Gary Gatten wrote:
OT now, but in high i/o envs with high concurrency needs, RAID5 is
still the way to go, esp if 90% of i/o is reads. Of course it depends
on file size / type as well... Anyway, let's sum it up with "a
storage subsystem is only as fast as its slowest link"

It's not just the balance of reads over writes.  It's the size and sequential
location of the IO requests.  RAID5 is good for sequential reads -- eg.
streaming a video -- where the system can read whole blocks from all the
drives involved, calculate parity over the whole lot and then push all that
blob of data up to the CPU.

RAID5 is pretty pessimal if your usage pattern is small reads or writes
randomly scattered over your storage area -- eg. typical RDBMS behaviour
-- which works a great deal better on RAID10.

I'd also contend that the essential difference between a really good fast
hardware raid controller and something disappointingly mundane is a decent
amount of non-volatile cache memory.  For most H/W raid that equates to
using a battery backup unit.  I've been thinking though that a few GB of
fast solid-state hard drive configured as a gjournal for a RAID10 (ie gstripe
+gmirror) might achieve the same effect for rather less outlay...  It
would probably not be too shabby with RAID5 even, but of course you'ld
lose the benefit of offloading parity calculations onto the RAID controller's
CPU. Still, modern multi-core CPUs are probably fast enough nowadays to
make that viable for many purposes.


Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                 Flat 3
PGP:     Ramsgate
                                                 Kent, CT11 9PW

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