VNX vs. XIV

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.  Note:  The first comment on this blog post states that this is not true, you can read the reply below.

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.  Also, once you choose a drive size in the initial config you’re stuck with that size. The maximum raw capacity (using 3TB drives) for the VNX 5700 is 1485TB, the VNX 7500 is 2970TB.

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4 thoughts on “VNX vs. XIV”

  1. I have extensive knowledge of EMC’s CX and VNX arrays and recently had the pleasure of implementing, supporting, upgrading, and adding modules to a Gen 3 XIV array.
    Overall, in our POC testing, we were unable to bring the XIV down. This involved pulling doubledigit drives with extensive iops running, among other tests. Our POC stress test on 6 modules put a peak of 80,000 iops against the XIV. In production, we ran the 6 module disk pool 100% full as we received a Write error on our esx envvironment when the disk pool went full. No data loss and no loss in performance as compared to an empty pool versus a full pool.
    Ading 3 modules was seamless and the data rebalance completed in less than 2 days.
    During production, we had a full module failure with no impact to production. Your negative remarks “1. tierless storage with no real configuration choices, 2. its block only, 4 entirely NL-SAS array, and 6. limited usable capacity on one array” I feel are pointless. Running VNX unified arrays was something good in the recent past but today, I prefer to put my nas on Isilon instead of Celerra. The unified approach of a VNX is no longer a ‘plus’ to me. That also negates #2 about being block only. The NL-SAS debate I feel is negated with the vast amount of front-end cache in the XIV. Running 60 ESX servers in production on the XIV was easy and we did not see any latency issues from the XIV. It just performed. The usable capacity? How many floor tiles do you need to get comparable amount of storage in a VNX7500? the XIV will fit its max capacity in a single floor tile. Would it be nice to be able and cluster multiple XIV’s together? You bet! But it’s not a negative in my book.
    Increased risk of data loss? I disagree. Although I can’t spout the IBM technical details, my experience running an XIV for a production ESX environment was fabulous. It handled the performance, simplified the storage administration, and the web gui simply worked.
    The performance measurement tools aren’t as robust as the analyzer enabler on a VNX, but they came in the base purchase and didn’t require an optional enabler license.
    Overall, the XIV is easy to manage with a web user interface that simply works. My only complaint would be the number of SAN ports that are required to fiber attach the array.

    If I had a choice for a mid-tier array and the choice was either VNX or XIV, my choice would be the XIV.

    1. Thanks for your Reply, Jim, it’s good to hear from someone who has practical experience with both platforms. It’s also good to hear that pulling double digit drives did not bring the XIV down, that was something I was going to try and do in a future POC. I’ll edit my original post and point out your comment. My comments that you thought were “pointless” are my perception of the platform based on what I’ve read and learned about so far, not on any real world experience. I’ll review my thoughts again, along with a few reasons as to why I think some of those items might still be considered negative to some people.

      Does tierless storage with no configuration choices matter? Not necessarily, it’s just a different approach and different from what I’m accustomed to. I like being able to configure and tweak an array to suit different types of workloads using various storage pool and tiering options. The XIV could potentially make all of that unnessary, yes, but some may want to retain that level of control. Does the fact that it’s an all NL-SAS array really matter? Again, not necessarily, and as you point out the large cache in the XIV pretty much makes up for it on the performance side. Some may have concerns about the reliability (MTBF) of NL-SAS disks, however, they do fail more often. Does being block only matter? Not to everyone, no, but some may appreciate that flexibility and the majority of midrange arrays from other vendors offer it as an option. Isilon is certainly better than using VNX data movers, no argument there. The usable capacity issue? Whether or not it’s a negative is really just an opinion. You’re correct that it will fill it’s maximum capacity in only one floor tile and the VNX would take several, but some may prefer the flexibility of a larger maximum capacity in one array regardless of how many floor tiles it uses. Regarding the performance measurement tools, I could go either way on that issue. I appreciate the extreme level of detail I can get from EMC’s Control Center & Performance Manager. I use that information to make informed decisions about managing workloads on our arrays. With the XIV that level of detail may not be needed, as there isn’t really much tweaking to be done on the array itself. I imagine the available performance tools for XIV are more than adequate. One point you didn’t mention in your reply is the lack of data integrity checking, that’s a definite concern.

      I think you’ve hit the nail on the head on the XIV’s biggest strengths. It’s easy to implement and manage, it performs well, and you just drop it in and it works. I’m not convinced yet that it’s the right choice for my organization, but that’s what POC’s are for, right?

  2. We have been running XIVs since the gen 1s, and before IBM bought them. They have always performed well with zero downtime. The Gen3s are even better. I now run a multipetabyte organization without a storage engineer using XIV and 3PARs. Our workload would see little if any benefit from tiering. It does depend on what you are doing with the arrays, but EMC would not even be I my top 5 as far as choices go.

    1. Thanks for posting. I was trying to be as objective as possible in my post, and I mentioned that I have no first hand experience with XIV (in my first comment). The info I posted was all based on what I’ve read and learned from IBM reps and reading info posted from other storage professionals. My company did do a POC with a Gen 1 XIV before I was hired along with the N-Series and both performed exceptionally well but evidently IBM couldn’t compete on price. That may be different in 2014. At the time (around 2009-2010), the CX4 was very competitive in the performance tests that were done and based on the price differential the XIV and N-Series couldn’t be justified. We’ll likely have another POC with IBM this year as it hasn’t been ruled out as a VNX replacement. I’ll post my first hand experiences later this year.

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