Bill Eldridge <bill@rfa.org> wrote: > I'm currently testing out some SCSI->IDE RAID 5 systems
> (not banging on them too rigorously at the moment, but
> no obvious flaws so far).
>
> Basically, you get a SCSI interface that the computer sees,
> but the enclosure holds 3 IDE drives, and the whole thing
> snugs into a tower chassis. RAID 5 at about $2200 for
> 12 Gigabytes (using 6 Gig Seagates).
Not too shabby!
A caveat, though, about speed vs capacity: RAID
5, when writing, is roughly as fast as a single disk.
Reading, it's a factor of how many disks you have.
I usually estimate read speeds of 7200 rpm disks
as (number of disks * 560 KB/S), so with three
drives in raid 5, read speed is 2*560 or 1120 KB/S
(I don't count the parity drive, of course).
For example, a simple stripe of 6 7200 rpm disks
implies 560 * 6 = 3,360 KB/S reads of random data.
Raid 5 will give you 2,800 KB/S reads and ~360 KB/S
writes (modulo buffering & write-behind, which trade off
reliability for speed)
This is slower than the transfer speeds you usually see quoted,
because it's based on observed start-i/o rates on real
equipment, and 8 KB buffering. In turn, these are limited
by physics: the number of bits the head flies over per second,
which are strictly limited by RPM and bit density.
Vendors may disagree (:-))
--dave
--
David Collier-Brown, | Always do right. This will gratify some people
185 Ellerslie Ave., | and astonish the rest. -- Mark Twain
Willowdale, Ontario | http://java.science.yorku.ca/~davecb
Work: (905) 477-0437 Home: (416) 223-8968 Email: davecb@canada.sun.com
