Thin Pools and Exchange DB vs. Logs

Question

Hypothetical question to help me get some better under standing of Thin Pools.

With a CX4-120 running ver 29. I want to migrate an Exchange environment to a new CX, normally I would put the Exchange DB's on different RAID groups than their corresponding Trans Logs.

Say I have 10 FC disks that are available to use.

Traditionally I would create two RAID groups and put the Exchange DBs on different RAID groups than their corresponding Transaction Logs. I would do this to help spread the log, but also from a data recovery perspective in case a raid group failed and then failed again during a rebuild, i would always have at lease the logs or dbs.

But if I go down the Thin Pool route and put 10 disks into a single Thin Pool, if there were a raid failure and the rebuild of the raid had a failure would the whole thin pool be unavailable?

Any suggestions, I can't seem to find these answers in the documentation?

Thanks.

AranH1 · Accepted Answer

If I understand the pool architecture correctly with regard to Thin Pools though is that the entire pool is not one "RAID" group, so a group can survive multiple disk failures. I think I remember reading that it was similar to RAID5 4+1. If it was otherwise a Thin Pool would be less redundant than a traditional RAID group.

I personally woldn't use 10 disks only to create a thin pool, that is too few for a good distribution of the data. The best practices (again if I remember correctly) was to use a minimum of 20 disks in a pool and to add disks in increments of 5 disks at time. So with only 10 disks to use I would stick with a more traditional RAID group approach.

kenn2347 · Answer

You can create thin pools in to flavors. Raid5 & Raid6. In your question, you would be in trouble if you had created it as raid5. Raid6 can withstand 2 disk failures. It is an extra layer of protection.

You can find a document on Virtual Provisioning here

https://community.emc.com/docs/DOC-6260

AranH1 · Answer

I just looked through the best practices guide for release 29 and found this tidbit in the Virtual Provisioning section:Interesting....

AranH1 · Answer

Also keep in mind that Thin Pools use the global hot spares and proactive sparing so any disk failure is handled in the same manner as traditional RAID Groups.

mtg82814 · Answer

Thanks AranH, that was the answer I was looking for. So I guess if there is a RAID failure in the Thin Pool then another raid failure in the same RAID group, then the pool would be unavailable.

Also, the 10 disks was just for hypothitical purposes, with FLARE 29 you can now use 40 disks in a Thin Pool with CX-120, which is what I would use.

Thanks!

kenn2347 · Answer

I think the number of disks vary depending on whether you use R5 or R6. Page 19 of that doc shows the min/max for pools. the recommended for R6 is 4+2, 6+2 if i remember right. we have a few 6+2 R6 pools. I read somewhere that that was the best config (Performance wise) for R6.

AranH1 · Answer

Kenn,

You are correct, I was referring to just RAID5. I should have clarified that.

As far as the configuration the only option is RAID5 or RAID6, we do not have the option to select the configuration (3+1, 4+2, 6+2). The recommendation is increments of 5 disks for RAID5 and 8 disks for RAID6. But you can add disks in any increment you want up to the pool maximum.

AranH1 · Answer

Yeah, interesting isn't it. Following the logic in the few examples they gave us, maybe it creates one 5+1 R5 group?

mtg82814 · Answer

Just curious, if it is increments of 4+1 RAID 5 groups for a RAID 5 Thin Pool. How is it possible that I am allowed to select 6 disks for the pool, wouldn't that create a 4+1 R5, but then only 1 disk remaining to create the second RAID group in that pool, what would the array do with the remaining disk?

Thanks.

AranH1 · Answer

I don't know how many physical disk failures a thin pool can withstand before you lose data. That is a good question though. Look through the document that Kenn referenced, that is the one I was referring to.

kenn2347 · Answer

Like you example stated, i think for R5, if you select disks in multiples of 5, it creates those 4+1 RG's inside the pool. if you select any number of disks below the next multiple of 5, then it just creates a RG with those disks. In your case you had 6 disks so it made one pool of 6 disks. i think it would have done the same if you had 7,8,9 disks. if you had 10 disks, it would have created a thin pool with (2) 4+1 groups. You get the best performance with 4+1 group so the pools try to create them when they can.

I hope that sounds right.

rupak1 · Answer

A couple of points that I would like to add in this discussion:

- There is seperate internal RAID structure in Thin Pools. FLARE takes care of that automatically without any user intervention. If you select 5 drives R5 pool, it creates a 4+1. Select 6, it will do a 5+1 and so on. However, if you create with 8 drives, it creates a 4+1 and another 2+1.

- It's always recommended to start 5 drives in R5 (or multiples of it). This is for optiomal performance. Also while adding drives to the Thin Pool, suggested method is to select 5 or multiples of it.

-rupak

CLARiiON

Thin Pools and Exchange DB vs. Logs

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