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Re: [PATCH v3 1/2] quorum: Implement bdrv_co_block_status()

From: Max Reitz
Subject: Re: [PATCH v3 1/2] quorum: Implement bdrv_co_block_status()
Date: Fri, 13 Nov 2020 12:36:56 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0

On 11.11.20 17:53, Alberto Garcia wrote:
The quorum driver does not implement bdrv_co_block_status() and
because of that it always reports to contain data even if all its
children are known to be empty.

One consequence of this is that if we for example create a quorum with
a size of 10GB and we mirror it to a new image the operation will
write 10GB of actual zeroes to the destination image wasting a lot of
time and disk space.

Since a quorum has an arbitrary number of children of potentially
different formats there is no way to report all possible allocation
status flags in a way that makes sense, so this implementation only
reports when a given region is known to contain zeroes

If all children agree that a region contains zeroes then we can return
BDRV_BLOCK_ZERO using the smallest size reported by the children
(because all agree that a region of at least that size contains

If at least one child disagrees we have to return BDRV_BLOCK_DATA.
In this case we use the largest of the sizes reported by the children
that didn't return BDRV_BLOCK_ZERO (because we know that there won't
be an agreement for at least that size).

Signed-off-by: Alberto Garcia <berto@igalia.com>
Tested-by: Tao Xu <tao3.xu@intel.com>
  block/quorum.c             |  52 +++++++++++++
  tests/qemu-iotests/312     | 148 +++++++++++++++++++++++++++++++++++++
  tests/qemu-iotests/312.out |  67 +++++++++++++++++
  tests/qemu-iotests/group   |   1 +
  4 files changed, 268 insertions(+)
  create mode 100755 tests/qemu-iotests/312
  create mode 100644 tests/qemu-iotests/312.out

Reviewed-by: Max Reitz <mreitz@redhat.com>

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