[Libguestfs] [PATCH] file: Zero support for block devices and NFS 4.2

[Richard W.M. Jones]

On Mon, Jul 30, 2018 at 10:01:47AM -0500, Eric Blake wrote:
> On 07/29/2018 07:04 AM, Nir Soffer wrote:
> >If we may not trim, we tried ZERO_RANGE, but this is not well supported
> >yet, for example it is not available on NFS 4.2. ZERO_RANGE and
> >PUNCH_HOLE are supported now on block devices, but not on RHRL 7, so we
> >fallback to slow manual zeroing there.
> >
> >Change the logic to support block devices on RHEL 7, and file systems
> >that do not support ZERO_RANGE.
> >
> >The new logic:
> >- If we may trim, try PUNCH_HOLE
> >- If we can zero range, Try ZERO_RANGE
> >- If we can punch hole and fallocate, try fallocate(PUNCH_HOLE) followed
> >   by fallocate(0).
> >- If underlying file is a block device, try ioctl(BLKZEROOUT)
> >- Otherwise fallback to manual zeroing
> >
> >The handle keeps now the underlying file capabilities, so once we
> >discover that an operation is not supported, we never try it again.
> >
> 
> >
> >Issues:
> >- ioctl(BLKZEROOUT) will fail if offset or count are not aligned to
> >   logical sector size. I'm not sure if nbdkit or qemu-img ensure this.
> 
> qemu-img tends to default to 512-byte alignment, but can be told to
> follow 4k alignment instead. nbdkit includes a filter that can force
> 4k alignment on top of any plugin, regardless of client alignment.
>
> Someday, I'd like to enhance nbdkit to support block size
> advertisement (qemu-img already knows how to honor such
> advertisements). It's on my todo queue, but lower in priority than
> getting incremental backups working in libvirt.

The VDDK plugin actually requires 512 byte alignment.  If a client
issues < 512 byte aligned requests it returns an error :-(

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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