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Re: [PATCH for-5.1 1/2] block: Require aligned image size to avoid asser

From: Max Reitz
Subject: Re: [PATCH for-5.1 1/2] block: Require aligned image size to avoid assertion failure
Date: Mon, 13 Jul 2020 13:52:41 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

On 13.07.20 13:19, Max Reitz wrote:
> On 10.07.20 16:21, Kevin Wolf wrote:
>> Unaligned requests will automatically be aligned to bl.request_alignment
>> and we don't want to extend requests to access space beyond the end of
>> the image, so it's required that the image size is aligned.
>> With write requests, this could cause assertion failures like this if
>> RESIZE permissions weren't requested:
>> qemu-img: block/io.c:1910: bdrv_co_write_req_prepare: Assertion `end_sector 
>> <= bs->total_sectors || child->perm & BLK_PERM_RESIZE' failed.
>> This was e.g. triggered by qemu-img converting to a target image with 4k
>> request alignment when the image was only aligned to 512 bytes, but not
>> to 4k.
>> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
>> ---
>>  block.c | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
> (I think we had some proposal like this before, but I can’t find it,
> unfortunately...)

(Ah, here it is:


(Which interestingly teases yet another mysterious “we had a discussion
on this before”...))

> I can’t see how with this patch you could create qcow2 images and then
> use them with direct I/O, because AFAICS, qemu-img create doesn’t allow
> specifying caching options, so AFAIU you’re stuck with:
> $ ./qemu-img create -f qcow2 /mnt/tmp/foo.qcow2 1M
> Formatting '/mnt/tmp/foo.qcow2', fmt=qcow2 cluster_size=65536
> compression_type=zlib size=1048576 lazy_refcounts=off refcount_bits=16
> $ sudo ./qemu-io -t none /mnt/tmp/foo.qcow2
> qemu-io: can't open device /mnt/tmp/foo.qcow2: Image size is not a
> multiple of request alignment
> (/mnt/tmp is a filesystem on a “losetup -b 4096” device.)
> Or you use blockdev-create, that seems to work (because of course you
> can set the cache mode on the protocol node when you open it for
> formatting).  But, well, I think there should be a working qemu-img
> create case.
> Also, I’m afraid of breaking existing use cases with this patch (just
> qemu-img create + using the image with cache=none).
> Max

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