Re: [Qemu-trivial] [Nbd] [Qemu-devel] [PATCH] nbd: fix trim/discard commands with a length bigger than NBD_MAX_BUFFER_SIZE

[Quentin Casasnovas <quentin.casasnovas@oracle.com>]

On Tue, May 10, 2016 at 05:54:44PM +0200, Quentin Casasnovas wrote:
> On Tue, May 10, 2016 at 09:46:36AM -0600, Eric Blake wrote:
> > On 05/10/2016 09:41 AM, Alex Bligh wrote:
> > > 
> > > On 10 May 2016, at 16:29, Eric Blake <eblake@redhat.com> wrote:
> > > 
> > >> So the kernel is currently one of the clients that does NOT honor block
> > >> sizes, and as such, servers should be prepared for ANY size up to
> > >> UINT_MAX (other than DoS handling).
> > > 
> > > Interesting followup question:
> > > 
> > > If the kernel does not fragment TRIM requests at all (in the
> > > same way it fragments read and write requests), I suspect
> > > something bad may happen with TRIM requests over 2^31
> > > in size (particularly over 2^32 in size), as the length
> > > field in nbd only has 32 bits.
> > > 
> > > Whether it supports block size constraints or not, it is
> > > going to need to do *some* breaking up of requests.
> > 
> > Does anyone have an easy way to cause the kernel to request a trim
> > operation that large on a > 4G export?  I'm not familiar enough with
> > EXT4 operation to know what file system operations you can run to
> > ultimately indirectly create a file system trim operation that large.
> > But maybe there is something simpler - does the kernel let you use the
> > fallocate(2) syscall operation with FALLOC_FL_PUNCH_HOLE or
> > FALLOC_FL_ZERO_RANGE on an fd backed by an NBD device?
> > 
> 
> It was fairly reproducible here, we just used a random qcow2 image with
> some Debian minimal system pre-installed, mounted that qcow2 image through
> qemu-nbd then compiled a whole kernel inside it.  Then you can make clean
> and run fstrim on the mount point.  I'm assuming you can go faster than
> that by just writing a big file to the qcow2 image mounted without -o
> discard, delete the big file, then remount with -o discard + run fstrim.
> 

Looks like there's an easier way:

 $ qemu-img create -f qcow2 foo.qcow2 10G
 $ qemu-nbd --discard=on -c /dev/nbd0 foo.qcow2
 $ mkfs.ext4 /dev/nbd0
 mke2fs 1.42.13 (17-May-2015)
 Discarding device blocks: failed - Input/output error
 Creating filesystem with 2621440 4k blocks and 655360 inodes
 Filesystem UUID: 25aeb51f-0dea-4c1d-8b65-61f6bcdf97e9
 Superblock backups stored on blocks:
	   32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632

 Allocating group tables: done
 Writing inode tables: done
 Creating journal (32768 blocks): done
 Writing superblocks and filesystem accounting information: done

Notice the "Discarding device blocks: failed - Input/output error" line, I
bet that it is mkfs.ext4 trying to trim all blocks prior to writing the
filesystem, but it gets an I/O error while doing so.  I haven't verified it
is the same problem, but it it isn't, simply mount the resulting filesystem
and run fstrim on it:

 $ mount -o discard /dev/nbd0 /tmp/foo
 $ fstrim /tmp/foo
 fstrim: /tmp/foo: FITRIM ioctl failed: Input/output error

Quentin