I recently started researching and learning more about IBM’s XIV as an alternative to EMC’s VNX. We already have many VNX’s installed and I’m a very happy EMC customer, but checking out EMC’s competition is always a good thing. I’m looking at other vendors for comparison as well, but I’m just focusing on the XIV for this post. On the surface the XIV looks like a very impressive system. It’s got a fully virtualized grid design that distributes data over all of the disks in the array, which maximizes performance by eliminating hotspots. It also offers industry leading rebuild times for disk failures. It’s almost like a drop in storage appliance, you choose the size of the NL-SAS disks you want, choose a capacity, drop it in and go. There’s no configuration to be done and an array can be up and running in just hours. They are also very competitive on price. While it really is an impressive array there are some important things to note when you compare it to a VNX. Here’s a brief overview list of some interesting facts I’ve read about, along with some of my opinions based on what I’ve read.
Good:
1. It’s got more CPU power. The XIV has more CPU’s and more overall processing power than the VNX. It’s performance scales very well with capacity, as each grid module adds 1 additional CPU and cache.
2. Remote Monitoring. There’s an app for that! IBM has a very nice monitoring app available for iOS. I was told, however, that IBM does not provide the iPad to run it. 
3. Replication/VMWare Integration. VNX and XIV have similar capabilities with regard to block replication and VMWare integration.
5. Granularity & Efficiency. The XIV stores data in 1MB chunks evenly across all disks, so reading and writing is spread evenly on all disks (eliminating hotspots). The VNX stores data in 1 GB chunks across all disks in a given storage pool.
6. Cloning and Rebuild Speed. Cloning and disk rebuilds are lightning fast on the XIV because of the RAID-X implementation. All of the disks are used to rebuild one.
7. Easy upgrades. The XIV has a very fast, non disruptive upgrade process for hardware and software. The VNX has a non-disruptive upgrade process for FLARE (block), but not so much on DART (file).
Bad:
1. No Data Integrity Checking. The XIV is the only array that IBM offers that doesn’t have T10-DIF to protect against Data Corruption and has no persistent CRC written to the drives. EMC has it across the board.
2. It’s Block Only. The XIV is not a unified system so you’d have to use a NAS Gateway if you want to use CIFS or NFS.
3. It’s tierless storage with no real configuration choices. The XIV doesn’t tier. It has a large SSD Cache, similar to the VNX’s FastCache which is supported by up to 180 NL-SAS drives. You have no choice on disk type, no choice on RAID type, and you must stay with the same drive size that you choose from the beginning for any expansions later on. It eliminates the ability of storage administrators to manage or separate workloads based on application or business priority, you’d need to buy multiple arrays. The XIV is not a good choice if you have an application that requires extensive tuning or requires very aggressive/low latency response times.
4. It’s an entirely NL-SAS array. In my opinion you’d need a very high cache hit ratio to get the IO numbers that IBM claims on paper. It feels to me like they’ve come up with a decent method to use NL-SAS disks for something they weren’t designed to do, but I’m not convinced it’s the best thing to do. There is still a very good use case for having SAS and SSD drives used for persistent storage.
5. There’s an increased Risk of Data Loss on the XIV vs VNX. All LUNs are spread across all of the drives in an XIV array so a part of every LUN is on every single drive in the array. When a drive is lost the remaining drives have to regenerate mirror copies of any data that was on the failed drive. The probability of a second drive failure during the rebuild of the first is not zero, although very close to zero due to their very fast rebuilt times. What happens if a second drive fails before the XIV array has completed rebuilding the first failed drive? You lose ALL of the LUNs on that XIV array. It’s a “FUD” argument yes, but the possibility is real.
6. Limited Usable Capacity on one array. In order to get up to the maximum capacity of 243TB on the XIV you’d need to fill it with 180 3TB drives. The maximum raw capacity (using 3TB drives) for the VNX 5700 is 1485TB, the VNX 7500 is 2970TB.
