Re: [Qemu-devel] [PATCHv3 1.8 8/9] qemu-img: increase min_sparse to 128 sectors (64kb)

[Peter Lieven]

> 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
>