Date: Tue, 2 Jan 2001 23:58:26 -0800 (PST) From: Matt Dillon <dillon@earth.backplane.com> To: Greg Lehey <grog@lemis.com> Cc: Zero Sum <count@shalimar.net.au>, Jim King <jim@jimking.net>, Alfred Perlstein <bright@wintelcom.net>, Thomas Seck <tmseck@web.de>, freebsd-stable@FreeBSD.ORG Subject: Re: RAID costs (was: Vinum safe to use for raid 0?) Message-ID: <200101030758.f037wQZ45135@earth.backplane.com> References: <20010102230107.A559@basildon.homerun> <01010313305000.03936@shalimar.net.au> <00e301c0752e$7ba65e60$04e48486@marble> <01010315274900.04373@shalimar.net.au> <20010103172132.T4336@wantadilla.lemis.com>
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Like everything, topologies have their strengths and weaknesses.
RAID-5 is excellent for read-centric operations (which large data stores
tend to be, I will note) and, as Poul reminded me a few days ago...
stripe-sized block-write operations can be made optimal. RAID-5 is
pretty aweful for random-write operations.
Of course, it has to be reliable to be useable, which is really the
crux of the current thread. Someone needs to buy Greg some faster
machines to play with :-), as the current vinum issues appear to be
related to timing.
There are other big differences between software and black-box RAID
solutions. For example, what happens when the machine crashes right
smack in the middle of a write? Hardware RAID (e.g. RAID-5) solutions
have NVRAM to hold the log. Software RAID either has to be extremely
careful in the sequencing of the data, play serial number tricks
(which is why you sometimes see disks with weird physical sector sizes),
or write a separate log and delay the actual disk updates until the log
write has been confirmed.
There are other solutions as well.. distributed data replication, for
example. What use is a RAID-5 disk array if a lightning hit takes out
the whole machine room?
-Matt
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