[Libguestfs] [PATCH nbdkit v5 FINAL 15/19] file: Implement extents.

[Eric Blake]

On 4/25/19 5:37 AM, Richard W.M. Jones wrote:
> On Wed, Apr 24, 2019 at 02:39:03PM -0500, Eric Blake wrote:
>> On 3/28/19 11:18 AM, Richard W.M. Jones wrote:
>>> This uses lseek SEEK_DATA/SEEK_HOLE to search for allocated data and
>>> holes in the underlying file.

>> Should we also return 0 if the lseek() returned the offset of EOF?

>> reason against: if we expect NBD_CMD_TRIM/NBD_CMD_WRITE_ZEROES or even a
>> third-party to be punching holes in the meantime, then opting out of
>> future lseek() won't see those later additions of holes.
> 
> This last point seems crucial - the file could become sparse, even if
> it starts off as non-sparse.
> 
> The test is designed really to eliminate the case where SEEK_HOLE
> isn't supported and returns an error.

Okay, then let's leave this one as-is.

> 
> However you rightly point out that there's another case we don't
> cover: what if we're using tmpfs where SEEK_HOLE is supported but has
> poor performance?  I would say the best solution is to fix tmpfs!  But
> if that's not possible, perhaps we can do {f,}statfs(2) on the file
> and blacklist certain combinations of f_type and kernel version?

Nah, it's easier to just tell the users to apply the noextents filter in
that case.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 488 bytes
Desc: OpenPGP digital signature
URL: <http://listman.redhat.com/archives/libguestfs/attachments/20190425/ee2d38ac/attachment.sig>