I believe you will actually get the read IOP's rate of 4 disks, rather
than 8. ZFS doesn't load balance reads accross mirrrors like raid does,
rather it reads from both mirrors so it can compare checksums.
But happy to be shown wrong if someone can provider references? :-)
Tristan
Ross wrote:
Rokka, this really isn't a ZFS problem. 8 SATA disks in a raid-z2 or raid-6
array is going to be horible for eith exchange or SQL, let alone both running
together.
ZFS has a lot of flexibility, and a lot of tools that will let you run these
systems off one pool, but you really need to do a lot more research on the
different performance aspects of different raid levels.
To give a quick example:
raid-6 or raid-z2 gives high throughput, but low iops. Your 8 drives can
stream data at around 6x the speed of a single drive, but when it comes to iops
it's no faster than a single disk.
A 4x2 raid10 set of mirrors gives completely different performance. Yes, you
loose space, that is the tradeoff you get for higher performance. In this
configuration you can write at 4x the speed of a single disk (since writes will
be striped over the 4 sets). Individual reads can run at 1-8x the speed,
depending on how many disks they are striped across. However it's iops where
things really improve. You have write performance of 4x the iops of a single
disk, and read performance of 8x.
So for iops bound loads such as exchange or sql, using raid10 with your disks
will be 4-8x faster than raid-6 / raid-z2.
And don't assume that SSD's will automatically fix the problem. Yes, they can
bring a massive improvement but they are even more complicated than disks in terms
of performance. You need to understand sequential and random read & write
speeds, and the speed at different block sizes. An SSD that can perform very fast
random reads might be horribly slow for writes, and wind up giving you worse
performance than your disk array.
You also have to consider the interface to your server. If you're running over
iscsi, you are probably limited to 90MB/s throughput at best. A single
physical disk can go faster than that so it's unlikely you're even seeing the
throughput benefits of your raid array.
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