Re: [Veritas-bu] Architectural question (staging)
2010-04-27 14:24:34
Excellent information, thanks. In theory, from what I am reading in this thread, it is desirable to stage backups for clients that can't stream the tape media.
What is the minimum speed for streaming an LTO3 drive?
Thanks, Vic On Tue, Apr 27, 2010 at 6:59 AM, WEAVER, Simon (external) <simon.weaver AT astrium.eads DOT net> wrote:
Have to agree with Ed here. I spent many, many months mucking
around with SAN Media Servers, and configuring Disk Staging Units, that then
write to tape!
I came across 2 major problems:
1) Performance was slower than LTO3 drives (FC attached, with
SSO)
2) Writing from disk to tape (using the DSSU config) was
quick, but and this is an annoying BUT, it would randomly fail to write to
tape!!
3) The time difference between writing to Disk and Tape was 2
mins. This difference was mainly where the Robot was picking the tape and
mounting it !
I have stopped going down this route, but although I have my
DSSU's ready and a good 4TB storage available, if the drives went pearshaped, I
could still do the backups to disk, short term !
Simon
On Sun, Apr 25, 2010 at 2:14 PM, Victor Engle <victor.engle AT gmail DOT com>
wrote:
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.
We worked with disk staging units for at least a year before we mostly
gave up. The biggest challenge we ran into was that destaging was too
slow. Even though we proved to Symantec that we could read from those disk
drives at over 100MB/sec, we could never destage even half that fast. We
had an open case with Symantec for a VERY long time before we agreed that it
wasn't going to get fixed.
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.
There are times when
DSSUs make sense. 1. If you don't have a tape drive free but want to
do a backup anyway - we still use DSSUs for things like small backups of Oracle
archive logs. 2. If you need to throttle your backups, especially across
things like a bunch of virtual servers on the same physical server. NBU
only allows you to set the maximum jobs per client name, not per client.
DSSUs make an acceptable choke point for clusters. 3. If you have
small backups, but don't have a lot of them at once, multiplexing may not buy
you enough performance boosts. Use DSSUs to write those little jobs to
disk and then destage them at once. If you currently multiplex, realize
that your restores are going to be slower than if you don't multiplex. All
tapes created from a DSSU destage are non-multiplexed so your restores can go
faster. DSSUs also give you a staging area for restores. If your
tapes go offsite, you may still be able to do a restore from the staging unit
the next day (or longer) depending on how big your stagig units are. NBU
is smart enough to realize that if the same data is on both disk and tape and
you kick off a restore, the restore will automatically come from disk. In
general, I'd say that there is a place for DSSUs but it's not the great benefit
we thought it was going to be. .../Ed
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