I believe this is a common misunderstanding. When you have drives which
stream faster than 10 MB/s and clients which cannot push data faster
than 10 MB/s (a 100 Mb/s connection), you need to multiplex your backups
to stream your tape drives. Not doing so reduces performance for your
backups since your tape drive now has to stop and start in order to
write data continuously. Additionally, this has a detrimental effect on
your tape drives, since they are not meant to start and stop. Some of
the newer tape drives (LTO2 for instance) have the ability to adjust
their speed to the incoming data stream, but this can also be
problematic, since the adjustment (I believe) is always to slow down the
drive. Once it has slowed down, it won't speed up.
More importantly, if you have 100 Mb/s connections, there is no sense in
multi-streaming. Now, you are asking your host to split its 10 MB worth
of bandwidth among multiple jobs, instead of sending the maximum amount
of data from one job from that host. I think there could be a rare
situation where you are unable to send 10 MB/s of data in one stream,
but can with multiple streams, but I believe this is extremely rare. In
my opinion, multi-streaming should only be used for hosts which have
adequate bandwidth to push multiple tape drives at once.
We have had several issues in the past because we were unable to stream
drives with slow clients. Even with multiplexing, it is very difficult
to manage your jobs so you never have a single job writing to a single
tape. Because of this, we were having tape drive failures nearly once a
month. Our solution has been to use virtual tape for slow clients (less
than Gb connections) and duplicate the images to tape, thus using the
fastest method possible to get the data to the tape drives. Since we
switched to this methodology, we have not had a single failed drive. As
we look to move to LTO2 and its 32 MB/s native speed, we will be moving
even more jobs to disk to make sure we can push the drives to maximum
efficiency.
Matthew Dobbertien
Chicago Tribune
Sr. Systems Analyst
-----Original Message-----
From: Weber, Philip [mailto:Philip.Weber AT egg DOT com]
Sent: Wednesday, September 15, 2004 6:36 AM
To: 'veritas-bu AT mailman.eng.auburn DOT edu'
Subject: [Veritas-bu] Multiplexing Question
Hi all,
During ongoing performance investigations I'm coming to the conclusion
that
in our environment single-streaming with multiplexing gives us better
performance than multi-streaming with multiplexing. The individual
client
backups may take longer but the overall usage of the tape drives is
better.
Two reasons for this :
1. Most of the clients are on 100/mb interfaces & aren't able to supply
multiple streams fast enough concurrently.
2. When multiple streams are multiplexed, typically multiple streams
from
the same client go to the same tape drive, so a slow client will really
drag
the tape drive throughput down.
My question is, whether I can set NetBackup up so that with multiplexing
at
the storage unit and schedule level, and multiple streams from each
client,
only one stream from any one client will go to any one tape drive? i.e.
one
stream per client per drive, but multiple streams possible from one
client
across the robot. I think this would be an optimal compromise but don't
at
present think it can be done.
Not sure if I've explained this very well...!?
regards, Phil
Phil Weber
Egg UNIX Technologist
Phone: 01384 26 4136
Mobile:
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