Re: RAID 5 q - in simpler terms - THANKS!
From: Hunter, Mark (Mark.Hunter_at_ANHEUSER-BUSCH.COM)
Date: 02/15/05
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Date: Tue, 15 Feb 2005 11:31:15 -0600 To: aix-l@Princeton.EDU
On SSA, we use 0+1, all software based. Using mklv with -u, -S, and -c. This
tends to balance I/O better than we can by hand. The complexity increases
because certain commands change behavior if it's a striped logical volume. For
example, if its striped on disks 1-4, then all expansion will be on disks 1-4.
Also, if you lose one disk in a stripe, the sync requires all disks in the stipe
to be resynced. The -P option to syncvg is your friend.
We are in the process of moving to hardware based Raid-5 for almost all of our
systems for non rootvg data. We have an enterprise Hitachi SAN. The economy of
scale helps the cost plus it saves a lot of floor space.
On your statement that no downtime is ever involved on mirroring, that's not
strictly true. If the wrong two drives fail, you will have downtime. We have
had situations where both the primary and secondary copies of a mirror have
failed at nearly the same time. 4 drives mirrored to 4 other drives, but the
"wrong" two drives failed. With raid-5, its any two drives, so it's a greater
chance, but with autospare we have not seen it happen.
We have found that the Hitachi Raid-5 SAN solution is at least as reliable as
most mirroring options in practice.
Mark Hunter
Anheuser-Busch Cos.
MIS Consultant, ES&SO Server Planning and Integration
*Office: (314) 632-6663
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-----Original Message-----
From: IBM AIX Discussion List [mailto:aix-l@Princeton.EDU] On Behalf Of Green,
Simon
Sent: Tuesday, February 15, 2005 4:52 AM
To: aix-l@Princeton.EDU
Subject: Re: RAID 5 q - in simpler terms - THANKS!
That's generally my view. Of course, when you start talking about
multi-terabyte arrays the potential cost savings are considerable. But for
smaller stuff, if you're just using SSA or something like that RAID-0 is much
simpler to manage and gives excellent results.
Some people favour RAID 0+1, where you stripe the data - simple RAID 1 with no
parity - then mirror it, (or vice versa: it can make a difference). This gives
you something of the best of both worlds but it adds to the complexity.
-- Simon Green Altria ITSC Europe s.a.r.l. AIX-L Archive at https://new-lists.princeton.edu/listserv/aix-l.html New to AIX? http://publib-b.boulder.ibm.com/redbooks.nsf/portals/UNIX N.B. Unsolicited email from vendors will not be appreciated. Please post all follow-ups to the list. > -----Original Message----- > From: IBM AIX Discussion List [mailto:aix-l@Princeton.EDU] On Behalf > Of Leyden, Joseph > Sent: 14 February 2005 18:56 > To: aix-l@Princeton.EDU > Subject: Re: RAID 5 q - in simpler terms - THANKS! > > > Thanks! You've answered my question with the overhead being the parity > data. I mentioned the word 'compressed' so we can talk strictly in > terms of bytes. > > Therefore (on the array of 5 disk say 1 disk is 9 GIG) I can not have > 45 gig of application data but only about 32 GIG. AND some time > overhead in rebuilding the replaced failed disk. > > I'm inclined to think that if I can afford disk (1 to 1) mirroring I > should stick with it since NO downtime is ever involved.
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