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If You Thought the Term “Midrange Array” was Nebulous Try Defining what a “Snapshot” Really Is

The overwhelming success of the 2010 Midrange Array Buyer’s Guide that DCIG released in May 2010 did not come without some caveats. One of the specific areas in the Buyer’s Guide that merited closer attention was in the area of replication software. It is not that midrange array replication software was ignored in the Buyer’s Guide. But it quickly became evident that in order to do this topic justice replication software required its own dedicated Buyer’s Guide which is what DCIG will be releasing in the first half of 2011.

After the 2010 DCIG Midrange Array Buyer’s Guide was complete, there were two general comments that I received from both end-users and vendors alike. First, there was too much emphasis placed on storage capacity and not enough on storage efficiency. That I hope to rectify in next year’s 2011 Midrange Array Buyer’s Guide. Second, the analysis on replication software lacked sufficient depth.

So now having had a chance to review replication software more in depth, I could not agree more with that assessment. However now having had a chance to review replication software in much more depth, it also became apparent to me that a totally separate Buyer’s Guide was needed to properly describe, evaluate and rank this feature on midrange arrays.

One of the comments I made early on in the 2010 Midrange Array Buyer’s Guide was that the term “midrange arrays” has become a nebulous one at best considering all of the new architectures and designs that have emerged in the last decade. That same sentence could be applied when trying to describe replication software on midrange arrays because, after all, what does it really mean when a vendor says, “Our array supports replication software”?

In doing my preliminary research, it turns out it could mean a whole host of things. It could mean the array does snapshots. It could mean it does asynchronous replication. It could mean it does synchronous replication. But even in cases where the vendor affirms that it can perform all three of these features that still does not really mean it has authoritatively answered the question as even these three terms are far too nebulous for users to make any type of informed decision.

Take the term “snapshot.” That term seems pretty innocuous, does it not? After all, when a vendor says it can do a snapshot, surely it means its replication software is just doing a snapshot, right? Ah, if only it were that simple. Because in the hazy world of replication where fuzziness rules, a snapshot can mean any number of things as it can be implemented in at least five different ways that fall into two broad categories.

Some midrange arrays only do differential snapshots of which they are two kinds: allocate on write and copy on write. As to which one implementation is best depends more on what your business requirements are than what the vendor says is best and having the option to select either one depending on the application requirements may actually be the best implementation.

But differential snapshots are only one category of snapshot. There is also a category of snapshots known as full snapshots or clones. Here again, there is more than one type of full snapshot. (I have identified at least three.)
The scary part from an end-user perspective is that vendors conveniently lump both of these categories of snapshots with their five different types under the broad “snapshot” label. So unless someone really digs deep to understand all of these different ways in which snapshot may be implemented and how it is implemented on the midrange array that they are looking to buy, they may end up with a snapshot implementation that is totally inappropriate for their environment or can only be used in part because of the overhead associated with implementing and managing.

It is for reasons like this that I decided a totally separate Buyer’s Guide on Midrange Array Replication Software was needed. It will look at all of the different ways replication software is implemented on midrange arrays as nearly every midrange array supports some form of replication software (over 97% of those midrange arrays included in the 2010 DCIG Midrange Array Buyer’s Guide supported one or more forms of replication software.)

However this Midrange Array Buyer’s Guide will go into much more detail and expose exactly how each form of replication software is implemented on midrange arrays. So unlike the Midrange Array Buyer’s Guide where asynchronous replication, synchronous replication, clones and snapshots were “yes/no” type answers, this Buyer’s Guide will peel back the onion and let users know what they are really getting. Further, this Buyer’s Guide will rank these replication software features from top to bottom so users will know which ones have the best implementation of each of these replication software features.

Also, for any midrange array vendors reading this blog entry, if you support replication software on your midrange array and would like to be included in this forthcoming Buyer’s Guide, please let me know. While I know who many of you are, there may be some I am sure I am overlooking so do not hesitate to email me.

Surveys will be going out late this week and early next and will be due back to DCIG before Christmas so we can put out this Buyer’s Guide in the first half of 2011. Also, please note, this Buyer’s Guide will exclusively look at replication software on SAN-attached midrange arrays so those midrange arrays that only supprt NAS will NOT be covered in this particular Buyer’s Guide.


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