Wanda, We appear to have very different sets of data, because what I've seen in the VTL and backup world is very different than what you're describing. With respect to what you've seen, let me describe what I've seen.
VTLs are using the same type of disks that any other ATA-based storage arrays are using. If you're saying they're not using Fibre-Channel - of course they're not. But do you need Fibre Channel drives to receive 10-20 streams of data? I don't think so. Fibre Channel drives are perfect for random I/O, which means they're probably perfect for a TSM disk storage pool, as it may be receiving hundreds of simultaneous backup jobs, which is very different than receiving 10-20-50 streams that were designed to go to tape. ATA drives do very well in streaming operations -- exactly what backups going to tape generate. In fact, I've seen tests where ATA drives actually outperform Fibre Channel drives in streaming ops. Yes, tape is still cheaper, but if you compare the price of a large VTL with de-dupe to an equivalently sized tape library, they'll be a lot closer than you think -- and the power and cooling will surprise you too. I've seen many scenarios where the VTL's power and cooling needs were less than the tape library. The people that I'm working with are not buying VTLs because they're cheaper; in many cases they're more expensive than an ATA-based array of equivalent capacity. They're buying a VTL for the things VTLs bring to the table, and those things come in play much more when we are talking about large environment. The first is ease of management. It's one thing to buy a 20 TB array and put that behind a single TSM server. It's another to buy that 20 TB array and split it up into properly sized partitions for several TSM servers. Provisioning is just as big of a pain in the TSM world as it is in the online world, and VTLs remove that problem. They use thin and/or over-provisioning where each server only consumes the amount of storage it sends to the VTL, not the amount you said it could have. The second thing VTLs bring to the table is hardware compression. Notice I said hardware compression. I'm not a fan of VTL software compression. So I get to buy a 20 TB VTL that's a little more expensive than a 20 TB ATA disk array, but what I get is 30-40 TB VTL, depending on my compression ratio -- and I don't lose performance. Then, of course, there's de-dupe, which most surveys are showing to be the got-to-have technology of this year. It's here. It's real. And it really does shrink the amount of disk you need to use by a factor of 10-20:1, and even more depending on how you do your backups. VTLs do indeed increase the speed of almost anyone's backups. It's not that disk/VTL is technically faster than tape. IMO, tapes are now much faster than disk. The reason that VTL/disk can outperform tape is that disk can go whatever speed your backup is going and tape cannot. If I send an 80 MB/s tape a 10 MB/s backup, it will shoe-shine and actually write 5 MB/s. A VTL would write 10 MB/s. Most environments never get anywhere near their tape's capabilities and about half or so are getting a small fraction of their tape drive's capabilities. VTL and disk bring THIS to the table. And yes, what ends up happening in almost every environment I've put a VTL in is that backups go faster. (There are some bad VTLs that actually slow down backups.) Finally, to one of the original questions of "why would anyone use a VTL in a TSM environment," I say the following. Most VTL companies are telling me that a significant proportion of their customers are TSM customers. Why is that? First, TSM customers have the same reasons that other products' customers have for going to VTL. Second, TSM customers can benefit from a significantly higher number of tape drives being available -- without having to pay for those tape drives. Reclamation goes much faster with two virtual drives than two physical drives, and you can throw as many drives at reclamation as you wish. Just my $.02. --- W. Curtis Preston -----Original Message----- From: ADSM: Dist Stor Manager [mailto:[EMAIL PROTECTED] On Behalf Of Prather, Wanda Sent: Friday, June 08, 2007 11:25 AM To: [email protected] Subject: Re: [ADSM-L] How to Incorporate a CDL into TSM environment? You go John! (And a BIG ditto on the compression rate issue - I've NEVER had a customer that got 3:1 over the whole TSM environment.) And let's step back a minute for a sanity check and ask, what IS a VTL anyway? It's disk with some cache and software in front. So if you need to back up 20 TB of disk, why not do as Kelly says and just buy another 20 TB of disk? Answer: In most cases, people buy a 20TB VTL because it's cheaper than adding another 20 TB in their disk array of choice. Why do you think that is? Is it because the vendors are really nice guys? Well, they may be really nice guys, but it's not because they want to give disk away. It's because the VTLs are built of A LESS EXPENSIVE KIND OF DISK. The cheaper disk is slower. Using cheaper disk, the VTL vendors have made it practical and cost effective to eliminate tape backups, FOR SOME CUSTOMERS. When people say they can back up or restore with a VTL faster than tape, it may mean 1) they are replacing slow tape drives 2) they are eliminating tape mount times 3) they no longer have to wait for a tape drive It doesn't mean there aren't cases where tape is faster. There are cases where a VTL really rocks. My favorite is using a VTL for OFFSITE storage and backing up to it directly over fibre. In case of a major problem, you aren't limited in the number of tape drives you have available for restore (you ARE still limited by the size of your fibre pipe). You don't have to physically move tapes around, and the media never leaves your control (If I never spend another minute doing a manual audit looking for misplaced tapes...etc.). And you don't have to collocate in a VTL, since there is zero effective tape mount time. And it is a good solution for people who want to do more Lan-free backups, and are short of tape drives. But you should be buying a VTL for one of THOSE reasons, not for raw speed. You can always create a scenario where you get down to the actual device speed of the underlying technology and hit that bottleneck. Many people