Arne Jansen wrote:
David Magda wrote:
On Wed, June 16, 2010 10:44, Arne Jansen wrote:
David Magda wrote:
I'm not sure you'd get the same latency and IOps with disk that you can
with a good SSD:
http://blogs.sun.com/brendan/entry/slog_screenshots
[...]
Please keep in mind I'm talking about a usage as ZIL, not as L2ARC or main
pool. Because ZIL issues nearly sequential writes, due to the
NVRAM-protection
of the RAID-controller the disk can leave the write cache enabled. This
means
the disk can write essentially with full speed, meaning 150MB/s for a 15k
drive.
114000 4k writes/s are 456MB/s, so 3 spindles should do.
Yes, I understood it as suck, and that link is for ZIL. For L2ARC SSD
numbers see:
http://blogs.sun.com/brendan/entry/l2arc_screenshots
oops, sorry, I should at least scrolled down a bit on your link... Nevertheless
I don't find it improbable to reach numbers like that for a proper RAID-setup.
Of cause it will take more space and power. Maybe someone has done some testing
on this.
You don't need a fast disk. It just needs to be at least as large as the
write cache on your RAID controller, and that needs to be large enough
to handle your SLOG needs.
For example, you can get an Areca RAID controller with 4 GB of cache for
about USD$1k. Hook any >4GB disk to it, and you have a _very_ fast 4GB
SLOG device with battery back up.
Of course this is less attractive now that other, less astronomically
expensive options are becoming available.
I'm not sure how that compares in performance to an Acard ANS-9010,
which you can populate with 16GB of RAM + flash backup for about the
same price.
--
Carson
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