Disk/Tape TCO Whitepaper Seriously Flawed

I received an email today telling me about a whitepaper sponsored by the LTO group and written by The Clipper Group.  I'm used to seeing such whitepapers, and used to seeing them state things in such a way that makes the point the sponsor of the paper is trying to make.  I've even written a number of these whitepapers myself.  But this one just takes the cake, and I'd like to tell you why.

Feel free to take a look at the whitepaper itself and/or the webcast that talks about it.  (If the ultrium.com link doesn't work, it's not my fault.  It wasn't working for me either tonight.)

First, I am no tape hater, nor am I being paid to promote disk over tape.  I also don't have a problem with using tape for long term archiving. 

But I really don't like it when whitepapers use statements to back up their claims when those statements are either untrue or seriously weighted in their favor.  Here are some examples from this paper.

  1. They use an average compression rate of 2:1, despite the fact that most customers get much less than that on average.
  2. They use hardware compression in their figures for tape, but ignore the fact that all major VTLs have hardware compression.
  3. Their per-TB pricing for disk is only found in the most expensive disk systems.
  4. Their TCO includes replacing the disk arrays at the end of their 3-year warranty. Are you SERIOUS?
  5. They use the latest tape drive (LTO-4), but do not use the latest disk drives in use (1 TB).
  6. RAID configuration seems to be configured to increase cost (two RAID-5 LUNs per drawer, spare disk driver per drawer, no such concept as global spares, etc)
  7. They use 85% of LTO-4's write speed plus compression, ignoring the FACT that 90% of customers get less than 50% of the rated throughput of their drives. (Based on my personal observation of data from over 100 customers.)
  8. "Did not consider LTO-5," but they sure hint at it — a technology that is promised "sometime in the next two years."  If you're going to mention futures, make sure you mention everybody's futures, not just yours.
  9. None of the calculations (acquisition cost, power/cooling, floor space) take deduplication into account.  When they mention it in passing, they state incorrectly that while dedupe will reduce the power/cooling, it will increase the cost.  That is simply not the case.  The per-GB pricing of dedupe disk is significantly less than non-dedupe disk.

Their "other factors" completely ignore the benefits of disk, and state as benefits of tape things that are almost all also benefits of disk.  For example:

  • Tape is removable and portable
    • While disk is not removable and portable, it also can't be replicated, but disk can.  If you want backups offsite, you can replicate them there, especially if you have deduplication.  As to being susceptible to corruption, virtual tapes can be made read-only as well.
  •  Tape is fast
    • And disk is faster.
  • Tape is reliable
    • Not only are individual disk drives inherently more reliable than individual tape drives and tapes, they can be RAID-protected, where tape cannot.
  • Tape can be encrypted
    • So can disk
  • Tape has WORM
    • So do some VTLs

How can you release a whitepaper today that talks about the relative TCO of disk and tape, and not talk about deduplication?  Here's the really hilarious part; one of the assumptions that the paper makes is both disk and tape solutions will have the first 13 weeks on disk, and the TCO analysis only looks at the additional disk and/or tape needed for long term backup storage.  If you do that AND you include deduplication, dedupe has a major advantage, as the additional storage needed to store the quarterly fulls will be barely incremental.  The only additional storage each quarterly full backup will require is the amount needed to store the unique new blocks in that backup.  So, instead of needing enough disk for 20 full backups, we'll probably need about 2-20% of that, depending on how much new data is in each full.

TCO also can't be done so generally, as pricing is all over the board.  I'd say there's a 1000% difference from the least to the most expensive systems I look at.  That's why you have to compare the cost of system A to system B to system C, not use numbers like "disk cost $10/GB." 

The relative TCO of disk versus tape is something I've looked a lot at with customers.  Tape is still wining — by a much smaller margin than it used to — but it's not 23x or 250x cheaper.  If it were really that much cheaper, no one would even be looking at disk. 

----- Signature and Disclaimer -----

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.

14 thoughts on “Disk/Tape TCO Whitepaper Seriously Flawed

  1. tkimball says:

    Thank you – I got a good laugh out of that whitepaper.

    I will admit that those of us with bad D-D-T experiences (remember the STK BladeStore? I was the unlucky owner of a 12-tray unit – if you need war stories for another book drop an email) will likely be jaded enough to consider nothing but tape. However, disk has advantages far beyond TCO, such as much faster recovery times and buffering snail-slow backups of systems still on 10base-T (to eventually end up on said LTO-4 and later technologies).

    One argument that still has not totally settled out in disk is the effectiveness of ‘direct’ disk devices over VTLs.

