W dniu 2012-03-20 23:12, Bill Fairchild pisze:
New thread.
What exactly does "MB/second mean when referring to how much data can
be copied from a DASD to a tape?
To be more precise, I am not interested in big MB vs. little mib,
just the philosophy. Suppose I have a huge file on a "3390" virtual
thing and I can copy whole tracks to tape at the rate of 100 MB/sec.
Assume the tracks hold 50,000 bytes instead of 56,664 to make the
math easier. Does 100 MB/sec. mean that I am copying 2,000 tracks
per second? Maybe. What if there is nothing written on the tracks,
but I don't know that until I read them in and then write the
contents? Of course, there is always at least an R0 on every track,
so they are not completely empty. If all they have written on them
is R0, am I still transferring data at the rate of 100MB/sec? If
each track were half full, would my effective data transfer rate be
only 50 MB/sec?
Well, megabyte per second is million bytes transmitted/read/written
within one second. Nothing more, nothing less.
Remarks:
- For purists: 10^6 is MB, 2^20 is MiB, but it changes nothing in the
above.
- I think there is no big problem with definition of megabytes
transferred through some interface/cable.
- Your problem mainly regards "what to do with unused objects". The
object could be track, block, etc. What to do? IT DEPENDS!
Let's take memory, RAM: there is no "empty" cell in the memory, even
uninitialized cell/page contain some data, usually trash.
In case of 3390 track - when you perform some kind of physical copy, you
don't care about empty tracks, VTOCs, you just copy track by track.
However when you care about "never used" tracks", your copy can run much
faster because you omit some tracks. That's how EMC TF works. But - does
the disk work faster? NO! ABSOLUTELY NO! The software works SMARTER.
And that's the answer: the software used to copy data can be smart and
omit unused parts of data.
Another example: DFSMSdss by default does not copy space in extent past
EOF. So, for example 1% used 100MB file is really copied as 1MB file.
Smart, but don't do it for JES2 spool - in this case EOF is misleading,
you have to specify ALLDATA ALEXCP (one of them actually), and demand
unused space to be also copied. Reason: you know the unused space is not
actually unused. However when you specify the above options for
"regular" datasets, you decide to waste resources, not to use smarter
method and you probably get worse performance.
BTW: Tape controllers use compression, IBM claims 3:1 compression. So
100MB/s can be compressed to 30MB/s of physical tape recording speed.
So, in case DASD->TAPE copy you have to specify what do you mean by
saying 100MB/s.
In "optimistic" version you can backup 1000x100MB file 1% used (1MB of
data), compressed 3:1 (1000x0.333MB= 333MB). Did you back up 100GB or 333MB?
--
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Lodz, Poland
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