Equallogic partitioning: pools, allocations, etc

Question

I have some new budget coming up to buy an Equallogic PS6110xv. This unit has 24x600G disks and I'm looking at different ways of setting it up.

The two primary uses will be VMs and MySQL.

MySQL: currently uses about 2T on an older SAN. Expected 3y growth would be an additional 2T. Seems like I should create a 4T partition and be done with it.

VMs: Im consolidating about a dozen retired machines into VMs using two hosts. This is where I'm not fully clear on how much space to allocate and how. Do I create one block with all the expected space or just create space for the basics and then mount additional space from the SAN on an as-needed basis? IE create a volume with 50g x 12 for the OS and then another with 5-6T as a 'pool'? I could mount 'thin' volumes on this as needed.

Last: If I'm using RAID-6 is there any advantage to making the whole device into one raid group? Seems like having that many disks to read for a rebuild is asking for trouble. On the other hand, having multiple smaller groups implies a less efficient use of the media.

Just wondering what the conventional wisdom is.

EDIT: (found this post in a VM group):

No, you can't separate the disks in one array into multiple storage groups or raid levels. Here's a quick cheat sheet on EqualLogic architecture/terminology:

Group - a collection of physical arrays that can be accessed as one resource. They are all managed at the same IP and share the same iSCSI target IP's.

Group member - an entire physical array. The entire (usually 16 spindle) array is essentially one big virtual disk. For RAID-10, by default you have two hot spares which means you have a 14-spindle RAID-10 array. On our PS6000XV with 15K 450GB SAS drives we got about 2.72 TB of usable space with RAID-10.

Storage Pool - a way to logically divide members and volumes within a group. However, both group members and volumes can only be part of one storage pool at a time, meaning you can't take a PS6000XV and dedicate 7 spindles to one storage pool and 7 spindles to the other.

The architecture does seem a bit inflexible on the surface, and it is - at least compared to traditional arrays where you can make as many RAID groups as you want within a single enclosure. Keep in mind that EqualLogic is really good for two things - ease of setup and scaling out.

Ease of setup - With most traditional iSCSI SANs, you have a main enclosure which houses some disks and the controllers. If you need more capacity, you add another enclosure, which usually means bringing the SAN down, adding your enclosure, reconfiguring the array controllers, and finally bringing the array back up. With EQL you just plug it in, enter the groups network information and authentication credentials, and it comes online as a member of your group with no downtime.

Scale out - Again, with traditional iSCSI SANs when you add an enclosure you are essentially doubling (or tripling) the number of spindles that your controllers are responsible for. With EQL, as you add capacity you are also adding dedicated controllers. The marketing people will tell you that this leads to more linear scaling of throughput and latency as you add capacity.

So this implies that I will have one 24 member RAID-6 group. Or 23 with a hot spare...

Answer 1

ethrbunny,

The VMware community post you mentioned in your edit is a bit outdated... Dell Equallogic recommendations for RAID policy are laid out in the Choosing a Member RAID Policy document though.

With 24x600GB disks, it's presumable you'll be using either 10K or 15K SAS drives. If that's the case, RAID 6, 10, and 50 are the recommended options. In accordance with what tim mentioned, an Equallogic storage array with 24 drives only lets you chose one RAID type/policy for an enclosure; the actual RAID sets are managed by the array on the back-end with no real user visibility through the GUI. Due to the number of drives, you're correct in being wary of having that many drives in a single RAID set, which is why the array splits them into two separate RAID sets (which is both better for data protection/redundancy and performance).

RAID6 is by far the best RAID policy to select for data protection, assuming that you have the system in warranty and drive failures will be dealt with promptly rather than ignored. The aforementioned document details the statistical likelihood of data loss between the different policies available, and RAID6 is a clear win by this measure.

Performance-wise, RAID6 suffers greatly with random writes in comparison to RAID10. It also experiences a greater performance impact during a failure/rebuild in comparison (though this is almost entirely negated with the copy-to-spare operation introduced with new firmware revisions for handling of preemptive failures).

If your current storage solution incorporates 16 or fewer drives of the same or lesser speed, I would nearly guarantee that a RAID6 policy would provide ample performance and IOPS for your needs in addition to the best capacity and protection level you can get on that array.

However, you could also consider setting up all of your volumes with thin provisioning, and allocate a max capacity for each volume that give you plenty of room to grow (even if that means over allocating to some degree). Start with a RAID10, get your full production environment in place, and then use the SAN Headquarters software provided by Equallogic to measure your performance (feel free to contact support or a technical sales rep for more info on this - they're usually very helpful). If your IOPS on individual drives is sitting below 100 even at your peak utilization, then you can easily get away with converting to a RAID6 to gain some extra capacity. The catch to this is that you cannot convert back from a RAID6 to a RAID10 without performing a factory reset on the array (which is only realistic in large multi-member environments) so make sure to do your research before making the switch.

Summary RAID type recommendation: RAID 6 (verify this w/ Dell after having your capacity needs evaluated) Volumes: 4TB volume for your database + 3+ volumes (perhaps 2TB in size) for VMFS datastores (multiples recommended for various performance reasons), all with thin provisioning enabled

Note 1: RAID10 on this array would give you just under 6TiB of actual usable capacity, while RAID6 would give you just over 10TiB (possibly a touch lower for each after space taken up by array's metadata)

Note 2: These recommendations are all assuming you don't plan to make much use of the replication or snapshot features. If you do, you'll need to take the additional space requirements into consideration as well (making RAID6 an even more favorable option)

Answer 2

You have no influence on how the equallogic builds the raid set. For Raid 6 on PS61x0 it's:

(10+2)(9+2)+1(Spare)

which means you have effectively two Raid 6 sets with one spare disk.