SYS_ADMIN. This would allow a trusted process
> to fallocate blocks with the extent already marked initialized. I've
> had two requests for such functionality for ext4 already.
>
> (Take for example a trusted cluster filesystem backend that checks the
> object checksum before returning any data to the user; and if the
> check fails the cluster file system will try to use some other replica
> stored on some other server.)
Hum, could you elaborate a bit? I fail to see how above fallocate() flag
could be used to help solving this problem... Just curious...

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR