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