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Re: Block alignment of qcow2 compress driver


From: Vladimir Sementsov-Ogievskiy
Subject: Re: Block alignment of qcow2 compress driver
Date: Tue, 1 Feb 2022 17:13:35 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0

28.01.2022 14:07, Richard W.M. Jones wrote:
The commands below set up a sparse RAM disk, with an allocated block
at offset 32K and another one at offset 1M-32K.  Then it tries to copy
this to a compressed qcow2 file using qemu-nbd + the qemu compress
filter:

   $ qemu-img create -f qcow2 output.qcow2 1M
   $ qemu-nbd -t --image-opts 
driver=compress,file.driver=qcow2,file.file.driver=file,file.file.filename=output.qcow2
 & sleep 1
   $ nbdkit -U - \
            data '@32768 1*32768 @1015808 1*32768' \
            --run 'nbdcopy $uri nbd://localhost -p'

The nbdcopy command fails when zeroing the first 32K with:

   nbd://localhost: nbd_aio_zero: request is unaligned: Invalid argument

This is a bug in nbdcopy because it ignores the minimum block size
being correctly declared by the compress filter:

   $ nbdinfo nbd://localhost
   protocol: newstyle-fixed without TLS
   export="":
        export-size: 1048576 (1M)
        uri: nbd://localhost:10809/
        contexts:
   ...
                block_size_minimum: 65536          <----
                block_size_preferred: 65536
                block_size_maximum: 33554432

The compress filter sets the minimum block size to the the same as the
qcow2 cluster size here:

   
https://gitlab.com/qemu-project/qemu/-/blob/cfe63e46be0a1f8a7fd2fd5547222f8344a43279/block/filter-compress.c#L117

I patched qemu to force this to 4K:

-    bs->bl.request_alignment = bdi.cluster_size;
+    //bs->bl.request_alignment = bdi.cluster_size;
+    bs->bl.request_alignment = 4096;

and the copy above works, and the output file is compressed!

So my question is, does the compress filter in qemu really need to
declare the large minimum block size?  I'm not especially concerned
about efficiency, I'd prefer it just worked, and changing nbdcopy to
understand block sizes is painful.

Is it already adjustable at run time?  (I tried using --image-opts
like compress.request_alignment=4096 but it seems like the filter
doesn't support anything I could think of, and I don't know how to
list the supported options.)



Hi!

I didn't read the whole thread, so in case it was not mentioned:

There is a limitation about compressed writes in qcow2 driver in Qemu: you 
can't do compressed write to the same cluster twice, the second write will 
fail. So, when we do some copying (or backup) and write cluster by cluster (or 
at least cluster-aligend) everything works. If you write partial cluster (with 
compress filter) it may work due to automatic RMW, but when you than try to 
write second part of same cluster it will fail.


--
Best regards,
Vladimir



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