Are you implying that you can use more than 1 channel per tape drive? (effectively, you can't)
A task, in this case, a backup or restore, can only use 1 path to the DASD at the same time, and 1 path to the tape at the same time. So your thruput speed is limited to the slowest (disk/tape) path you have. In the case of 8 ESCON channels to the 3494ATL, you can have 8 backup/restores (or 4 tape to tape copies), going at the same time, without loosing performance on the tape side. But if you are actively using only 1 tape, you are only using 1 channel. So you are limited to 17 MB/s ESCON, being written to the tape at 9 MB/s (3590B) or 14 MBs (3590 E/H) without tape controller compression. If you are using tape controller compression, then the 17 MB/s will be your bottleneck. An that won't feed the beast. And the drive will shoe shine, which is very bad for performance. Tom Duerbusch THD Consulting >>> Alain Benveniste <[EMAIL PROTECTED]> 11/27/2008 12:54 PM >>> Tom, At restore time we have 8 ESCON per tape drives in a 3494ATL. alain Le 26/11/08 20:30, « Tom Duerbusch » <[EMAIL PROTECTED]> a écrit : > Channel/device speeds > > If you are on ESCON, with 17 MB/s and going to an IBM 3590 tape drive, for > example... > > The B drives write to tape at 9 MB/s. > The E/H drives write to tape at 14 MB/s. > > If you use tape controller hardware compression, say 3:1, you need to send at > 3 times the data rate as the drive can write to the tape. i.e. the controller > on a B drive, will have to receive data at a 27 MB/s, in order to be able to > write to the tape at 9 MB/s. Not going to happen with ESCON. The E/H drives > are even worse. You need FICON channels to handle these drives. > > And, you need FICON channels on the DASD to be able to read the data at these > high rates. > > Otherwise, you shoe-shine the drives. Very time expensive. > > For my 3590B drives with 10 GB tapes, it takes about 75 minutes to fill the > tape with tape controller compression turned on over ESCON channels. > > So, what kind of drives do you have and what kind of channels are they > attached to (both disk and tape)? > > Tom Duerbusch > THD Consulting > >>>> Alain Benveniste <[EMAIL PROTECTED]> 11/26/2008 11:35 AM >>> > We are several guys to misunderstand something about compression. > Here is the situation: > Our interrogations come from MVS but I think it is true fo VM too. > > We did 2 backups with DFDSS, 1 with the COMPRESS option and 1 without. > For the 1st one, the restore too 3h20, the 2nd 6hrs ! 25 output tapes for > both tests. This was made on 2084 with 10 procs,and really enough mips. > > Ok the software compression is very cpu consumming, hardware no. The > question is what a software compression does, IDRC doesn't. Several of us > think the algorithm used is very similar from one to the other, so !. Why > that difference ! Others think, that IDRC is off, even if it is coded in the > system (I verified this twice). Others says, we didn't pay to use IDRC. > Others remember we had to install IDRC in the old past. So we don't have it. > > I'm looking for a firefly in this dark mess. > > Alain Benveniste >
