Hi,
On Sat, 24 Apr 1999 21:09:05 +0200 (MEST), Francisco Jose Montilla
<[EMAIL PROTECTED]> said:
> Hi, I happen to came across a couple of statements that somewhat
> involves the use of RAID, statements that I believe are not absolutely
> correct, if not false, or half truths.
> -----------
> [...]
> Keep in mind that 99 percent of PC hardware is garbage. A friend of mine
> was a small-time Internet service provider. He was running BSDI, a
> not-quite-free Unix, on a bunch of PC clones. A hard disk was generating
> errors. He reloaded from backup tape. He still got errors. It turned out
> that his SCSI controller had gone bad some weeks before. It had corrupted
> both the hard disk and the backup tapes. He lost all of his data. He lost
> all of his clients' data.
> Lesson 1: You are less likely to lose with a SCSI controller designed
> by a real engineer in the Hewlett-Packard Unix workstation division than
> you are with one thrown in on a $49 sound card.
> Lesson 2: Mirrored disks on separate SCSI chains. Period.
> ------------
No. Lesson number zero: check the consistency of your backups.
Regularly.
> I know the HP part is gonna make Dietmar's delights :). Apart from
> that, I wonder:
> - Doesn't SCSI controllers use parity? (Although you have to
> enable it, of course)
Yes, if the controller supports it, and all modern controllers do. I
don't even think any of our drivers let you disable it any more.
However, most of the cheapo sound-card-based scsi controllers (which
were first designed as a cheap way of interfacing to a cdrom) don't do
parity. Run raid on that? Yeah, right...
> - I agree on using two *controllers* (not two channels on the same
> controller) gives appropiate redundancy if one of they go mad, but
> nonetheless, although we use only one, shouldn't data corruption be
> detected by the controller parity?
No. Errors generated on the cable will be detected. Bus/memory errors
will not; soft errors in the controller will not; and errors in the disk
itself will not.
> One step further, how will the soft RAID code handle this? does it
> have some heuristics to detect that, or is completelly the task of the
> controller and imposible for soft RAID to detect that?
If the IO completes with the status "OK, all IO finished fine", the RAID
code believes it.
> ----------
> Why would I want a two channel RAID card for RAID one?
> By putting each harddrive on a separate channel, you can ensure that even
> if a cable or terminator on one channel were to go bad, the system would
> continue to function.
> When hot-swapping a harddrive, the RAID card must temporarily stop the
> SCSI channel the drive is attached to. If the other drive in a RAID one
> array is connected to a different channel, the computer can operate
> completely normally during the hot-swap.
> ------------
> I agree completely with the first statement. But the second sounds
> somewhat odd to me. I can hotadd or hotremove a disk on linux with sw RAID
> and a non-hot swappable capable controller, maybe this is another feature
> of sw RAID over hw RAID?
You can try, but if the bus is active while you do it, chances are
you'll corrupt data. There _are_ specially designed raid cabinets
which electrically isolate the bus so that you can do this safely, but
that's not the case for your typical scsi bus.
--Stephen
