[OpenIndiana-discuss] Inefficient zvol space usage on 4k drives

[Jason Lawrence]

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 at hostname:~# zfs create -V 20g -o volblocksize=32k storage/testbed/32k
root at hostname:~# zfs create -V 20g -o volblocksize=8k storage/testbed/8k

root at 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 at 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 at 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...

--jason