On Thursday 11 January 2018 14:43:49 Luc Lalonde wrote:
> Hello Folks,
>
> I'm wondering if there's a bug with 'Amtapetype' (version 3.5.1).
>
> We're migrating from LTO5 to LTO7 and I'm getting strange results when
> I run 'amtapetype'.
>
> Here's what I get after roughly 35 hours:
>
> define tapetype LTO7 {
> comment "Created by amtapetype; compression enabled"
> length 5874932832 kbytes
> filemark 1813 kbytes
> speed 118310 kps
> blocksize 32 kbytes
> }
>
> Why is it saying that it can write roughly 6Tb if it's supposedly
> hardware compressed? LTO7 is supposed to give a compression ration of
> 2.5 to 1.
That can vary widely, dependent on the data. If already compressed, don't
waste your cpu's power trying to gain another 5%, 'tain't worth it.
>
> I've disabled compression... Here's the ouptut of 'tapeinfo:
>
> [root@ulysses ~]# tapeinfo -f /dev/sg12
> Product Type: Tape Drive
> Vendor ID: 'IBM '
> Product ID: 'ULTRIUM-HH7 '
> Revision: 'G9Q1'
> Attached Changer API: No
> SerialNumber: '123456789A'
> MinBlock: 1
> MaxBlock: 8388608
> SCSI ID: 4
> SCSI LUN: 0
> Ready: yes
> BufferedMode: yes
> Medium Type: 0x78
> Density Code: 0x5c
> BlockSize: 32768
> DataCompEnabled: yes
_________________________
> DataCompCapable: yes
> DataDeCompEnabled: yes
> CompType: 0xff
> DeCompType: 0xff
> Block Position: 2
> Partition 0 Remaining Kbytes: -1
> Partition 0 Size in Kbytes: -1
> ActivePartition: 0
> EarlyWarningSize: 0
> NumPartitions: 0
> MaxPartitions: 3
>
> Am I missing something?
>
> Thanks!|
Yes, you wrote that tapes label with compression enabled, so despite your
turning it off, when the drive scanned that tape to mount it, it found
the compression was enabled in a hidden header only the drive can see,
located in front of the label block, so it re-enabled it. BTDT, had fun,
finally developed a method to turn it off:
1. rewind the tape.
1a. Do NOT remove tape from drive, or cause it to read the tape other
than what I write here until after step 5.
2. read the label out to a 32k file.
3. rewind the tape.
4. Turn the compression off, probably with mt.
5. Immediately re-write that label while the tape is rewound, and the
hidden tape id block in front of the label will get rewritten too, with
that compression flag off.
6. You must do this with every tape that was previously labeled with
compression enabled else its a virus that will re-infect every tape
after the one you missed.
I do not personally believe in using drive compression for the simple
reason that it hides the true tape capacity from amanda. If you use
local compression, gzip or ???, amanda counts bytes sent down the cable
to the drive and this is already compressed data if your dumptype in use
says to, and can then know pretty precisely how much more data this tape
can hold. Otherwise its a SWAG*, and we all know just how far off that
can be under the wrong circumstances.
*SWAG, thats a Scientific Wild Assed Guess.
In any event, I've likely started a flame war over where to do the
compression, so I'll go get my nomex underwear out.
Cheers, Gene Heskett
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>
