On Aug 8, 2013, at 11:34 AM, Lionel Cons <[email protected]> wrote:
> On 8 August 2013 17:11, Richard Elling <[email protected]> > wrote: >> On Aug 7, 2013, at 2:50 PM, Jason Lawrence <[email protected]> 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? In my experience, most iSCSI targets advertise 512 byte sectors, regardless of the back end. For COMSTAR, this can be changed, so it is an easy way to experiment with different "physical" block sizes. -- richard -- [email protected] +1-760-896-4422 _______________________________________________ OpenIndiana-discuss mailing list [email protected] http://openindiana.org/mailman/listinfo/openindiana-discuss
