zfs discuss - Jul 2010 - Performance advantages of spool with 2x raidz2 vdev"s vs. Single vdev

Hi guys, I am about to reshape my data spool and am wondering what performance
diff. I can expect from the new config. Vs. The old.

The old config. Is a pool of a single vdev of 8 disks raidz2.
The new pool config is 2vdev''s of 7 disk raidz2 in a single pool.

I understand it should be better with higher io throughput....and better
read/write rates...but interested to hear the science behind it.

I have googles and read the ifs best practice guide and evil tuning, but neither
cover my question.

Appreciate any advice.

FYI, it''s just a home server....but I like it.
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This message posted from opensolaris.org

On Mon, 2010-07-19 at 01:28 -0700, tomwaters wrote:
> Hi guys, I am about to reshape my data spool and am wondering what
performance diff. I can expect from the new config. Vs. The old.
> 
> The old config. Is a pool of a single vdev of 8 disks raidz2.
> The new pool config is 2vdev''s of 7 disk raidz2 in a single pool.
> 
> I understand it should be better with higher io throughput....and better
read/write rates...but interested to hear the science behind it.
> 
> I have googles and read the ifs best practice guide and evil tuning, but
neither cover my question.
> 
> Appreciate any advice.
> 
> FYI, it''s just a home server....but I like it.

Very simple.  2vdevs gives 2 active "spindles", so you get about twice
the performance of a single disk.

raidz2 generally gives the performance of a single disk.

For high performance, if you can sacrifice the storage, I recommend a
vdev made up of two-drive mirrors.  This gives pretty good resilience,
and good performance.  (Its not as "safe" as say raidz2, though,
because
with raidz2 you can lose two drives per vdev, where as with mirrors, you
have many more vdevs and can only lose one drive.)

With those same 14 drives, you can get 7x the performance instead of 2x
the performance by using mirrors instead of raidz2.

	-- Garrett

Thanks, seems simple.
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This message posted from opensolaris.org

On Mon, 19 Jul 2010, Garrett D''Amore wrote:
>
> With those same 14 drives, you can get 7x the performance instead of 2x
> the performance by using mirrors instead of raidz2.
This is of course constrained by the limits of the I/O channel. 
Sometimes the limits of PCI-E or interface cards become the dominant 
factor.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

On Mon, 2010-07-19 at 12:06 -0500, Bob Friesenhahn
wrote:
> On Mon, 19 Jul 2010, Garrett D''Amore wrote:
> >
> > With those same 14 drives, you can get 7x the performance instead of
2x
> > the performance by using mirrors instead of raidz2.
> 
> This is of course constrained by the limits of the I/O channel. 
> Sometimes the limits of PCI-E or interface cards become the dominant 
> factor.
> 
> Bob
Of course. ;-)  One *hopes* that if you have direct connects (not via
expanders) that each of the sata channnels has its own channel to the
hba, and the hba has enough pcie bandwidth to support all of its
channels being used fully.  This may or may not be the case in you
system.

	- Garrett

On Mon, Jul 19, 2010 at 01:34:58AM -0700, Garrett D''Amore wrote:
...snip...
> 
> Very simple.  2vdevs gives 2 active "spindles", so you get about
twice
> the performance of a single disk.
> 
> raidz2 generally gives the performance of a single disk.
> 
> For high performance, if you can sacrifice the storage, I recommend a
> vdev made up of two-drive mirrors.  This gives pretty good resilience,
> and good performance.  (Its not as "safe" as say raidz2, though,
because
> with raidz2 you can lose two drives per vdev, where as with mirrors, you
> have many more vdevs and can only lose one drive.)
> 
> With those same 14 drives, you can get 7x the performance instead of 2x
> the performance by using mirrors instead of raidz2.
> 
> 	-- Garrett
I frequently see things like this stated but it doesn''t seem like the
whole
story.  A single raidz vdev may limit the IOPS but it can get fairly good
throughput with linear access.  For instance a single raidz3 vdev of 16
HD203WI Samsung drives (which would then have 13 disks worth of striping)
can write linearly with a dd process at easily over 700 MB/s, which, while
not being the speed of 13 disks (each disk alone can do about 100 MB/s),
is much better than a single disk, and actually the speed of about 7 disks
in this case.  I would venture to guess that most home servers have little
need for lots of random I/O from many clients simultaneously and don''t
need
the high IOPS of several mirrors over one or two vdevs, and such cases might
be better served by the safety of raidz2/3 where you have more flexibility
in which disks are allowed to fail...

Chad

> Hi guys, I am about to reshape my data spool and am wondering what 
> performance diff. I can expect from the new config. Vs. The old.
> 
> The old config. Is a pool of a single vdev of 8 disks raidz2.
> The new pool config is 2vdev''s of 7 disk raidz2 in a single pool.
> 
> I understand it should be better with higher io throughput....and 
> better read/write rates...but interested to hear the science behind it.
> 
> ...
> 
> FYI, it''s just a home server....but I like it.
Some answers (and questions) are here:
http://www.opensolaris.org/jive/thread.jspa?threadID=102368&tstart=0


*** We need this explained in the ZFS FAQ by a Panel of Experts ***

Q: I (we) have a Home Computer and desire to use ZFS with a few large, cheap,
(consumer-grade) Drives. What can I expect
from 3 Drives, would I be better off with 4 or 5. Please note: I doubt I can
afford as many as 10 Drives nor could I stuff them
into my Box so please suggest options that use less than that many (most
prefefably less than 7).

A: ?


Thanks,
Rob
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