Re: [Nbd] NBD, sparse files & TRIM

[Wouter Verhelst <w@...112...>]

Hi Folkert,

(List added)

On Tue, Dec 28, 2010 at 04:23:06PM +0100, folkert wrote:
> Hi,
> 
> Please consider adding TRIM support to NBD.

I'd been thinking that would be a great option, too; it's just that I
wasn't aware of a userspace interface to tell the kernel to revert a bit
of a file back to a sparse state.

> Since XFS can "punch hole in a file" and it seems other Linux
> filesystems may be able to do so too in the near future (see
> http://lwn.net/Articles/415889/ ) this can be nicely integrated.

This of course fixes that problem, and I'd be happy to do something like
that.

It would, however, require an update of the protocol, with an extra
packet to allow the client to communicate to the server that a given
region of the device is not needed anymore.

Since nbd 2.9.18, I've changed the handshake protocol a little, which
allows me to more easily provide backwards compatibility with previous
versions of NBD, even as I add options. So I could imagine the
following:

- The server sets a flag if it supports a TRIM command. "Supports" can
  be interpreted fairly broadly here: it might know about the command,
  but also know that the filesystem on which we're running does not
  support sparse files; or it might be compiled on a system (FreeBSD or
  some such) that doesn't have the fallocate() option in question. In
  such cases, nbd-server might not set the flag.
- If set, nbd-client performs another ioctl() to communicate to the
  kernel that the TRIM command is supported. Since this is just a flag
  we're setting, I suggest a somewhat generic NBD_SET_OPTS ioctl which
  takes a bitmask or something (so that it can easily be extended in the
  future, should the need arise).
- The kernel can now issue TRIM packets.

The packet itself could be something like:

.type: 3 (NBD_CMD_TRIM, or perhaps NBD_CMD_DELETE, doesn't really matter
all that much)
.from: offset to trim
.len: length of stuff to trim

Thoughts?

-- 
The biometric identification system at the gates of the CIA headquarters
works because there's a guard with a large gun making sure no one is
trying to fool the system.
  http://www.schneier.com/blog/archives/2009/01/biometrics.html