On 12/13/2011 9:43 AM, Josef Weingand wrote:

Hi,
you are right that LTO4 drives are slowing down the speed to 40 or 30
MB/sec. Below this speed the drive will start/stop (shoe shining), however
this will not impact your performance. Because the LTO drive, at least the
IBM LTO drive has a 256 MB Buffer. During the start/stop, repositioning,
show shining the server/client writes the data to the buffer. With an 256
MB Buffer you can write with 40 MB/sec up to 4 sec to the buffer. The shoe
shining / repositioning takes less the 4 sec.

Hope this helps.


Mit freundlichen Grüßen / Kind regards


Josef (Sepp) Weingand

Leading Technical Sales Professional - Data Protection&Retention / Storage
Platform
Certified IT Specialist / IBM System Storage
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From:   Dag Nygren<dag AT NEWTECH DOT FI>
To:     NETWORKER AT listserv.temple DOT edu,
Date:   10.12.2011 10:35
Subject:        Re: [Networker] Drive speed?
Sent by:        EMC NetWorker discussion<NETWORKER AT listserv.temple DOT edu>



fredag 09 december 2011 17:51:36 skrev  George Sinclair:

A basic question here on drive speed, but maybe not a simple answer as
there are undoubtedly numerous variables involved.

Let's say you have an LTO-4 drive (SAS connection to the tape library)
with a single stream (one save set) clocking in around 4-10 MB/sec,
coming in over the network. You then start another backup (also a single
stream) from the same client to the same drive, and now it jumps up to
70+ MB/sec, and remains at that speed until that second save set
completes, and then quiets down to 4-10 MB/sec again. I've seen this
happen with a number of other streams, too, wherein running just one of
them from that same client, concurrent with the already running stream,
cranks the speed up considerably, until it's done, at which point the
original stream is reported again to be running at the same slow pace.

We all know that a drive will come closer to performing optimally when
you can keep it streaming, and you can do that by keeping its buffer
full. OK, so having more concurrent streams - up to a point - will
improve drive performance, BUT does it affect the speed at which the
slow stream runs?

In other words, when the reported write speed jumps up to 70+ MB/sec
because you're now sending another stream (possibly one that compresses
well), is the original stream (possibly one that does not compress so
well) now increasing its write speed as a result? Or is it instead the
case that while the drive is now functioning more optimally, and writing
more data per second, that first (slow) stream is still clunking along
at its original speed, and sending more streams will not increase the
speed of any one of them?

I'm inclined to think that the increase in speed is only affecting the
additional stream(s) and not that original one.

This seem to be a typical example of dropping the drive below streaming
speed.
The LTO-4 has a native speed of 120 MB/s. The LTO technology can provide
for
slower "feeding" speeds than that by slowing down the reels, but only down
to
1/4 of the max. That will give us about 30 MB/s as the minimum ingestion
speed. Below that the drive starts "shoeshining" heavily and that is a
killer
for the performance. Compression on the drive will increase this minumum
by
the compression ratio, which again is depending on the compressability of
the
data.
As the number you gave falls exctly into these figures I think this is
what is
happening.
If you have access to the drive you can easily verify by listening to it
:-)

Best
Dag

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