In this episode of EMC CLARiiON VNX7500 Configuration Guidelines, I thought it would be useful to discuss Storage Pools, RAID Groups and Thin things (specifically Thin LUNs). But first you should go away and read Vijay’s blog post on Storage Pool design considerations. While you’re there, go and check out the rest of his posts, because he’s a switched-on dude. So, now you’ve done some reading, here’s a bit more knowledge.
By default, RAID groups should be provisioned in a single DAE. You can theoretically provision across buses for increased performance, but oftentimes you’ll just end up with crap everywhere. Storage Pools obviously change this, but you still don’t want to bind the Private RAID Groups across DAEs. But if you did, for example, want to bind a RAID 1/0 RAID Group across two buses – for performance and resiliency – you could do it thusly:
naviseccli -h <sp-ip> createrg 77 0_1_0 1_1_0 0_1_1 1_1_1
Where the numbers refer to the standard format Bus_Enclosure_Disk.
The maximum number of Storage Pools you can configure is 60. It is recommended that a pool should contain a minimum of 4 private RAID groups. While it is tempting to just make the whole thing one big pool, you will find that segregating LUNs into different pools may still be useful for FAST cache performance, availability, etc. Remember kids, look at the I/O profile of the projected workload, not just the capacity requirements. The mixing of drives with different performance characteristics in a homogenous pool is also contra-indiciated. When you create a Storage Pool the following Private RAID Group configurations are considered optimal (depending on the RAID type of the Pool):
- RAID 5 – 4+1
- RAID 1/0 – 4+4
- RAID 6 – 6 + 2
Pay attention to this, because you should always ensure that a Pool’s private RAID groups align with traditional RAID Group best practices, while sticking to these numbers. So don’t design a 48 spindle RAID 5 Pool. That will be, er, non-optimal.
EMC recommend that if you’re going to blow a wad of cash on SSDs / EFDs, you should do it on FAST cache before making use of the EFD Tier.
With current revisions of FLARE 30 and 31, data is not re-striped when the pool is expanded. It’s also important to understand that preference is given to using the new capacity rather than the original storage until all drives in the Pool are at the same level of capacity. So if you have data on a 30-spindle Pool, and then add another 15 spindles to the Pool, the data goes to the new spindles first to even up the capacity. It’s crap, but deal with it, and plan your Pool configurations before you deploy them. For RAID 1/0, avoid private RAID Groups of 2 drives.
A Storage Pool on the VNX7500 can be created with or expanded by 180 drives at a time, and you should keep the increments the same. If you are considering the use of greater than 1TB drives use RAID 6. When FAST VP is working with Pools, remember that you’re limited to one type of RAID in a pool. So if you want to get fancy with different RAID Types and tiers, you’ll need to consider using additional Pools to accommodate this. It is, however, possible to mix thick and thin LUNs in the same Pool. It’s also important to remember that the consumed capacity for Pool LUNs = (User Consumed Capacity * 1.02) + 3GB. This can have an impact as capacity requirements increase.
A LUN’s tiering policy can be changed after the initial allocation of the LUN. FAST VP has the following data placement options: Lowest, Highest, Auto, no movement. This can present some problems if you want to create a 3-tier Pool. The only workaround I could come up with was to create the Pool with 2 tiers and place LUNs at highest and lowest. Then add the third tier and place those highest tier LUNs on the highest tier and change the middle tier LUNs to No Movement. What would be a better solution is to create the Pool with the tiers you want, put all of your LUNs on Auto placement, and let FAST VP sort it out for you. But if you have a lot of LUNs, this can take time.
For thin NTFS LUNs – use Microsoft’s sdelete to zero free space. When using LUN Compression – Private LUNs (Meta Components, Snapshots, RLP) cannot be compressed. EMC recommends that compression only be used for archival data that is infrequently accessed. Finally, you can’t defragment RAID 6 RAID Groups – so pay attention when you’re putting LUNs in those RAID Groups.
