July 28, 2011 at 9:33 am
Hi all,
Just trying to clarify. Im talking to our SAN guy and getting wierd answers (Or im wrong and would to get a confirm on that!)
1: He says that its impossible to have RAID1 with more than 2 disks. As soon as you go more it becomes RAID 10...
To quote"RAID 10 is RAID 1 across more than 2 disks, by definition. i.e. RAID 1 can only be two disks and RAID 10 is a mirrored stripe of 4 or more disks "
Most things dont really mention using RAID 1 in more than a 2 disk config but wiki has:
"In RAID 1 (mirroring without parity or striping), data is written identically to multiple disks (a "mirrored set"). While any number of disks may be used, many implementations deal with only 2."
Its wiki so not like i can really believe what it says. But First i think that RAID 1 can be on more than 2 disks and it just does the same thing as with 2 (Can it be done with odd number of disks? im guess it can?) Copies data to every disk. Then can read from any or use multi is the controller is smart enough to?!
Im trying to find a comparison of RAID 1 vs RAID 10 to find the difference between them?
I think RAID 10 should be a lot quicker at writes? and should be same at reads?
If any one could do a little break down for me that would be great 😀
Thanks
S
July 28, 2011 at 10:31 am
My understanding has always been that RAID-1 can be set up using two or more disks, all mirrored. I think generally most people look at other RAID levels when you use more than 2 disks, simply for the capacity gains.
RAID-10 is 4 drives, with the drives mirrored in pairs (so 2 RAID-1 arrays) then the RAID-1 arrays are striped (RAID-0)
Here's a couple sites with good information on the various RAID levels, that aren't Wikipedia;-)
JetStor: http://www.acnc.com/raid
PCMag: http://www.pcmag.com/article2/0,2817,2370235,00.asp
And here's a RAID capacity calculator:
http://www.z-a-recovery.com/art-raid-estimator.htm
Jason
July 28, 2011 at 11:49 am
It all depends on the final amount of disk being made available. Using 4 disks, if the "virtual drive" space made available to the user is the same as a single drive, then all of the drives are involved in mirroring so RAID = 1 (one is being written to, and that one is being copied 3 more times).
If on the other hand - your virtual driver roughly represents the amount of space for say 2 of the 4 drives, then the virtual drive is "striped" (you're writing to 2 of the disks directly at all times) AND mirrored, so RAID is 1+0 or 10.
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July 28, 2011 at 1:15 pm
RAID-1 can definitely be more than 2 disks. It's just more redundancy, not more space. It means sequential writes, but some controllers will be able to do parallel reads (but keep in mind that read/write conflicts can ruin that concept pretty quickly). No upper limit to the number of disks in RAID-1, except what your controller can deal with.
The reason many/most think of RAID-1 as only being 2 disks is because, most often, adding more doesn't really increase DR capabilities in a cost-effective manner, so it's not used often.
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July 29, 2011 at 7:49 am
Hi all,
yeah thats a really simple way that i should have thought off (Size of the volume)
So it is in RAID10 but we are just getting performance fitting RAID1 ..
I thought Write performance would be a lot better on RAID 10 and read performance would be improved due to the more disks.
Call is open to the SAN company but sadly its LSI who have just been brought out by NetAPP and im having trouble getting support while the NetAPP guys sort out how things will go.
Thanks
s
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