On 19.07.2013 07:58, Paolo Bonzini wrote:
Il 18/07/2013 21:28, Peter Lieven ha scritto:
thanks for the details. I think to have optimal performance and best
change for unmapping in qemu-img convert
it might be best to export the OPTIMAL UNMAP GRANULARITY
Agreed about this.
as well as the write_zeroes_w_discard capability via the BDI
But why this?!? It is _not_ needed. All you need is to change the
default of the "-S" option to be the OPTIMAL UNMAP GRANULARITY if it is
nonzero.
2 reasons:
a) Does this guarantee that the requests are aligned to multiple of the -S
value?
b) If I this flag exists qemu-img convert can do the "zeroing" a priori. This
has the benefit that combined with bdrv_get_block_status requests before it
might
not need to touch large areas of the volume. Speaking for iSCSI its likely that
the user sets a fresh volume as the destination, but its not guaranteed.
With the Patch 4 there are only done a few get_block_status requests to verify
this. If we just write zeroes with BDRV_MAY_UNMAP, we send hundreds or
thousands of writesame requests for possibly already unmapped data.
To give an example. If I take my storage with an 1TB volume. It takes about
10-12
get_block_status requests to verify that it is completely unmapped. After this
I am safe to set has_zero_init = 1 in qemu-img convert.
If I would convert a 1TB image to this target where lets say 50% are at leat
15MB
zero blocks (15MB is the OUG of my storage) it would take ~35000 write same
requests to achieve the same.
Peter
Paolo
and than zero
out the whole device with bdrv_write_zeroes and the BDRV_MAY_DISCARD
flag.
the logic for this is already prepared in patch4 (topic of this email). except
that
i have to exchange bdrv_discard with bdrv_write_zeroes w/ BDRV_MAY_DISCARD.
what do you think?
On Thu, Jul 18, 2013 at 11:43 AM, Peter Lieven <p...@kamp.de> wrote:
Am 18.07.2013 um 16:35 schrieb Paolo Bonzini <pbonz...@redhat.com>:
Il 18/07/2013 16:32, Peter Lieven ha scritto:
(Mis)alignment and granularity can be handled later. We can ignore them
for now. Later, if we decide the best way to support them is a flag,
we'll add it. Let's not put the cart before the horse.
BTW, I expect alignment!=0 to be really, really rare.
To explain my concerns:
I know that my target has internal page size of 15MB. I will check what
happens
if I deallocate this 15MB in chunks of lets say 1MB. If the page gets
unprovisioned
after the last chunk is unmapped it would be fine :-)
You're talking of granularity here, not (mis)alignment.
you are right. for the target i am talking about this is 30720 512-byte blocks
for the granularity (pagesize) and 0 for the alignment.
i will see what happens if I write same w/unmap the whole 30720 blocks in
smaller blocks ;-) otherwise i will have to add support for honoring this
values in qemu-img convert
as a follow up.
Peter
--
Mit freundlichen Grüßen
Peter Lieven
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