DSSUs are OK but not as useful as e.g. AdvancedDisk pools for staging.  The 
algorithm for how it selects items to delete is very different.  I think you 
need to turn the question on its head.  Disk staging is not useful if your 
backups  (including incremental) can always stream the drive.  Ours cannot, 
Windows VMs are just too slow.  Disk staging allows the slow backups to be 
decoupled from the tape drives.  You don't have to dream up multiplexing levels 
that are different in full backups and incrementals; I doubt even if we set MPX 
to 32 the Windows VM incrementals could stream the LTO3/4, and LTO5 started 
shipping.

Using staging, disk does not care how slow the data arrives.  When it destages, 
you get proper use of the tape drives.  More importantly, the drives are not 
hogged by slow show-shine jobs, leaving other jobs queuing and in some cases 
hitting the end of the window. With SSO shared drives this can be painful.  So, 
what I'm saying is don't just think about your fast clients, think how to 
prevent the slow ones being a pain.

I'd even suggest considering sending FULL backups direct to tape and the INCR 
via staging.  With INCR the time taken to tee up the tape drive can be out of 
proportion to the backup duration, and it allocates the drive before it even 
starts to decide what files to backup...which may be very few.  Backups to disk 
start running so much faster, no mount time.  So for a backup that runs for < 5 
minutes tape is painful.

But more broadly, identify the slow backups that hog the drives, and if the 
total size will fit your staging, do it.  Destaging to tape makes the tape 
drives sing (OK, poetic licence there).  But you can use fewer drives and that 
means less $$ on annual maintenance.

William D L Brown


-----Original Message-----
From: veritas-bu-bounces AT mailman.eng.auburn DOT edu 
[mailto:veritas-bu-bounces AT mailman.eng.auburn DOT edu] On Behalf Of Victor 
Engle
Sent: 25 April 2010 20:15
To: veritas-bu AT mailman.eng.auburn DOT edu
Subject: [Veritas-bu] Architectural question (staging)

Hello List,

Just wanted to get some opinions about whether disk staging units are
worthwhile. My backup server has two BasicDisk staging units with the
storage units configured such that the data goes to disk and is then
moved to tape. I have a tape library with four LTO-3 drives connected
via FC. So what I'm wondering is, since the LTO drives are reasonably
fast, and since I'm writing the data ultimately to tape anyway, would
it be better to just write directly to tape. The disk is just old
fashioned spinning disk with no de-duplication so there are
operational costs for the disks. All tape and disk storage units are
local to the backup server. I'm thinking it would be better to add LTO
drives and eliminate the disk for now and maybe later add a
de-duplicating disk unit.

Under what circumstances does it make sense to stage data on disk. I
would appreciate hearing what your thoughts and experiences are with
regard to disk staging.

Thanks,
Vic
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