Re: [Qemu-devel] [PATCH 1.8 5/6] qemu-img: add option to align writes to cluster_sectors during convert

[Paolo Bonzini]

Il 25/11/2013 16:32, Peter Lieven ha scritto:
>>
>> Also, a couple of ideas for separate patches.  Perhaps the default value
>> of "-S" could be cluster_size if specified?  This would avoid making raw
>> images too fragmented, and compounding filesystem-level fragmentation
>> with qcow2-level fragmentation.  And 4K is too small a default in my
>> opinion; it could be easily changed to 64K, though 4K was of course an
>> improvement compared to 512 before commit a22f123 (qemu-img: Require
>> larger zero areas for sparse handling, 2011-08-26).
> I would vote for 64K or 256K, we already use the first for some time.
> However, it turned out
> that (much) bigger values decrease performance. Setting it
> to cluster_size can be dangerous. As described in my case its 15MB and
> I think for vhd its 1MB. This can be a lot of zeros that have to be
> written.

What about max(4096, min(bdi->cluster_size, 1048576))?

Paolo