On 01/17/2013 10:35 AM, Alexander 'Leo' Bergolth wrote: > If using some kind of container like lvm or dm-crypt that has a > data-offset inside my md device... > Do I need to align the data offset to the stripe size (chunk-size*number > of data-bearing disks) or just to the chunk size? > > If aligning to the chunk size is sufficient, could anyone give an > explanation? Doesn't this break filesystem alignment inside the > container? (I.e. the --stripe-width option) I've done some benchmarks using a raid-6 md-device (5 disks, 64k chunksize) with lvm (different data-offsets) and ext4 on top. The ext4 filesystem is always aligned to the array (stride=16,stripe-width=48) but the data offset of the lvm physical volume is either the default of 2048 sectors (which isn't a multiple of the stripe size of 3*64k) or 384 sectors (3 data bearing disks * 64k chunks). The results are available at http://leo.kloburg.at/tmp/raid-dataoffsets/ It looks like it doesn't make any difference if the data offset is a multiple of the stripe size or just of the chunk size. But why doesn't a wrong data-offset break the --stripe-width hint given to mkfs.ext4??? I'd really appreciate if someone could shed some light on that. Thanks, --leo P.S.: The benchmarks were done with bonnie++, details can be found in bench.sh at the above URL. -- e-mail ::: Leo.Bergolth (at) wu.ac.at fax ::: +43-1-31336-906050 location ::: IT-Services | Vienna University of Economics | Austria -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
