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Really simple RAID question

Posted on 2011-09-13
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A really simple question.  I have four disks in a server and would like reasonable performance and redundancy but I'm not too worried about space.

Is RAID5 still considered slow?  I figure RAID5 + spare would allow two disks to fail at the same time (live and spare on rebuild) and I still get 50% of space.  But is RAID1+spare or RAID1+0 faster/better solution?
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Question by:RenovoIT
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by:dlethe
dlethe earned 72 total points
ID: 36528062
No, RAID6 allows 2 disks to fail at the same time.   There is no free lunch.  You want data integrity & availability, go RAID6.  You want performance as measured in both I/Os per second and throughput  .. go 2 x RAID1 or 1 x RAID10.   Decide.

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by:Miguel Angel Perez Muñoz
Miguel Angel Perez Muñoz earned 72 total points
ID: 36528074
RAID 5 is slow comparing with other options because must to calculate parity when writes data. IF hasn´t got space problems, think that better solution is RAID 10, it provides better protection against disk failures.
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by:siht
siht earned 71 total points
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In a RAID 5 + spare you are at risk during the rebuild process. If you lose a second disk during the rebuild you lose the array.

Raid 6 will allow any two disks to fail without data loss.

RAID 10 will allow 1 disk from each mirrored set of 2 to fail without data loss. RAID 10 also offers better performance than RAID 5 or RAID 6. This is at the expense of 50% of the member disks capacity.

Whats better depends entirely on your requirements.

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by:McRonis
McRonis earned 142 total points
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RAID10 offer better IOPS than RAID5, and it also more reliable than RAID5.
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by:FarWest
FarWest earned 71 total points
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I wonder why experts use single word "Performance" when they talk about storage and IO,
performance must be compined with Read or Write.
so do you need Higher READ performance, or Higher WRITE Performance, or Balanced
also  it very much depend on RAID controller specially Cache size.
with 4 disks if you need balanced you can use RAID 5 or RAID 10,
for higher READ perofrmance you can use RAID 1, for Write Performance (without high availability) RAID 0
check this
http://www.ahinc.com/raid.htm
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Author Comment

by:RenovoIT
ID: 36528553
This all seems to match with what I assumed.  Perhaps it would also help if I explained what I was doing :)

I'm building a few MS Hyper-V boxes that will use a SAN to store the VMs.  Therefore the local disks will really only be used for the MS Hyper-V OS and perhaps a few test VMs that won't be run on the SAN.  So I want the stability for the OS but I suspect it doesn't need fast write access as once it's loaded it shouldn't need to do much writing.  But If I'm running a few test environment VMs of the local disks, if possible a bit of write performance would be nice.

I'm currently thinking RAID6 would probably be the safest route to go down.  Probably both the slowest for read and write, but I like the idea of being able to lose two active disks (not just one active and the spare on rebuild).
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by:McRonis
McRonis earned 142 total points
ID: 36528586
With RAID10 you can also loose 2 hdd .
Remember, for shared resources, like SAN, Virtualization, IOPS is critical.
The best option is do some testing before any production setup.
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Author Comment

by:RenovoIT
ID: 36528730
The SAN itself is two arrays setup as RAID5+1 (a couple of HP LeftHand SAN boxes).

Previous ESX boxes I've setup that use SAN for VM storage have only had two local disks, so RAID1 and purchase a spare disk to sit in a cupboard that will work in all servers.

Having 4 disks in an existing server just got me thinking if I could get a good setup that also allows stability and the ability to run a couple of test VMs if required from the local disk.
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kevinhsieh earned 72 total points
ID: 36540413
If you have several Hyper-V hosts clustered you don't need much availability for a single host because your cluster should be able survive losing the host for a while. I wouldn't do RAID 0, but a single disk or any RAID level that can survive a disk failure would be okay. The test VMs aren't important either; if they were they would be on the SAN. If the IOPS requirements of the test VMs were high they would also be on the SAN.  

I would recommend RAID 1 with a cold spare on the shelf. Three drives in RAID 5 would be okay if you needed more space (which you probably don't), so would a 4 drive RAID 10, 4 drive RAID 5 or a 4 drive RAID 6.
The more drives you have spinning, the more you are spending on power and air conditioning.
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Author Closing Comment

by:RenovoIT
ID: 36548586
Thanks for the response, all  similarly valid answers I guess.  As mentioned above "no free lunch".  For the servers that support it I've used RAID 6 over the four disks and for the server that doesn't support RAID 6 I've used RAID1+0.  Given that they're Hyper-V hosts I'm hoping there won't be much disk access.
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