Date: Tue, 27 Oct 1998 10:57:51 -0400 (EST) From: Simon Shapiro <shimon@simon-shapiro.org> To: Keith Anderson <keith@apcs.com.au> Subject: RE: DPT Hangs at Boot (Especially with 2.2.7) Message-ID: <XFMail.981027105751.shimon@simon-shapiro.org> In-Reply-To: <XFMail.981027221902.keith@apcs.com.au>
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Keith Anderson, On 27-Oct-98 you wrote:
>
> Dear Simon
>
> Thanks for your information today.
>
> But could you please point me in the right direction.
>
> I use ccd to join drives
>
> ie 3 * 4Gig drives gives me 12 gig in /usr/local/squid/cache
>
> I looked at the FAQ with no luck
>
> what other way do i join drives ?
With a DPT controller, you can create a RAID-0 (thisd is what you describe
in your CCD setup). This RAID array will then appear to Unix (and indeed
to the BIOS too) as a single, large, fast disk.
The difference between a DPT RAID array and a CCD array can be brifly
summarizes as:
* CCD can be done with any device attached to FreeBSD
* DPT arrays only work with DPT controllers
* CCD array causes the kernel to serve individual I/O requests; If you
access something that spans 3 drives, Unix will perfrom 3 distinct I/O
operations, serve at least three interrupts, etc.
* DPT arrays are implemented in the hardware; There is only one
``physical I/O'' per logical I/O from the O/S point of view.
* CCD arrays are only usable from FreeBSD.
* DPT arrays are visible to any O/S that looks at the drives. Actually
DPT arrays look like a single disk to the BIOS itself.
* CCD only implements variations on RAID-0; It is a performance
enhancement but a single drive (partition) failure renders the entire
array useless.
* DPT arrays can be of type 0, 1, or 5, thus allowing for redundancy and
fault resistance in the data. A single DPT controller will run any mix
of drives, array types, and other SCSI devices.
* A CCD device can span partitions, thus allowing stiping across portions
of devices.
* A DPT array always spans whole devices. A disk that is a part of a DPT
array is invisible to the O/S.
* CCD implements the striping in theO/S kernel, thus giving you intimate
control of its behavior at the cost of CPU cycles consumed to implement
this loic.
* DPT implements the array logic in the board firmware, thus relieving the
CPU from RAID operation management and control, at the cost of incrreased
controller price.
There is another set of benefits, unique to a DPT SCSI subsystem:
* ECC error correction code across the SCSI cables and drives protects the
data stored and retrieved from controller memory errors, cable errors and
drive errors.
* True hot plug operation, including hot standby drives, hot swap, etc.
Keith, since this seems a generic question, I am forwarding it to
freebsd-questions. I hope you do not mind...
Sincerely Yours, Shimon@Simon-Shapiro.ORG
770.265.7340
Simon Shapiro
Unwritten code has no bugs and executes at twice the speed of mouth
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