On 12/9/2011 10:51 AM, George Sinclair wrote:

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.

Thanks.

George

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