SATA RAID question... basic RAID question

I notice on the motherboard I'm buying (Asus SK8V for AMD FX-51 64-bit CPU),
it has 4x SATA - 2 from south bridge and 2 from promise controller)...

MY QUESTION:
Will I notice a performance decrease if I run whatever RAID config just does
a constant backup - "mirroring" ... basically running two identical SATA
drives and I want to have the "safe" config (mirroring) as opposed to the
"speed" config (striping).

Thanks!

 

Hmm. Well, yes, sortof.

I say "sortof" because if you haven't run for awhile on a RAID0/striped
array...THEN switched to RAID1...you won't have a point of reference against
which to compare the performance [decrease] of a mirrored array.

Is a striped array faster than a mirrored array? Absolutely. But as you
already know, the latter is safer.

That said - and I know there are those that disagree - my counsel would be
to go with the RAID0. The likelihood of a failure on one out of two drives
which are in a RAID1 array is still pretty slim. And if the failure of one
of those two drives ends up taking 2 years...then you will have lost out on
the significant performance boost offered by a striped array for 2 years. A
bit like buying insurance, eh? You just pay your price; just in case.
Except in THIS case, a failure means a bit of time to re-install software,
not a loss of life. Besides, if you set up a striped array, you can always
maintain a clone of the array on a separate drive in the system!

Please post a follow-up.
Ron

 

Hey Ron -
Thanks for your response.
However, my question has nothing to do with striping. I tried striping long
ago and like you say, I found it to be too dangerous.
My question here is this:
Will I take a performance hit by going raid (mirroring) using two drives vs.
just no raid at all with one drive. In other words, when the raid controller
splits the data to write to two devices instead of one, is there a
performance hit?
Thanks!
Scott

Hmm. Well, yes, sortof.

I say "sortof" because if you haven't run for awhile on a RAID0/striped
array...THEN switched to RAID1...you won't have a point of reference against
which to compare the performance [decrease] of a mirrored array.

Is a striped array faster than a mirrored array? Absolutely. But as you
already know, the latter is safer.

That said - and I know there are those that disagree - my counsel would be
to go with the RAID0. The likelihood of a failure on one out of two drives
which are in a RAID1 array is still pretty slim. And if the failure of one
of those two drives ends up taking 2 years...then you will have lost out on
the significant performance boost offered by a striped array for 2 years. A
bit like buying insurance, eh? You just pay your price; just in case.
Except in THIS case, a failure means a bit of time to re-install software,
not a loss of life. Besides, if you set up a striped array, you can always
maintain a clone of the array on a separate drive in the system!

Please post a follow-up.
Ron

 

Raid 1 is slower than raid O and a single drive, in writing because it has to
write to 2 drives. Raid 1 doesn't split the data between the 2 drives, but it
does take longer to write to the 2 drives vs one drive. DOUG

 

Yeah. What he said!

(Merry Christmas, BTW)
Ron

 

Ron,

You can get the answer and that is by running a benchmark, or locating some
benchmarks.

My experience with installing raid 1 on servers is that I am impressed with
their performance, reliability (of the raid mechanism itself - the raid 1 is
self apparent), and ease of setup (Intel ICH5R).

With the Intel ICH5R controller - from experience - it is an absolute
breeze: It does everything as it should. If you get a drive failure, you get
a flashing icon on the task bar (the intel documentation is crappy in the
extreme). Rebuilding a broken array is automatic - just don't delete the
array or fiddle with the config (ever). Notification options are needed:
email is in my view essential, along with Event Log records since a flashing
icon requires you to be at the server to see it (or use terminal server).

I would have no qualms about raid 1 at all. If the performance is not good
enough, then consider the more advanced IDE / SATA raid controllers that are
out. Some include raid 5, cache memory, and increased drive connectivity.
While it is certainly true that raid 1 requires 2 x writes, both writes can
occur at the same time. If both drives are healthy then responses are
received by the controller in close succession, so the elapsed time for a
write is about the same as per a single drive, not twice.

