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Performance when pooling LUNs

Posted on 2011-02-15
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We currently have a small ESX cluster running off an iSCSI san with 2 nodes.

We are looking at an expansion this year of 2 more nodes.

I plan on making a number of smaller LUNs across all the nodes to take advantage of the extra spindles, but my question is does anyone have an experience with pooling or using metaluns in VMware? I see I have the option of pooling multiple smaller LUNs into a larger one using the storage console. I can see how that would be easier from a management perspective but I have a few questions with how that works.

Do VMDK files get spread across the multiple smaller luns that make up the pool? do the snapshots? or do the files stay together?

What happens if we have a node failure? do just the VMDK files in the small LUN go offline or do we lose the whole pool?

How does VMware allocate space in the pooled lun? Does it fill one of the smaller luns first and then move onto another or does it attempt spread the VMDKs out?
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Question by:PerimeterIT
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Duncan Meyers earned 500 total points
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You must have EMC CLARiiON storage...

Do VMDK files get spread across the multiple smaller luns that make up the pool? do the snapshots? or do the files stay together?
There's two types of MetaLUN - concatenated and striped. A concatenated LUN is created by attaching a lump of storage on the end of an existing LUN. You get the extra space immediately, and it gets used when the original LUN fills up. A striped metaLUN, on the other hand, is created by adding another lump of disk and then restriping (or redistributing) all the data from the original LU across the original and the new LUN. A striped metaLUN gives you more performance as you are now working across more disk spindles. The caveats: the LUNs must be identical size for a striped metaLUN, have the same protection (ie the array won't let you create a striped meta with a RAID 0 and RAID 5 set), be of the same drive types (ie no SATA + FC metas allowed), and you have to be careful how you lay out the LUNs. You musn't put two parts of a metaLUN on the same disks as you end up with disk contention.


What happens if we have a node failure? do just the VMDK files in the small LUN go offline or do we lose the whole pool?
By node, I assume you mean metaLUN component: yes, if you lose a component of the metaLUN, you're in a world of hurt. If you configure you're CLARiiON and metaLUNs according to best practices, that's extremely unlikely.

How does VMware allocate space in the pooled lun? Does it fill one of the smaller luns first and then move onto another or does it attempt spread the VMDKs out?
See above - for a concatenated meta, yes - it fills the first LUN then uses the others. For a striped meta, all LUNs are used equally.
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by:PerimeterIT
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HP Lefthand actually, but same idea.

We have different types of storage nodes with different drive types and RAIDs (bought in different years) so striping is out.

I think I'll avoid the metaluns for now. I'm more worried about maintaining uptime in case of a single storage node failure right now then making things convenient.

Thanks for the help
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by:Duncan Meyers
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Thanks! Glad I could help.
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