On Fri, Jun 04, 2010 at 08:23:32AM -0600, Eric Blake wrote: > On 06/04/2010 02:28 AM, Daniel P. Berrange wrote: > > On Thu, Jun 03, 2010 at 11:57:33PM -0400, Laine Stump wrote: > >> See https://bugzilla.redhat.com/show_bug.cgi?id=599091 > >> > >> Saving a paused 512MB domain took 3m47s with the old block size of 512 > >> bytes. Changing the block size to 1024*1024 decreased the time to 56 > >> seconds. (Doubling again to 2048*1024 yielded 0 improvement; lowering > >> to 512k increased the save time to 1m10s, about 20%) > > > > Surely we should have gone for 1024*1024 in this case ? > > > > NB, since our XML header gets rounded up to a multiple of the block > > size, smaller is better, because we're filling the disk with zeros > > here :-) > > In that case, it may be better to have two constants - the preferred > transfer size (1M), and the XML padding block size (512 as before, or > perhaps 4k given newer disk architectures that prefer 4k), along with > code that allows the final transfer to be shorter than the preferred > size so long as it is still a multiple of the block size. That doesn't work because dd can't seek into the file a size less than its transfer size. ie the seek=NN parameter is specifying a multiple of the bs=XX value, not an absolute seek position in bytes. Daniel -- |: Red Hat, Engineering, London -o- http://people.redhat.com/berrange/ :| |: http://libvirt.org -o- http://virt-manager.org -o- http://deltacloud.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: GnuPG: 7D3B9505 -o- F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :| -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list