building a RAID system

Daniel Fernandez daniel at labtie.mmt.upc.es
Thu Oct 9 14:50:17 EDT 2003


Hi again,

Thanks for the advice, also it has started an interesting thread.

On Thu, 2003-10-09 at 01:39, Mark Hahn wrote:
> > 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 ;)
> 
I cannot get the point here, the dedicated processor should take all
transfer commands and offload the CPU why it would run slower ? In some
tests a raid system for a single workstation ( no networking ) it's a
bit useless (slower) unless you want to transfer really big files. In a
networked environment there could be a massive number of I/O commands so
should be critical.

> 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!
> 
Agreed, our server would not be doing anything more than managing NFS
so, there is power to spare, where talking about an Athlon XP2600+
processor. But, a really good  Parallel ATA 100/133 controller is
needed, and 4 channels at least... 4 HDs in 2 master/slave channels
reduces drastically performance
¿ any controller recommended ?

But must be noted that HW RAID offers better response time.

HW raid offers hotswap capability and offload our work instead of
maintaining a SW raid solution ...we'll see ;)

> concern for PCI congestion is a much more serious issue.
> 
We're limited at 32 bit PCI, we cannot get around this unless spend on a
highly priced PCI 64 mainboard.

> 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.

There would not be noticeable difference between SW/HW mode here. The
clients would be doing write bursts of 2-5Mb per second so there must
not be any problem.

> > 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.
> 

In fact we have two choices:

	- Use an spare existing ( relatively obsolete ) computer and couple it
with a HW RAID card.

        - Spend on a fast CPU computer and a good but cheap Parallel ATA
controller.


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