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RAID Group Size

I've been browsing through all of the technical documentation and am still trying to figure out the best RAID size to go with.  The currently size is set at the deafult of 16.  We started with 2 3240 heads connected to 2 DS4243 shelves each half populated with 12 drives.  We have just purchased another 24 drives to fully populate each shelf.  Before we add the drives we were trying to figure out the best RAID group size.  We are using the storage for a VMWare ESXi and CIFS environment.  Any recommendations would be greatly appreciated.


Thanks,
Andrew

Re: RAID Group Size

Hi Andrew,

Are these SATA or SAS drives?  Keeping the RG size @ 16 would give you three RGs of equal size (48/3=16).  With only 12.5% parity overhead, which is pretty good. I don't see a great deal of motivation to go to anything larger, as long the default provides an equal distribution of disks.

Hope that helps!

- Dan Isaacs - NetApp

Re: RAID Group Size

Daniel,

These are 15K SAS 600GB drives.  I should have mentioned that 1 shelf is assigned to head a and the second shelf is assigned to head b.  We were initially thinking a RAID group size of 22 in order to fully utilize all 24 drives per shelf.

Thanks,

Andrew

Re: RAID Group Size

That is good to go, you will reap the following with it.

Raid size: 22

Number of raid groups: 1

Drives in last raid group: 22

Data drives: 20

Parity drives: 2

usable capacity: 9.62 TB

Thank you,

AK G

Re: RAID Group Size

Hi,

Well, it is a compromise between max capacity & RAID rebuild time  - whilst the RG size of 22 addresses the former, indeed, Synergy tool defaults to RG size of 11 in this scenario, to improve the latter.

Regards,

Radek

Re: RAID Group Size

A raid group size of 22 is outside of best practice. For SAS this should be between 12 and 20.

I'd go as follows:

Controller A: 32 disks assigned, 1 spare, so use a RG size of 16 and use a single aggregate

Controller B: 16 disks assigned 1 spare, RG size of 15 or 16 use a single aggreagate

Re: RAID Group Size

I don't like the idea to loose 2 disks. I only have one SAS shelf and I will never add another shelf with the same disk size (we need new space every 4-5 years).

Is there really a problem using 22 (or 23) disks in the RAID group?

It's a performance degradation issue? How much?

Re: RAID Group Size

There should be no performance impact during normal operation, you will get longer rebuild time (and performance will be degraded during rebuild).

Re: RAID Group Size

Thx!