On 30/12/2010 13:11, Stan Hoeppner wrote:
John Robinson put forth on 12/30/2010 5:58 AM:
On 30/12/2010 10:39, Stan Hoeppner wrote:
[...]
Any RAID scheme that uses parity is less than optimal, and up to
horrible, for heavy random IO loads. As always, this depends on "how
heavy" the load is. For up to a few hundred constant IOPS you can get
away with parity RAID schemes. If you need a few thousand or many
thousand IOPS, better stay away from parity RAID.
Sorry, I have to disagree with this, in this situation. RAID-6 over 4
discs will be just as fast for reading multiple small files as RAID-10
over 4 discs, and a web server is a read-mostly environment, while at
the same time I can't imagine any RAID schema ever giving thousands of
IOPS over 4 discs, parity or no.
That's because you apparently didn't learn about paragraph's in English
class: http://en.wikipedia.org/wiki/Paragraph Do you Brits use
paragraphs differently than we do here in the states?
No, but apparently we use apostrophes correctly over here.
My first paragraph dealt with general performance of parity vs non
parity RAID WRT high IO loads.
Yes, and I suppose that I should have pointed out that the OP's friend
had been given slightly inappropriate advice, since a web server doesn't
do small file I/O like a mailserver. You expanded on a general situation
which didn't apply, and the statement you made was wrong, or at least
not correct in all circumstances.
My second paragraph covered the downside
of the redundancy methods of RAID 3/4.
You were wrong again there: if you lose the parity disc in RAID 3/4 you
don't lose the array, as the data discs are all still there. It is true
that with modern huge (1TB+) drives where the error rate per bit read is
still much the same as when drives were tiny (1GB+) that a recovery is
much more risky than it used to be due to the dramatically increased
chance of a second disc failing, but that is equally true of RAID 5.
My third paragraph dealt
specifically with Roger's web server.
The third and the fourth; jolly good.
Note that nothing in my first paragraph mentioned a web server workload.
Also note that nowhere did I mention a count of 4 drive, nor commented
regarding the suitability of any RAID level with 4 drives.
No indeed, but that was the context of the question; why give entirely
general advice when a specific usage applies?
Also note there were two "situations" mentioned by Roger. The first
referenced a previous thread which dealt with a high transaction load
server similar to a mail server, IIRC.
I see no such reference, apart from noting that "when asking for help,
everybody pounced on us: - NEVER use raid5 for a server doing
small-file-io like a mailserver. (always use RAID10)" which as I say is
in my opinion inappropriate advice, since they're not trying to run a
mailserver and won't have heavy random writes.
My first paragraph related to
that. The second "situation", to which you refer, dealt with Roger's
web server.
I had surmised from the original question about using RAID-10, RAID-4
etc that there was a desire to have more storage than a single drive
mirrored twice, so I didn't think plain mirroring would suit, but
perhaps that wasn't the intention and your solution would work.
Cheers,
John.
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