Thanks, Richard, for your comments.
Richard Elling wrote:
so much data, so little time... :-)
:) indeed.
Adam Lindsay wrote:
Clearly, there are elements of the model that don't apply to our
sustained read/writes, so does anyone have any guidance (theoretical
or empirical) on what we could expect in that arena?
I have a model for the disk/media bandwidth. In this model, the bandwidth
limit is the media speed of the disk, as determined by the disk vendor's
data sheet. I then apply the RAID configuration to determine the range of
maximum, sustainable, logical data, read, media bandwidth (whew! :-).
For example, consider an X4500 with 6 Hitachi E7K500 (500GByte) disks.
config min (Mbytes/s) max (MBytes/s)
--------------------------------------------------
RAIDZ2 (4d+2p) 124 259
RAID1+0 (2d * 3) 186 389
And, extrapolating (and by implication from Bart's comments), this
scales linearly as you add data (non-parity) spindles? Even the minimum
figures are much more inviting than what I was guessing from the random
read figures.
This will give you a sense of the maximum media bandwidth capabilities
assuming you will blow by all caches. But this does not identify
bottlenecks.
Indeed, and you quite nicely get into the other questions I had
regarding my proposed server: what bottlenecks am I going to run into,
practically? I suspect it's best off in another thread, with your
indulgence.
We know that channels, controllers, memory, network, and CPU bottlenecks
can and will impact actual performance, at least for large configs.
Modeling these bottlenecks is possible, but will require more work in
the tool. If you know the hardware topology, you can do a
back-of-the-napkin
analysis, too.
. Saturate a 1GigE link for sustained reads _and_ writes
... (long story... let's just imagine uncompressed HD video)
This shouldn't be too hard, but you'll need a bunch of disks.
Okay, so I won't be shy about aspiring to saturate an aggregated 2x GigE
link. :)
In the above example, the Hitachi E7K500 is 7,200 rpm, 3.5"
If you want to blaze, then the Seagate Saviio 2.5", 15krpm
disk should be able to do something like 60-95 MBytes/s sustained
(I'm speculating, the last time I checked, they hadn't published
the data sheet yet)
. Do it cheaply
Fast disks aren't inexpensive :-(.
Indeed. That's why I want to scale out to lots of spindles, and was
hoping ZFS would help make up for the other failings of using the cheap
stuff.
My strong desires:
. ZFS for its reliability, redundancy, flexibility, and ease of use
. Maximise the amount of usable space
My resources:
. a server with 16x 500GB SATA drives usable for RAID
That should work, at least as far as the disk media bandwidth requirements.
You'll need to make sure that you have plenty of CPU power to drive the
rest of the system.
That's the first decision I made when speccing out the system, really:
the local vendor (experienced with running linux on this chassis)
originally proposed two fast single-core Opterons. I suggested two
slower (2.2GHz) dual-core ones.
Cheers,
adam
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