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

[Sriram Narayanan]

Thanks for the insight, Richard.

-- Sriram


On Thu, Aug 8, 2013 at 8:41 PM, Richard Elling <
richard.elling at richardelling.com> wrote:

> On Aug 7, 2013, at 2:50 PM, Jason Lawrence <jjlawren at 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 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...
>
> 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.
>  -- richard
>
> --
>
> Richard.Elling at RichardElling.com
> +1-760-896-4422
>
>
>
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