Virtual Tape and DPM

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I've started using DPM 2010 for DRing to another site. I've bought a cheapish Promise 16 disk ISCSI controller to hold the data and I'm about to buy 16 2T disks The server has 4.6T dedicated to DPM which is completely full with only 1 days backup.

The question is should I buy virtual tape software or just raid 5 all the disks?

cheers Mark
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Most Valuable Expert 2015
Commented:
A virtual only really makes sense when you actually do have tapes to which you eventually want to backup your "Virtual Tapes" to. If you aren't working with any real tapes then I suggest just raiding the disks.
DPM is a backup-to-disk solution which manages it's own disk-structure, so you can give it 16 individual 2TB-disks, or you can put the disks in a RAID5-setup and then presenting LUNs to DPM. Both will work.
The advantage of the RAID5-setup is it can withstand a drive failure, and potentially it might give you better performance, but that depends on the RAID-controller.

If DPM has filled 4.6TB after 1 day, it doesn't mean it will use 9.2TB after 2 days, 18.4TB after 4 days and so on. DPM works with a replica and sync points, which are comparable to a full and incremental backups. The initial replica-creation (~full backup) will consume most space, about the same as the original data, but after that, only the changes are saved (like incremental backups), making disk-requirements depending on activity. If a lot of files are changed with big differences, you will need more diskspace compared to if it were static data which doesn't change that much.

DPM isn't ONLY backup-to-disk, it supports backup-to-tape (or virtual tape) too, but there's no need for that.

Author

Commented:
Would there be some advantage of virtual tape with compresion? Also I thought thta maybe some data on a raid 5 set of say 6 disks giving 9-10 TB of disk protection and then more adhoc tape protection (i.e. you can pull a disk out without it breaking a whole set)

cheers Mark
Most Valuable Expert 2015

Commented:
No. As I mentioned earlier the only reason to use Virtual Tapes would be if you intend to copy the backups to tape at a later stage. If there is no real tape drive involved don't bother with it.
I'd suggest looking at your restore times before treating DPM as a DR system rather than a backup system.

We use DPM for backing up Exchange and SQL but the Exchange is a geo split CCR cluster across two sites, and although the SQL is citical, the data size is onder 10GB. For our main file server (~7 TB live data) we currently use rsync to a Linux box at a secondary location (with samba for SMB/CIFS access), but have started to look at Archiware Presstore with the synchronise module as we could then be completely on Windows. We looked at DPM for file server backup, but to achieve our restore window we would have to have replaced our file server storage with a much faster disk system, and put in the same for DPM, running a live mirror was significantly less expensive and files are immediately accessible in a DR situation.
Most Valuable Expert 2015

Commented:
I gave the correct answer to your Question, you should accept it.

Author

Commented:
There are several possible advantages to virtual tape

1: you can use compression
2: if you loose 1 disk (V tape) you won't currupt an entire disk pool (DPM puts bits of backup all across the disk pool)
3: you could pull disks in and out and store them much like tapes

You didn't give the correct answer
just to put my 2p in in answer to your points

1/ yes you can. can your DPM server take the processor hit of doing it ?
2/ no, if you use RAID6 you can lose 2 disks, if you use JBOD, then unless you use small VTL "tapes" (which can have other side effects) you're going to have a fun time with data distribution in each disk, and the number of virtual tapes you'd be managing will be high
3/ yes, if you run your "RAID" as JBOD, with each individual disk served as an individual LUN, and you've just turned an automatic process into a manual one is you have to unmount the VTL tapes in DPM, then unmount the volume in Windows, and then pull the disk, which as per my point 2, is gong to get quite complex very quickly.

There is a point with using VTL without tapes, but as soon as you start gm a going down the JBOD route served over iSCSI from a "RAID" system, you're getting into a circular problem...

Author

Commented:
Thank you, answered my question :-)

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