never run into that scenario. But some do. Also, FWIW, tape is still cheaper per MB of storage than a VTL. There are price points where they are comparable, or where the benefits of a VTL outweigh the cost differential. But in general, the larger your site in terms of TB to store, the more difference you will see in cost if you go with a VTL vs. tape, with tape still being lower. You gotta first know what you are trying to do, THEN figure out where your bottlenecks are, THEN figure out what technology matches your need and fits your budget. Wanda (I think I'm done for the day now and I'm sure glad it's Friday) Prather ________________________________ From: ADSM: Dist Stor Manager on behalf of Schneider, John Sent: Fri 6/8/2007 1:00 PM To: [email protected] Subject: Re: How to Incorporate a CDL into TSM environment? Greetings, A lot of the chatter about VTL's being good or bad seems to stem from which vendors you listen to, and what they are trying to sell you. There are a lot of dogmatic statements made by people on both sides of this issue, usually by people with no personal experience about what they are talking about. Somebody has fed them a sales line and they dutifully parrot it back. EMC sold their CDL product for about two years before IBM entered the market. During that time you would not believe how many times I heard IBM pooh-pooh the CDL saying it wasn't a good fit for TSM, didn't perform well, whatever they had to say to compete against it. I even heard someone recently say it was against the law to use a CDL if you used the IBM drivers to talk to it. Against what law exactly? Then after two years IBM came out with their VTL the TS7510, and almost immediately came out with a Redbook about it with a TSM chapter explaining why the TS7510 was such a good fit for TSM! Huh? And not because it was a better product than the EMC one, it was actually slightly slower and only scaled to about a fourth the size of the largest EMC VTL. The only difference is that now IBM had something in the marketplace, and that changed everything. As Wanda has said, a lot of the distinctions fall down to how you use the VTL, and if your expectations are set correctly. It is easy for a vendor presentation to promise the moon without qualifying it's claims. A single-engine DL4100 from EMC can sustain a 1100MB/sec (3.7 TB per hour) write speed like they claim IF: 1) You are writing multiple simultaneous virtual tape streams (like 16 or more), 2) You balance the I/O across at least 4 FC streams coming in the VTL engine, 3) You have at least 5 or more disk drawers to spread out the I/O load. 4) You are not compressing at the VTL engine. If you compress at the VTL engine, your performance will drop off, perhaps as low as a third as fast. This is because the compression is done in software. If you want hardware compression, go with one of the DL6000 series that has an optional hardware compression engine. But the presentations only say 1100MB/sec performance, and so customers install one, set up a single backup to a single virtual tape drive, and when it pegs at ~100MB/sec they think they have been lied to. The other complaint I hear a lot is the claim of 3:1 compression. Almost every vendor puts that in their literature as if it is a solid fact, and not a typical value. I had a customer once get so mad they almost yanked the whole box out and made the vendor take it back because they bought a 10TB VTL, which they sized on the assumption of 3:1 compression. Never mind that the compression they were getting on their existing LTO tape library was on 1.2:1, they were told the VTL would do 3:1, so it should. I had another customer almost throw out IBM because they bought 12 new 3592 tape drives, and they wouldn't perform anywhere near their rated performance. Never mind the fact that data was coming in through a single GigE connection, and the 12 tape drives had an aggregate throughput rating at about times that. Customers looking to purchase any tape or disk technology would be wise to ask questions about how performance numbers were achieved, and look at their own situation to see what results they should expect. Best Regards, John D. Schneider Sr. System Administrator - Storage Sisters of Mercy Health System 3637 South Geyer Road St. Louis, MO. 63127 Email: [EMAIL PROTECTED] Office: 314-364-3150, Cell: 314-486-2359 -----Original Message----- From: ADSM: Dist Stor Manager [mailto:[EMAIL PROTECTED] On Behalf Of Prather, Wanda Sent: Friday, June 08, 2007 10:56 AM To: [email protected] Subject: Re: [ADSM-L] How to Incorporate a CDL into TSM environment? ...I understood restore performance suffered with a VTL - the way it has been described to me is that, should a restore need to come from a volume that has been destaged from disk to tape in the VTL, then a restore of a single file from the volume would first have to wait for the vtl to rebuild the tape on disk? Or have I got the wrong end of the stick? Um. Both. Most VTL's are disk-only devices that emulate tape, and do not have the staging issue you describe. Many VTL's will make restores FASTER because the tape mount time goes from potentially minutes to a second or less. (You also don't have to worry about collocating data in a VTL, so your migration times are generally faster as well.) Now that goes with a caveat - you have to PIN YOUR VENDOR TO THE WALL and get documentation about throughput rates. ALL VTL's work about the same way, but they all have different hardware inside the box, so you can get drastically different results. You can easily create a case where restoring 1 VERY LARGE file will take longer on a slow VTL than with fast tape (Say a TS1120, which run get more than 100MB/sec.) It depends on WHICH VTL you are talking about, the speed of the disk in it, the size of the cache in it the speed of your SAN connection and/or HBAs compared to which tape drive, and whether you are talking about restoring lots of little files or a few huge ones. A VTS (don't they make this confusing?) is an IBM-only mixture of disk/tape that emulates tape. It has to pull data off tape and stage it back to disk before you can restore. Normally the VTS is used in a mainframe environment. IBM also makes VTLs, the TS7510 and TS7520, for use in open environments. They are all disk.
