On 8 August 2013 17:11, Richard Elling <richard.ell...@richardelling.com> wrote: > On Aug 7, 2013, at 2:50 PM, Jason Lawrence <jjlaw...@gmail.com> wrote: > >> This might be a better question for the Illumos group, so please let me know. >> >> I have a zvol for a KVM instance which I felt was taking up too much space. >> After doing a little research, I stumbled upon >> http://support.freenas.org/ticket/2383 and repeated the test on my machine. >> I'm running a RAIDZ2 pool with eight "advanced format" 4k sector drives. I >> created the pool with ashift=12 with this in mind. >> >> root@hostname:~# zfs create -V 20g -o volblocksize=32k storage/testbed/32k >> root@hostname:~# zfs create -V 20g -o volblocksize=8k storage/testbed/8k >> >> root@hostname:~# dd if=/dev/zero of=/dev/zvol/rdsk/storage/testbed/32k >> bs=1048576 count=20400 >> 20400+0 records in >> 20400+0 records out >> 21390950400 bytes (21 GB) copied, 493.836 s, 43.3 MB/s >> >> root@hostname:~# dd if=/dev/zero of=/dev/zvol/rdsk/storage/testbed/8k >> bs=1048576 count=20400 >> >> 20400+0 records in >> 20400+0 records out >> 21390950400 bytes (21 GB) copied, 548.916 s, 39.0 MB/s >> >> root@hostname:~# zfs list | grep testbed >> storage/testbed 64.2G 5.79T 307K /storage/testbed >> storage/testbed/32k 21.3G 5.79T 21.3G - >> storage/testbed/8k 42.8G 5.79T 42.8G - >> >> >> The 8k blocksize zvol takes up twice the space of the 32k blocksize zvol. As >> 8k is the default, this must be affecting others... > > This is expected. For 4K sector sizes and > 9 disks per raidz2 set: > volblocksize = 8k, raidz2 writes 2 data (8k) + 2 parity (8k) [like > mirroring] > volblocksize = 32k, raidz2 writes 8 data (32k) + 2 parity (8k) [like > RAID-6] > > What most folks miss is: > volblocksize = 4k, raidz2 writes 1 data (4k) + 2 parity (8k) [like > triple mirroring] > > so, don't do 4k recordsize with 4K sector disks unless you are mirroring.
What about disks (i.e. iSCSI connected to a hardware raid; the larger sector size is intentional to improve throughput) which have even larger sectors, e.g. 8k sectors? Lionel _______________________________________________ OpenIndiana-discuss mailing list OpenIndiana-discuss@openindiana.org http://openindiana.org/mailman/listinfo/openindiana-discuss
