“My operations people want me to move off of VTLs as quickly as possible.” This was something someone said at a recent Dedupe School that I was speaking at in Seattle last week. The person who said this — I’ll call him Greg — happens to work for a very large company that uses well over 100 VTLs and well over 100 NAS-based target dedupe systems. I’ve had the opportunity to chat with Greg several times. He is what my Boston friends would call “wicked smart;” he knows his stuff.
Why are his ops people so down on VTLs? What does this say about FalconStor, SEPATON, or IBM that produce products that are VTL only? Do they have a future?
I have defended the VTL industry more than most pundits I know, so I thought it only appropriate to revisit the topic in light of this feedback from a real VTL customer. The good news is that the reason that I push certain people to VTLs still exists; the bad news is that the bad things about VTLs haven’t gone away either — or at least with this guy’s particular VTL.
VTLs have one major advantage over their NAS counterparts — Fibre Channel. Even with advancements like 10 GbE Ethernet, TCP Offload Engines (TOEs), iSCSI & FCoE, Fibre Channel is still a more efficient way to move large chunks of data than any of the alternatives. Greg told me that they have tested all of these newer technologies and agreed with the above statement. 10 GbE Ethernet may sound faster than 8 Gb Fibre Channel, but when it comes to backups it just isn’t. You can argue that Fibre Channel is more expensive, or that it’s the protocol of the previous decade and not the next, but it’s still the best thing going for bulk transfers of data like backups.
For the last 10 years, backup experts have recommended moving larger servers to LAN-free backups, where backups are sent over Fibre Channel. We’ve also been recommending using disk as the initial target for backups, even if you ultimately copy your backups to tape. (It solves the shoe-shining issue by using the disk as a cache.) For the past few years, we’ve also been recommending using deduplicated disk, but we’ll set that aside for the moment.
Assume that you want to follow the first two of these recommendations: LAN-free backups and disk as your initial backup target. You need a disk device that you can share with multiple servers over Fibre Channel, and there are two ways to do that: a global SAN-based file system and a VTL. Greg and I agree that the VTL is by far the best choice. The concept of a SAN-based, globally-writeable filesystem may seem nice but it has not gained market acceptance. Even a guy whose company doesn’t like VTLs thinks they’re better than a SAN file system. That’s something if you ask me.
When you add deduplication into the mix, it’s the final nail in the coffin of the discussion. If you want disk as a target, Fibre Channel as a transport, and deduplication features, your only choice is a VTL. (There are no globally-writeable SAN filesystems with dedupe yet.)
The statement above is referring to the options available to a person whose backup software does not have a built-in deduplication option, or to a person who who believes that their backup software’s dedupe option isn’t ready for their needs. Products like TSM, ARCServe, NetBackup, and CommVault do offer built-in dedupe that allow customers to use both Fibre Channel and disk and have dedupe as well.
FalconStor might claim that they are the only exception to this rule, since they offer OST (NetBackup/Backup Exec Open STorage) over Fibre Channel. (Everyone else does OST over IP.) However, since they ultimately store that OST data on virtual tapes in their VTL, they should continue to have the same challenges that other VTLs have.
So what’s wrong with a VTL?
When VTLs first came out, it was thought that the similarity to real tape libraries would make them easier for customers to integrate into existing backup systems. It’s just like what you’re already using — just better! The problem is that pretending to be tape also comes with some of its disadvantages:
- Tape drive operations sometimes hang, requiring SCSI resets
- Even virtual tapes get stuck in virtual tape drives (this is a variation of the previous issue)
- 50 virtual drives do indeed take more work to manage than 10
- You cannot read and write to a single tape at the same time
Greg told me that the first two issues caused enough hassles for his operations team that they wished they could get rid of VTLs altogether. He said they don’t have any of those challenges with their NAS-based dedupe appliances. Because of this, he feels very strongly that he wants to limit the number of virtual tape drives that he needs to manage. But since the NAS-based systems can’t meet the performance requirements of his larger systems, he’s forced to deploy the VTLs.
I did push back a little, asking him which brands of VTLs he had seen this on. (His experience was with one VTL that is no longer on the market, and one other mainstream product that is a VTL-only product.) I suggested that had he tested and deployed a different model of VTL that things might have been different. He disagreed, feeling that the problem is primarily a SCSI/tape drive thing, not something any particular VTL vendor could fix. I still can’t help but wonder if his opinion about VTLs would be different if he had over 100 VTLs of somebody else’s brand. They aren’t all created equal, after all. If any VTL customers want to reach out to me privately to confirm or deny that you’re experiencing the same problems as Greg, I’d really like to hear it.
The final disadvantage (not being able to read/write at the same time) is one that causes many people grief, because it causes conflicts for various processes. Perhaps you want to copy last night’s backups to real tape, or perhaps your virtual tape needs to be read by a post-process deduplication process. Either way, it is a pain that you can’t write and read to/from the same tape at the same time.
I actually have a bet with Marc Staimer about the future of VTLs. He said they will be on the decline in five years and I said they would be on the rise. The weird thing is that we made it on Valentine’s day and the bet was for dinner. 😉 We’re almost 3/5 of the way through the five years and VTL-only products continue to flourish. They may not be perfect, but I still think they’re the best thing going if you want dedupe and Fibre Channel in a single appliance. (They’re actually the only thing going.) Ultimately the market will decide.
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Written by W. Curtis Preston (@wcpreston). For those of you unfamiliar with my work, I've specialized in backup & recovery since 1993. I've written the O'Reilly books on backup and have worked with a number of native and commercial tools. I am now Chief Technical Evangelist at Druva, the leading provider of cloud-based data protection and data management tools for endpoints, infrastructure, and cloud applications. These posts reflect my own opinion and are not necessarily the opinion of my employer.