W dniu 22.04.2021 o 16:38, Ed Jaffe pisze:
On 4/22/2021 1:31 AM, (K.K.Paradox)T.Kobayashi wrote:
There is a tape where a DFDSS dump was created on a 3592 drive
(TS1130) with the COMPACT feature enabled.
Q1. Can I read this tape on another 3592 drive (such as TS1130 or
TS1140)?
Are the 3592 hardware compressions compatible?
IDRC is IDRC. At only about 3:1, It ain't great. But, it's compatible...
Gentlemen,
Drive compression is *unpredictable* unless you know data to be written.
The true is same data on same drive will be compressed with same ratio.
BTDT. I spend a lot of time playing with compression ratios and
preparing data compressible 3:1, 6:1, 1:1, etc.
Long time ago I've got F37 abend, because my tape dataset exceeded 6TB -
but it was on JA tape in J1A drive. Native capacity is 300GB, so I got
over 20:1. I don't know exact value, because RMM was unable to record
such big numbers - that was a reason of F37.
3:1 is marketing value, which is more or less based on typical data.
However "typical" is not well defined, and you may not know how much
typical is your dataset.
Remark:
We know marketing 3:1 value. There is another one: speed. Is your drive
capable to write 360MB/s? Great, but you actual throughput will be
lower. Nominal value is true for big blocksize *and* proper compression
ratio, usually nominal one.
Worse compression (i.e. 1:1) mean worse throughput, usually you will get
same compressed stream, so in case of 1:1 it would be 3 times worse.
Better compression ...can be even worse, much worse! Very good
compression means very little compressed stream. Tape drive can slow
down, there are several gears (speeds), but when the stream is not
enough to satisfy the lowest gear the data buffer is exhausted, drive
stops, rewinds a little and starts again. Veeeery slow.
Of course the above covers real tape drives only, so it is rather
historical.
--
Radoslaw Skorupka
(looking for new job)
Lodz, Poland
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