    Between Sun ZFS’ compression and the eventual integration of Avamar’s dedupe, I suspect that Networker at least will eventually be giving users the option of still using their old adv_file devices instead of going out and getting new hardware (an important consideration given the state of the economy right now). It would not surprise me if Symantec was doing the same thing, if already done.

    Really, its too bad that dedupe VTL didn’t come out a couple years earlier than it did – we probably would have switched to that from the old BladeStore and ditched adv_file. I’m still happy with the current solution, even if dedupe is not in my immediate future.


  2. stu52 says:

    you state:

    “tape is fast and disk is faster”

    OK, if that’s true, then how come you can’t keep a LTO-4 (or LTO-3)drive streaming from a single disk? LTO-3 is 80MB/sec and LTO-4 is 120MB/sec (you already know that). With compression the speeds are going to be a bit faster (depends on the numbers you use, so I’ll stick with native speeds).

    A Seagate Barracude ES.2 1TB drive sustained transfer rate is 116MB/sec (slower than the LTO-4). The Seagate Cheetah 15K.6 is 171 to 110 MB/sec Sustained (close, but the disk wins I guess). I think the answer is “it depends”.

    So…your blanket statement about disk being faster than tape just makes me a bit nervous.


  3. cpjlboss says:

    An individual tape drive will beat an individual disk drive any day of the week. But, unlike tape, disk drives can be put in a RAID array that can go much faster than any individual tape or disk drive.

  4. tkimball says:

    In most environments, the ‘disk’ will be made of LUNs split out from multiple disks in RAID configurations – usually RAID5.

    Lets use my current D-D-T disk environment as an example – a STK B280 (FlexStore) FC-AL controller using multiple 3+1 RAID5 across four trays (400 gig each disk, all SATA), and the LUNs concatenated using Veritas VxVM/VxFS (due to Solaris 8 limitations at the time, and no ZFS back then). Over the 16 disk trays we have for this configuration (staggered RAID/LUN generation down the trays and starting over again at the top), I’m very happily streaming four LTO-2 drives and the servers (E450 and V240) are currently the bottleneck – which should be fixed soon.

    Even Sun-STK’s ‘children’ of the above technology (6140 and smiliar disk) is running at or better than the above speeds for general and heavy I/O (again using RAID5 configurations).


  5. esherril says:

    I remember back from the early 90’s we actually did have RAID tape arrays – Data General (pre – EMC buyout for the Clariion) had a 5 x DAT DDS RAID tape array for backup of the Aviion boxes. I wonder why nobody else ever picked up on that for newer generation drives; you’d think since it’s all basically just block data and SCSI commands, that RAIDing tapes would be as easy as disks. Maybe EMC sat on the patents when they bought DG, since they are obviously pro-disk & anti-tape?

  6. cpjlboss says:

    Man, you guys are ON me!

    Yes, there was such a thing as RAID tape, and it is technically possible. (The one you list is not the only one I knew about. ARCserve even had a software RAIT.)

    The problem is that it exacerbates tapes core difficulty, the inability of incoming data streams to go fast enough to keep the drive happy. If you were to create a RAID1 stripe of five LTO-4 drives, you would need almost 1000 MB/s to stream it! That thing would ALWAYS shoe-shine.

    Disk, on the other hand, can go both fast AND slow, so you can RAID as much of it as you want together and still have that array go as slow as you need it to go.

  7. ddierickx says:

    yickes! ok, so it is possible, that doesn’t make it a good idea or even make me want to use it… no way in hell.

    i guess there is a reason it hasn’t caugth on or isn’t used anymore or nobody speaks of it to this date.

  8. hmarks says:

    Back in the day I had a client using the CA tapeRAID. Of course when they had to do a restore they could only find 3 of the 4 tapes. At least if the backup was to 4 tapes sequentially they could have restored SOME of their data.

    If I remember right having a tape drive out of service made life difficult too.

  9. Rich says:

    I haven’t seen one TOC model that wasn’t weighted towards making a sale, or supporting some marketing position.

  10. ghoenig says:

    I accept the flaws in the Clipper report, but this begs the question of which research reports most accurately reflect the relative TCOs of disk and tape backup solutions.

  11. Greyhat64 says:

    I stumbeld on your site today and found this article. In it you state, “Not only are individual disk drives inherently more reliable than individual tape drives and tapes, they can be RAID-protected, where tape cannot.”

    Have you never seen a system that does paralllel writes to two tapes before? They’ve been around for years, and most backup software packages allow you to configure for this option.

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