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