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Moresukine
33 Posts
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2401
July 14th, 2010 06:00
RAID groups for backup to disk on CX4-240
Hello community,
I am going to set up a CX4-240 as a exclusive backup to disk solution in order to back up the data stored on our NS4-480.
Hardware is:
2 DAE filled with 15x 2TB SATA drive
1 DAE filled with 8x 2TB SATA drive
So the total drive count is 38.
searching the forum for some hints, I found that I have to read the docs below and did it.
EMC CLARiiON Storage System Fundamentals for Performance and Availability
EMC CLARiiON Best Practices for Fibre Channel Storage
After reading I considered to be on the safe side to use RAID6 in order to get double drive failure protection while the performance should still be good enough for backup to disk usage (sequential writes, large I/O and a steady I/O flow).
Unfortunately even after reading thoroughly I don't feel safe to decide on a RAID group size.
I thought about 3 RAID 6 Groups (10+2) using up the first 36 drives and to have 2 Spares.
I'd be happy if anyone would comment on my assumptions and point out if I missunderstood something.
Furthermore any recommendation on how to place the groups within the DAEs would be very wellcome.
Thanks in advantage and kind regards
Mike
1004
0
Posted July 14th, 2010 06:00
Welcome to the forums, Moresukine !
I think you prepared yourself well
What I did in the past to determine the optimal RG size is a bit sneaky: I looked at the config of an old CDL (Clariion Disk Library) and copy the RG setup from there (4+1 and 8+1), but nowadays with the very large disk sizes as you mention the old CDL setup is not be the optimal one anymore, since Raid6 is probably the better and safer choice.
Purely based on numbers I'd say 3 x (10+2) plus 2 HS's is a good choice, since you have no drives left over and the number of HS's is about right too, but I know there's a optimal stripe size which involves stripes to either 4 or 8 drives, so in your case the (8+2) is the cheapest (cheaper than (4+2) or something), but still safest way (Raid6), but that'll leave you with 3 x (8+2) plus 6 HS's, which is a bit overkill on the HS side.
Could you go to 4 x (8+2) plus 2 HS's ? You still need 3 DAE's, which you already have, so there's no extra cost there, just 4 extra drives.
I know this is only half the story
, so I'd like to know if this assumption of mine that you should choose 8+2 over 10+2 is valid. Anyone ? I can explain why smaller RG's for random I/O are better, but I always seem to get stuck in translation with sequential data streams....
1004
1
Posted July 14th, 2010 07:00
The perferred configuration is the 8+2. This will generally have better caching performance than the 10+2. BTW, this is the configuration that is used by EMC's backup suite.
1004
0
Posted July 14th, 2010 07:00
Mike,
You said that the CX4-240 has three DAEs populated with SATA drives. Are one of those DAEs the DAE-OS? It will be bus 0 encl 0. If so then I would not use disks 0-4 as part of a RAID group used as a backup to disk target.
Aran
1004
0
Posted July 14th, 2010 07:00
I thought so, but I had no proof to support my story
So the new DL's also have Raid6 in 8+2 ? Do you know if all raid groups are 8+2 or is it a mix like on the old CDL's (4+1 and 8+1) ?
1004
0
Posted July 14th, 2010 11:00
I don't know.
The beauty of the 8+2 is its 512KB stripe size. It aligns real well with a backups I/O size.
1004
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Posted July 14th, 2010 16:00
Aran, i think that the only CX4 array that allows SATA II in enclosure 0-0 is a CX4-120.
1004
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Posted July 15th, 2010 01:00
AranH was a bit confused about SATA drives in the CX4-240's first enclosure. Forgive him. I think he nees a vacation.....
1004
0
Posted July 15th, 2010 07:00
I do and I was ...
I couldn't remember which models could use SATA for the vault disks. Those models are just so small, I never work with them.
847
0
Posted July 16th, 2010 04:00
The CX3-10 can use SATAN drives as vault disks. All other models can only have FC in the first enclosure.
jps00
2 Intern
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392 Posts
1490
0
Posted July 16th, 2010 06:00
Mike,
The Hot Spare provisioning recommendation is 1:30.. That is one hot spare for every 30 workload drives. The reliablity of the 2TB drives is actually pretty high, and you won't be running them 7/24. You'll also be in RAID 6, which has you double failure protected. You can get by with a single hot spare, instead of two.