It took a while, but the nice folks over at NetApp have attempted to describe what exactly Tape Smart Sizing does. I could be wrong, but I think they agree with me. I guess since I agreed with them that using single parity RAID on a VTL with 1 TB drives is dangerous (yes their Near Store VTL does exactly that...) they decided to return the favor? Perhaps...
In any event, Absolute Architect decided to pen a response which was completely complementary to my analysis. To the best of my understanding, she agrees that Tape Smart Sizing is a "confusing ... technology that won't remotely impact your life" (her words). Which pretty closely echoes my claim that it is "unnecessary and irrelevant". And I mean that literally. For most, if not all organizations, the "feature" is either unnecessary, irrelevant, or both.
Much like a square wheel bike.
Do square wheels work? Yes, if the ground is shaped like an inverted catenary. Are square wheels necessary or relevant? Not even remotely.
The analogy is perfect. Does Tape Smart sizing work? Yes. Is Tape Smart Sizing necessary or relevant? Not even remotely.
The good stuff is always in the why.
There are two basic scenarios for how physical tape is created from virtual tape:
- The virtual tape appliance creates it.
- The backup application creates it.
This turns out to be a pretty important choice: if you choose #1, it might mean that your backup application loses the ability to track the location of physical tapes. And I emphasize might because this is not always the case--in particular, if your physical tape will always be at a single location, that is fine, because backup catalogs can only reflect a single location per barcode. However, if your physical tape might be in one of two locations (either inside the library and outside the library but in the data center, or inside the library and at a second site) then this is not a good approach.
As a result, you usually want your backup application to create physical tapes. Having a single, central point of control for monitoring the location of all backup media is a very good thing. Further, there is another good thing that I get out of choice #2: I can change the retention period of a backup object when I move it from virtual tape to physical tape using the backup application.
In summary: choice #1 means that you often can't track the location of physical media (bad), and that you can't have different retention periods for virtual and physical media (bad).
Despite all this, Tape Smart Sizing is only relevant for choice #1. If you opt for choice #2, then tape smart sizing just doesn't matter. Why? Because modern backup applications let me copy from a virtual media of any size to a physical media of any size. I can move 400 GB of data from 4 LTO1 virtual cartridges to 1 LTO3 cartridge, for example. No wasted media. So if you opt for choice #2, NetApp's claim that "all other VTLs" that use fixed capacity tapes waste media, and that Tape Smart Sizing is required, is neither true nor relevant.
But, what if you do opt for choice #1? There may be a small set of end users that are comfortable with this approach, and for whom it is the right choice. So, let's look again at NetApp's claim. They say that their VTL will take 400 GB of data, and store the data on 400 GB of disk (even though it can be compressed at 2:1). They will take that 400 GB of data and move it to a single LTO2 cartridge (200 GB, which can hold all 400 GB of data because the data compresses at 2:1). Lets ignore the fact that they just wasted expensive storage--disk--in order to save cheap storage--tape. The trick here is that the 400 GB of disk is represented to the external world, and to the backup application, as a single LTO2 virtual cartridge.
Wouldn't it be so much easier just to compress the data on disk? Wouldn't I rather just store that 400 GB of data on 200 GB of disk? You bet. And that is exactly what the EDL from EMC does. Thus, even if you opt for choice #1, Tape Smart Sizing is unnecessary. 400 GB of data is written to 200 GB of disk, and moved to 200 GB of tape. Neither disk nor tape is wasted. Everybody (except maybe NetApp) is happy.
Now, truthfully, lets deal with three last factors.
First, there is actually a third choice in addition to the two I listed above: let the backup application, which runs on the EDL, create the physical tape volume. This is a capability that is, I believe, unique to EMC. With an EDL, I can actually put a Storage Node (for EMC Networker) or a Media Server (for Symantec NetBackup) directly on the disk library engine of an EDL. Just like this:
By taking this approach, which is really a blend of the first two, I get the best of both worlds: I move all the I/O load of creating physical tape onto my virtual tape library, but I continue to let the backup application control the process, meaning that I get a single point of control, as well as full catalog consistency on media location, and the ability to have two retention periods (one for virtual and one for physical). There is really no downside here.
Second factor: I get that Tape Smart Sizing is different from compression, and exclusive of it. Which opens up a tiny door: what if the compression offered by your virtual tape array is different than what your drive would achieve, and therefore (but only for choice #1 does this apply) Tape Smart Sizing allocates a different amount of disk. (Trivia: there are a few different compression algorithms used by physical tape, and only one used by the onboard hardware compression card used in most virtual tape libraries today--including both NetApp's Near Store and EMC's EDL.) To which I can only offer the following: while it may be possible, in theory, that LZS and ADLC compression will result in different outcomes, in practice (across several hundred implementations) I have never observed this to be an issue. So true, but again, trivial and irrelevant.
Third factor: to avoid the inevitable NetApp collective hand-wringing, I am also smart enough to admit that many of the problems I describe above with choice #1 do not obtain if you use NetBackup (only) and Direct Tape Creation. That may be true. It still does not make Tape Smart Sizing any more necessary or relevant.
So at last, some wise words from Ella (yes it is a bit of a switch from my last musical citation, which was PWEI, but...)
Wise at last, my eyes at last,
Are cutting you down to your size at last
Bewitched, bothered and bewildered - no more
Burned a lot, but learned a lot
And now you are broke, so you earned a lot
Bewitched, bothered and bewildered - no more
Yep, she gets me every time.
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