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.
Both come from sources I believe to be respectable, but.. who
knows...
First: On Philip Greenspun's "Philip and Alex's Guide to Web
Publishing", http://www.photo.net/wtr/thebook/server.html, anyone reads:
-----------
[...]
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.
------------
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)
- 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? 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?
Second: I was wandering by PenguinComputer site, after looking at
that 8way Xeon beast (http://www.penguincomputing.com/8000.cgi), and
following the link related to RAID, (a good source of somewhat biased but
good theoretycal info) on the FAQ section I found
(http://www.penguincomputing.com/RAID.html)
----------
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?
greetings,
*****---(*)---**********************************************---------->
Francisco J. Montilla Systems & Network administrator
[EMAIL PROTECTED] irc: pukka Seville Spain
INSFLUG (LiNUX) Coordinator. www.insflug.org - ftp.insflug.org
