On Aug 4, 2009, at 10:22 PM, Bob Friesenhahn <bfrie...@simple.dallas.tx.us
> wrote:
On Tue, 4 Aug 2009, Ross Walker wrote:
Are you sure that it is faster than an SSD? The data is indeed
pushed closer to the disks, but there may be considerably more
latency associated with getting that data into the controller
NVRAM cache than there is into a dedicated slog SSD.
I don't see how, as the SSD is behind a controller it still must
make it to the controller.
If you take a look at 'iostat -x' output you will see that the
system knows about a queue for each device. If it was any other
way, then a slow device would slow down access to all of the other
devices. If there is concern about lack of bandwidth (PCI-E?) to
the controller, then you can use a separate controller for the SSDs.
It's not bandwidth. Though with a lot of mirrors that does become a
concern.
Well the duplexing benefit you mention does hold true. That's a
complex real-world scenario that would be hard to benchmark in
production.
But easy to see the effects of.
I actually meant to say, hard to bench out of production.
Tests done by others show a considerable NFS write speed advantage
when using a dedicated slog SSD rather than a controller's NVRAM
cache.
I get pretty good NFS write speeds with NVRAM (40MB/s 4k sequential
write). It's a Dell PERC 6/e with 512MB onboard.
I get 47.9 MB/s (60.7 MB/s peak) here too (also with 512MB NVRAM),
but that is not very good when the network is good for 100 MB/s.
With an SSD, some other folks here are getting essentially network
speed.
In testing with ram disks I was only able to get a max of around 60MB/
s with 4k block sizes, with 4 outstanding.
I can do 64k blocks now and get around 115MB/s.
There is still bus and controller plus SSD latency. I suppose one
could use a pair of disks as an slog mirror, enable NVRAM just for
those and let the others do write-through with their disk caches
But this encounters the problem that when the NVRAM becomes full
then you hit the wall of synchronous disk write performance. With
the SSD slog, the write log can be quite large and disk writes are
then done in a much more efficient ordered fashion similar to non-
sync writes.
Yes, you have a point there.
So, what SSD disks do you use?
-Ross
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