On 02/24/2011 03:53 PM, Zdenek Kaspar wrote:
And likewise, what if there are three clients, or four clients, ...,
all requesting different but large files simultaneously?
How does one calculate the drive's throughput in these cases? And,
clearly, there are two throughputs, one from the clients'
perspectives, and one from the drive's perspective.
we us Jens Axboe's fio code to model this.
Best case scenario is you get 1/N of the fixed sized resource that you
share averaged out over time for N requestors of equal size/priority.
Reality is often different, in that there are multiple stacks to
traverse, potential seek time issues as well as network contention
issues, interrupt and general OS "jitter", etc. That is, all the
standard HPC issues you get for compute/analysis nodes, you get for this.
Best advise is "go wide". As many spindles as possible. If you are
read bound (large block streaming IO), then RAID6 is good, and many of
them joined into a parallel file system (ala GlusterFS, FhGFS, MooseFS,
OrangeFS, ... ) is even better. Well, as long as the baseline hardware
is fast to begin with. We do not recommend a single drive per server,
turns out to be a terrible way to aggregate bandwidth in practice. Its
better to build really fast units, and go "wide" with them. Which is,
curiously, what we do with our siCluster boxen.
MD raid should be fine for you.
Regards,
Joe
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