RE: Storage array advice anyone?
- From: "Cary Millsap" <cary.millsap@xxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Wed, 15 Dec 2004 13:59:21 -0600
Stephen,
I think the way to determine whether such ideas have become outdated is to
test the idea. However, I would bet that the following are pretty resilient
ideas:
- When a cache gets full, the subsystem will operate at a speed that's less
than or equal to the speed of the devices beneath the cache.
- The probability of a device failure is proportional to the number of
devices within a system.
Cary Millsap
Hotsos Enterprises, Ltd.
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-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Stephen Lee
Sent: Wednesday, December 15, 2004 9:33 AM
To: oracle-l@xxxxxxxxxxxxx
Subject: RE: Storage array advice anyone?
=20
I appreciate the discussion on the topic. I think additional
considerations on this particular array (Hitachi TagmaStore 9990) are
that the "normal" configuration (according to Hitachi) is that the disks
are in groups of 8; each group is a stripe with parity; the parity
cycles around all drives. When a bad block occurs, the block is NOT
replaced by a spare block on the drive, but the drive is failed and
replaced by a hot spare, and phone home occurs. Which -- I guess -- is
a fairly aggressive drive replacement scheme.
There appears to be agreement that the best performance for most cases
(note: most cases) is to stripe everything across all drives. There
does appear to be some remaining discussion, from a fault tolerance
standpoint, about whether to go strictly with stripe + parity and trust
that Hitachi really has worked out the fault tolerance issues, or assume
that claims from Hitachi are just a bunch of sales hype and insist on
stripe + mirror. Healthy skepticism is useful, but one does not want to
be basing that skepticism on outdated ideas. That is what a lot of this
comes down to: Which ideas and rules are outdated -- given the
capabilities of this new gee whiz hardware -- and which still hold.
The astute reader will note that the stripe + parity is, more or less,
raid 5-ish. But yet again, we have a manufacturer who claims that in
their case the I/O speed penalty is no longer an issue. In the case of
this array, there appears to be some real world experience to support
that claim. Any comments from those who know otherwise, are most
welcome. Again, another one of those "Have some of the ideas about this
become outdated?" sort of thing.
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