Toms Hardware ran a review on one of these recently and as they do, gave it
their thumbs up.

If the performance is not good enough, but you still need that extra data
assurance, then I would suggest Raptors or SCSI with or without raid. SCSI
drives are substantially more reliable. Ultra 320 SCSI RAID 1 with a caching
controller is brilliant (and a little expensive).

Raid 0 is high risk - the probability of a logical drive failure is
multiplied by the number of drives in the RAID 0. IDE drives seem to be
dropping in reliability, so this is only good for scratch data EG video
editing.

At the end of the day, I suggest you look at the value of your data, your
charge rate, the cost of an outage or loss of data and ask yourself: How
much would I be willing to spend to prevent an outage or loss of data of say
1 day. If you earn $1000 / day (lets hope) then that is what you should
spend. If it took 5 days to recover lost data, then you should consider
spending more - get an AIT tape drive or something.

Final Note: I had 2 drives in a mirrored pair die the other day. First drive
was 100% shot, second came up with 1 bad sector (no earthquakes or anything
like that). The customer was *very* lucky and got everything back. This does
happen, so make sure you have good backups too.

- Tim

 

Nice response, Tim...although I seem to have become the "target" of the
info, rather than the OP. Nevertheless, I always learn something, and
appreciate *anyone's* thoughts.

I agree with most of what you said, but I would like to add an opinion.
(Naturally this is based upon my OWN adventures.)

If you create a striped array with two identical drives (+see note below+)
then you will be enjoying very decent performance. But, yes, you are at
greater risk of data loss. My suggestion is to get the array running
nicely, and then create a cloned image on a THIRD drive [which is outside
the array]. Then, in the event of a failure on one of the RAIDed drives,
you can replace the failed drive, (recreate the array) and restore the
cloned image.

Unfortunately, drives are like light bulbs; they can last for years &
years...or fail after 500 hours. You never know. And unfortunately
SMART-type sentries are not yet 100% reliable. So you have to weigh the
odds; how long will it be before one of two RAIDed drives fails? Six
months? Three years? I would suggest that the odds favour the latter.
Regardless, if you set it up as a mirror, you will be *without* the RAID0
advantage for all of the time that passes UNTIL a drive fails. I think that
a concientious backup program is sufficient "insurance" to warrant risking a
striped array. Heck - you could even create an image on TWO separate
[non-RAIDed] drives...and/or on an *external* drive, if that makes you feel
any safer!

Admittedly, it can get a bit silly. You have to balance these things. In
any case, it's always interesting to hear the logic that people employ
during these musings. (So, Scotter - please let us know, OK?)

Happy New Year, gentlemen.
Ron

 

geez. As usual, I forgot the note.

+Note: I've read a pile of literature about RAID creation, and I acknowledge
that the drives do not HAVE to be identical. But the consensus is that
identical drives leave much less opportunity for conflicts/errors/access
time discrepancies etc.

As always with things like this, there are extremes that must be considered.

Ron

 

Ron,

Sorry I meant to address Scotter....

The most important thing in all this? If you are specifying a config for
someone else, make sure the config is appropriate and the customer
understands or accepts the pitfalls / benefits of the options at hand.

You do seem to think that disc subsystem performance is *the* most important
thing. Often it is secondary to data integrity and reliability - or even
further behind than that.

Most of the machines I install are database servers, so raid 1 or 10 is
always the way to go. Most recent machine has 12 x seagate U320's in raid 10
Pity its not mine. If I had to specify a system for someone doing video
editing I would defintitely follow your leads.

Quote of the day "drives are like light bulbs"...

Does that mean we should buy Russian disc drives? Their light bulbs are
supposed to be extremely reliable.

Happy new year too!

- Tim

 

Lets not forget that the Western Digital Raptor drives come with a 5yr
warranty. That say's something about their perceived reliability.

It's a damn poor mind that can only think of one way to spell a word.
-- Andrew Jackson