building a RAID system

Mark Hahn hahn at physics.mcmaster.ca
Wed Oct 8 19:39:41 EDT 2003


> I would like to know some advice about what kind of technology apply
> into a RAID file server ( through NFS ) . We started choosing hardware
> RAID to reduce cpu usage.

that's unfortunate, since the main way HW raid saves CPU usage is 
by running slower ;)

seriously, CPU usage is NOT a problem with any normal HW raid,
simply because a modern CPU and memory system is *so* much better
suited to performing raid5 opterations than the piddly little
controller in a HW raid card.  the master/fileserver for my 
cluster is fairly mundane (dual-xeon, i7500, dual PC1600), and 
it can *easily* saturate its gigabit connection.  after all, ram
runs at around 2 GB/s sustained, and the CPU can checksum at 3 GB/s!

concern for PCI congestion is a much more serious issue.

finally, why do you care at all?  are you fileserving through
a fast (>300 MB/s) network like quadrics/myrinet/IB?  most people
limp along at a measly gigabit, which even a two-ide-disk raid0
can saturate...

> The server has a dozen of client workstations connected through a
> switched 100Mbit LAN , all of these equipped with it's own OS and

jeez, since your limited to 10 MB/s, you could do raid5 on a 486
and still saturate the net.  seriously, CPU consumption is NOT an issue
at 10 MB/s.

> machines tough, server only takes care of file sharing.

so excess cycles on the fileserver will be wasted unless used.

> Considering that not all the workstantions would be working full time
> and with cost in mind ¿ it's worth an ATA RAID solution ?

you should buy a single promise sata150 tx4 and four big sata disks
(7200 RPM 3-year models, please).

regards, mark hahn.

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