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

Ted Ts'o
2010-11-09T21:40:28Z

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.)

- Ted