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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