Thought I'd throw my 2 cents in with regard to VTL. My primary experience
is with EMC disk libraries (formally CDL)
I used to think the same way as Oscar - i.e. why bother with a VTL when you
can just use disk.
In a single networker server instance (or probably even one storage node),
or a small requirements e.g. up to a terrabyte or two I probably wouldn't
use a VTL. However, once you start getting storage nodes into the mix, they
start to come into their own. Consider the following
1. you can create as many virtual tape drives as you want
2. with backup to disk, you have to allocate this to a given storage node.
In a VTL, this one big pool of disk is carved up into X size chunks that
match a tape format. This disk is then availabel to ANY storage node that
you have allocated a virtual drive to (no need to script unmounting and
remounting filesystems between storage nodes)
3. your physical library for long term storage can be connected directly to
the storage node reducing load produced by cloning (Depending on make and
model) from both the server (cpu & IO) and network (bandwidth).
4. While the saveset is in the VTL you are still restoring from Disk
(NetWorker takes care of media management as usual)
5. If you add more storage nodes, you simply configure the disk library to
create another couple of virtual tape drives which share the same pool of
virtual tapes. This avoids the problem of tape drive contention that you
get with say 4 drives and 8 storage nodes.
The disadvantages as I see them are
1. As oscar said, you can only do one read/write to a virtual tape at a
time. If the tape is in use, and you need to restore, you've just gotta
wait.
2. The main limitation when using with NetWorker comes mainly when you are
wanting to keep browse policy equal to retention policy and you have to
clone data off to reclaim space in the VTL. This is currently a scripted
solution.
Cheers!
Brendan
On 4/24/07, Oscar Olsson <spam1 AT qbranch DOT se> wrote:
On 2007-04-23 14:15, Stan Horwitz revealed:
SH> We don't currently do any kind of disk-to-disk-to-anything backups. I
do not
Ive been following this thread for a while now. What I can't figure out is
why one would want to stage from disk to disk when it comes to backups?
Why not write to the secondary disk storage (i.e. cheap disk) right away?
There are several cheap storage systems (for instance the systems from
http://www.infortrend.com) where they just supply a disk array, where you
then buy off-the-shelf products such as actual disks and controller RAM
yourself. In terms of I/O performance, even these systems should be more
than enough for backups. So why do people use tiered disk for backups, or
VTLs? To me, the point of VTL is just to work around backup products that
aren't good at working with file systems or block devices as targets of
backup data. VTLs just seem to limit the functionality of the actual disk,
by emulating library behavior, with its set of limitations such as random
paralell I/O. Why not just use adv_file devices instead, and then just
stage that data to tape?
//Oscar
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