On 2007-10-30 08:31, Tim Verbois revealed:
TV> we bought a VTL recently, but now I'm asking myself: why a VTL, why not just
TV> Disks? Networker is perfectly capable of using Disk backups instead of the
TV> VTL. Disk backups have some advantages over VTL what I know:
TV>
TV> - when cloning, it is not needed to skip the not used sessions when
TV> parallelism is high
TV> - possibility of purging recycled save sets on a disk device (1 good saveset
TV> on a tape makes it impossible to recycle the tape)
TV>
TV> Why would you choose a VTL then? Any thoughts?
Interesting. I've had the same discussion with some VTL vendors recently.
They didn't manage to convince me either. The main arguments that have
been put forth are:
* VTLs can support data deduplication
- And so can raw disk with additional hardware. However, data dedup loses
its appeal as it can't handle several save streams to one device at once.
* VTLs act as a real tape library. Hence no need to modify your solution.
- And why would I want to put an expensive and at the same time limiting
emulation layer between disks and software, as long as my software can
support disk-based backups?
* VTLs can help you cirrcumvent networker licensing for AFTDs.
- Considering both that VTLs impose severe limitations on what can be
done with disk, at an additional cost to that, this argument becomes
invalid. Besides, isn't EMC going to create specific VTL licenses too?
* VTLs can clone virtual tapes to physical ones, thus ofloading I/O from
the server.
- Buy a better server then? Besides, I assume that cloning of virtual
tapes to physical ones put a constratint on how much data can be squeezed
into physical media, as I assume its hard to calculate how much
compression can be done when the data is written to physical tape.
So, the conclusion is: Don't bother with VTL. Get a cheap JBOD, and use
ZFS, or a cheap RAID-capable box (InforTrend EonStor, Fujitsu-Siemens
Fibrecat SX88 or similar) and invest your money in a diskbackup license
instead.
One other thing that's worth noting about diskbackup licenses for
networker is that the license puts restrictions on the actual amount of
data on disk, compared to library based licenses, who put restrictions on
the amount of slots instead. This means that unlike library licenses,
diskbackup licenses don't allow you to put more data in over time, as
technology evolves. For instance, a 32-slot license could be used for AIT
or Mammoth media 10 years ago, now it can be used for just as many LTO-4
tapes. Unlike library licenses, 50TB for a tier4 diskbackup license is
still 50TB in 10 years. This means that EMC will need to scale the limits
of the different tiers accordingly in order to make that license model
appealing over time. To my knowledge, no such plans exist today.
//Oscar
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