Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert

[Paolo Bonzini <pbonzini@redhat.com>]

Il 17/07/2013 12:21, Peter Lieven ha scritto:
>> > BTW, Peter and Ronnie: we were assuming that UNMAP with LBPRZ=1 always
>> > zeroes blocks, but is that true for unaligned operations?
> Good question, I will pass it to ronnie. My guess is that the command will fail with
> a check condition if it failed to unmap the data. From what Ronnie sent earlier
> it should be guaranteed that the blocks are at least zero after the unmap command.

I'm not so sure about that, SBC says explicitly that a misaligned unmap
request "may result in unmap operations on fewer LBAs than requested".

Perhaps it's better to do this:

- if LBPWS=1, use WRITE SAME(16).  For WRITE SAME it is explicit that
"if the device server does not deallocate or anchor the LBA, then the
device server shall perform the specified write operation".  Expose the
value of LBPRZ in the "discard zeroes" field of BlockDriverInfo.

- if LBPWS=0, you can still support discard iff LBPU=1 and use UNMAP.
But if LBPRZ=1 and LBPWS=0, you should not expose the "discard zeroes"
field of BlockDriverInfo.

> As for the qemu-img patch this shouldn't matter. It uses always blocks of bdi->max_unmap
> which should be a multiple of the alignment.

Also, the remainder itself (which SBC calls "UNMAP GRANULARITY
ALIGNMENT") may not be 0.  I think the above trick (only exposing LBPRZ
if you'll use WRITE SAME) will sidestep the problem.  It may also remove
the need to fall back to has_zero_init=1.

Paolo