When EMC announced that VNXe will also utilize storage pools my first thought was that it is similar to what CX/VNX has. Storage pool would consist of five disk RAID 5 groups and LUNs would be striped across all of these RAID groups to utilize all spindles. After some discussions with EMC experts I found out that this is not how the pool works in VNXe. In this part I will go a bit deeper into the pool structure and also explain how Storage Pool is created.
Hands-on with VNXe 3300 series [list edited 9/23/2011]:
- Initial setup and configuration
- iSCSI and NFS servers
- Software update
- Storage pools
- Storage provisioning
- VNXe performance
- Wrap up
VNXe 3300 can be furnished with SAS, NL-SAS or Flash drives. The one that I was configuring had 30 SAS disks so there were two options when creating Storage Pools: 6+1 drive RAID 5 groups or 3+3 RAID 1/0 groups. I chose to create one big pool with 28 disks (four 6+1 RAID 5 groups) and one hot spare disk (EMC recommends having one hot spare disk for every 30 SAS disks). EMC also recommends not putting any I/O intensive load on the first four disks because PSL (Persistent Storage Layout) is located on those disks. I wanted to test the storage pool performance with all the disks that were available so I ignored this recommendation and also used the first four disks in the pool too.
When LUN is created it will be placed on the RAID group (RG) that is the least utilized from a capacity point of view. If the LUN created is larger than the free space on an individual RG the LUN will be then extended across multiple RGs but there is no striping involved. So depending of the LUN size and pool utilization a new LUN could reside either in one RG or several RGs. This means that only one RG is used for sequential workloads but random workload could be spread over several RGs. Now if disks are added to the storage pool those newly added RGs are the least utilized and will be used first when new LUNs are created. So a storage pool on VNXe can be considered more as a capacity pool than a performance pool.
Before I wrote this post I was in contact with EMC Technology Consultant (TC) and EMC vSpecialist to get my facts right. Both of them confirmed that the LUNs in VNXe pool are not striped across RGs. Pool structure was explained to me by the EMC TC. Looking at the test results that I posted on part 6 and also looking at the feedback that I got the description above is not accurate. Here is a quote from Brian Castelli’s (EMC employee) comment:
“When provisioning storage elements, such as an iSCSI LUN, the VNXe will always stripe across as many RAID Groups as it can–up to a maximum of four.”
Based on Brian’s comment LUNs in VNXe pool are striped across multiple RGs. [Edited 9/15/2011]
Creating Storage Pools
Storage pools are configured and managed from System – Storage Pools. If no pools have been configured then Unconfigured Disk Pool is only shown.
Selecting Configure Disks will start disk configuration wizard and there are three options to select from: Automatically configure pools, Manually create a new pool, and Manually add disks to an existing pool. Quite easy to understand what each option stands for. I chose the Automatically configure pools option. When using the automatic configuration option 6+1 disk RAID 5 groups are used to create the pool.
Next step is to select how many disks are added to the new pool and you can see that the options are multiples of seven (6+1 RAID 5).
A hot spare pool will also be created when using the automatic pool configuration option.
When selecting Manually create a new pool there is a list of alternatives (see picture below) based on the desired purpose of the pool. This makes creating a storage pool easy because VNXe suggests the RAID level based on the selection that the user made. There is also an option further down on the wizard where the user can select the number of disks used and the RAID level (Balanced Perf/Cap R5 or High Performance R1/0).
It feels a little disappointing to find out that the pool structure wasn’t what I was expecting it to be. But maybe my expectations were also too high in the first place.
Creating a Storage Pool is in line with one of EMC’s definitions for VNXe: simple. When Automatic configuration option is selected Unisphere will take care of deciding what disks are used in the pool and what is the correct number of hot spares needed based on EMC’s best practices.
The next part will cover storage provisioning from VNXe and also using EMC’s VSI plug-in for vCenter.