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Date: Mon, 24 Jun 2013 18:13:05 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 7/8] iscsi: assert that sectors are aligned
	to LUN blocksize
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To: Peter Lieven <pl@kamp.de>
Cc: Kevin Wolf <kwolf@redhat.com>, ronniesahlberg@gmail.com, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>

Il 24/06/2013 18:10, Peter Lieven ha scritto:
> Am 24.06.2013 16:30, schrieb Paolo Bonzini:
>> Il 22/06/2013 22:58, Peter Lieven ha scritto:
>>> if the blocksize of an iSCSI LUN is bigger than the BDRV_SECTOR_SIZE
>>> it is possible that sector_num or nb_sectors are not correctly
>>> alligned.
>>>
>>> for now assert that there is no misalignment to avoid data
>>> corruption.
>> You should just fail to open the LUN instead.
> Ronnie added support for reading fragments of blocksize if the
> offset is aligned. I don't know what the idea was, maybe for qemu
> reading the boot sector. If the OS does only read multiple of
> blocksize at aligned offsets everything should work. I now that
> our storages support 4K blocksize. I can check if it is usable.
> I could also just fail the operations instead of asserting.

So far, 4K blocksize is usable if you also specify the same block size
for the guest device.  I have posted once the patches to do
read-modify-write, but I never really pursued inclusion of those.

In any case, the right place to fix this is the block layer;
driver-specific hacks are... hacks. :)

Paolo