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Re: [Qemu-devel] [PATCHv3 1.8 8/9] qemu-img: increase min_sparse to 128
From: |
Peter Lieven |
Subject: |
Re: [Qemu-devel] [PATCHv3 1.8 8/9] qemu-img: increase min_sparse to 128 sectors (64kb) |
Date: |
Thu, 5 Dec 2013 05:55:22 +0100 |
> Am 05.12.2013 um 03:12 schrieb Eric Blake <address@hidden>:
>
> On 12/04/2013 09:46 AM, Peter Lieven wrote:
>
>>> I guess a sane size would be cluster size. For a raw file 4 KB is
>>> reasonable since that's the file system block size.
>> in case of iscsi the cluster size could be much too high as for example
>> my storage has a cluster_size of 15MB.
>>>
>>> Is it necessary to increase to 64 KB here?
>> No, its indepent of the rest. Paolo suggested to increase it and I can
>> confirm
>> that for my usage case its faster than 4K.
>
> At least on NTFS file systems, 64k is the minimum size of a hole in a
> sparse file. While many file systems support smaller holes, there are
> definitely systems where trying to detect smaller holes only results in
> wasted efforts. Is it worth making the default dynamic based on stat()
> information regarding optimum IO size for the given destination file system?
it is definetely worth it, but i would require additional work and testing. the
current code does not create holes that are aligned to min_sparse and
min_sparse has to be limited to a reasonable size. and i wonder if the right
value is bs->bl.opt_transfer_lenght, bs->bl.discard_alignment or
bdi->cluster_size/9. maybe depepnding on if its a cow Image or not.
i can look at this. but i would leave the patch out for now.
Peter
>
> --
> Eric Blake eblake redhat com +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>