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

[Stefan Hajnoczi <stefanha@redhat.com>]

On Thu, Dec 05, 2013 at 05:55:22AM +0100, Peter Lieven wrote:
> 
> 
> > Am 05.12.2013 um 03:12 schrieb Eric Blake <eblake@redhat.com>:
> > 
> > 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.

Okay, I'll drop this patch for now.  It can be improved in a separate
series.

Stefan