On Tuesday 30 December 2003 22:10, Amanda wrote:
># ./amtapetype -c -f /dev/nst0
>Writing 256 Mbyte   compresseable data:  37 sec
>Writing 256 Mbyte uncompresseable data:  114 sec
>WARNING: Tape drive has hardware compression enabled
>Estimated time to write 2 * 1024 Mbyte: 912 sec = 0 h 15 min
>
># ./amtapetype -e 20g -f /dev/nst0
>Writing 256 Mbyte   compresseable data:  37 sec
>Writing 256 Mbyte uncompresseable data:  114 sec
>WARNING: Tape drive has hardware compression enabled
>Estimated time to write 2 * 20480 Mbyte: 1856 sec = 0 h 30 min
>wrote 524240 32Kb blocks in 80 files in 7414 seconds (short write)
>wrote 527436 32Kb blocks in 161 files in 7414 seconds (short write)
>
>
>define tapetype Dell-PV100T-DDS4 {
>    comment "just produced by tapetype prog (hardware compression
> on)" length 16432 mbytes
>    filemark 0 kbytes
>    speed 2269 kps
>}
>
>
>Question:  Is it better to have hw compression turned off?  Another
> poster got a measured length of 19457 mbytes.  Or is this deceptive
> since I'll be getting roughly 1.3 to 2 times the amount of space
> listed in my measured length due to the hw compression?  Just
> curious.

No, kill the hw compression.  There are at least 2 reasons for doing 
so.

1. (am)tapetype uses /dev/urandom as the data source.  This is not 
compressable data, and may expend as much as 15-20% in some hardware 
compressors, giving you falsely low estimates of the drive/tapes 
size, usually about in the range of what you got above.

2.  Using hw compression also hides the true size of the media from 
amanda, who counts bytes sent down the cable to the media after any 
software compression if its used. So amanda guesses based on your 
putting in the compressed rated size in the tapetype, and guesses 
itself right into an EOT occasionally.  This is not a Good Thing(tm) 
as it leaves you to clean up the resultant mess.

With good software compression, at the expense of those extra btu's 
the cpu will emit doing it, that nearly 20g (uncompressed rating) 
rated tape might have as much as 60Gb of data written to it.  I have 
only a DDS2 drive here, but I have received email reports from amanda 
indicating that the original data size was over 12Gb before gzip went 
to work.  Your hw compression cannot often match that.  Because 
amanda knows how much the tape can hold, I get fill ratios exceeding 
95% most of the time.

-- 
Cheers, Gene
AMD K6-III@500mhz 320M
Athlon1600XP@1400mhz  512M
99.22% setiathome rank, not too shabby for a WV hillbilly
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by Gene Heskett are:
Copyright 2003 by Maurice Eugene Heskett, all rights reserved.