RAID 6 on SAN vs. RAID 5 + hot spare

[hvk]

Hi guys,

because this is my 1st post here on the beginning I would to say "Hi" to everybody. I was reading this forum for a while, tried to find needed information but could not , so I thought it's right time to write something.

I am going to create array consisted from 9 scsi hdds (MSA1500 + MSA30), later it will be expanded to 14 drives, each 300GB 15K, and was wondering which RAID to choose. I heard a lot that writing on RAID 6 is really slow, but I think it is quite risky to create 14 drives array with about 3TB of data on RAID5 + hot spare, in case one drive will fail I won't have any redundancy during the time the RAID will be rebuild and I think it can take a while.

Could you please, give me some best practices or links regarding problem I have ? Maybe some links which will compare reading/writing on both RAIDs and maybe somebody found or did tests how long it will take to rebuild degradated RAID arrays on R5 and R6 with comparing data amount.

Could let me also know what do you think about RAID6 + 1 hot spare.

Thanks,

ML

[esiebert7625]

Welcome! Here's some helpful links...

http://www.vmware.com/community/thread.jspa?messageID=718641&#718641

SAN Configuration Guide - http://www.vmware.com/pdf/vi3_esx_san_cfg.pdf

SAN Conceptual and Design Basics - http://www.vmware.com/pdf/esx_san_cfg_technote.pdf

SAN System Design and Deployment Guide - http://www.vmware.com/pdf/vi3_san_design_deploy.pdf

Choosing and Architecting Storage for your Environment - http://download3.vmware.com/vmworld/2006/adc0135.pdf

Vmware Infrastructure 3, HP StorageWorks best practices - http://h71019.www7.hp.com/ActiveAnswers/downloads/VMware3_StorageWorks_BestPractice.pdf

RAID Recommendations for local disk - http://www.vmware.com/community/thread.jspa?messageID=622456

Fyi…if you find this post helpful, please award points using the Helpful/Correct buttons.

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Thanks, Eric

Visit my website: http://vmware-land.com

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[tmcd35]

We have gone for a 12 drive array. We are going to use RAID-50 with two hot spares. That will give us a full 4 drive redundancy. The striping across two RAID 5 arrays gives the same overall redundancy as RAID-6 but with an extra speed boost thanks to the stripping. My personal opinion is that RAID-50 is the best for a large number (8+) of drives.

[waynej]

Interesting to note is that EMC has added RAID6 support in FLARE 26. Just FYI.

http://www.emc.com/techlib/pdf/H2891_clariion_raid_6.pdf

[Mike_Deardurff]

Indeed they have. However, RAID6 also has an additional overhead applied to it, even more so with random i/o writes on the disks. Also remember that you have had proactive hot sparring since FLARE 24 too, which somewhat covers the whole dual parity issued applied with RAID6. Almost makes you wonder if it is worth putting in RAID6. Either way, I still think it has its place though.

[hvk]

Hi, thanks for all answers.

I have heard that it is not possible to set up RAID-50 on this array.

I am thinking 'bout 2 options:

1. 14x300GB as RAID6

or

2. 2x 6hdds in RAID5 + 2 global hot spare.

will be greateful for any pros and cons.

I have also problems to set up RAID from ACU GUI, I can't see MSA1500 controller in it, but I can see it in System Management Homepage. I don't know if it is common or do I have to set up something explicitly on FC switch or MSA ?

Thanks.

Hi I have noticed, that MSA30 has got limit to max. storage 2TB, so problably I'll go for option 2.

It would be fgrate if somebody could help with CU GUI problem.

Thanks

Message was edited by:

hvk

[Texiwill]

Hello,

It is possible to have RAID6 + hot spares.....

Raid5 at any time 1 drive can fail. So if I had a failed drive I pick up one hot spare, the array rebuilds... If the array fails during rebuild, that would be 2 Failed drives and even with a hot spare, the array is gone. I have seen this and it was a very very bad time for everyone.

Raid6 can have up to 2 failed drives... So RAID6 + 2 HOT Spares allows for somewhat safer redundancy.

RAID5+2 Hot Spares does NOT equal RAID6.

I like RAID6 for my main array but I use only 6 or so drives for my RAID6. I am not using the full 14 as I need multiple LUNs not one massive LUN. Remember LUN size is an interesting discussion. I rather have multiple 600GB LUNs than 1 massive 1TB LUN.

Best regards,

Edward

[whynotq]

EMC have indeed added Raid 6, primarily it is aimed at customers with Low Cost FC drives of high capacity. these drives are some times considered less reliable and with the introduction of support for these drives as the flare drives the increased redundancy Raid 6 provides is a good option. Performance is close to that of Raid 5 and i expect to see it more in an archive or large data store scenario I don't think it is ideally suited to for use in VMware unless it is used as a repository or large datastore rather than a VMFS for OS VMDKs.