Problem with hardware compression (Tape drive)

[skiran]

We are using Compaq Storageworks MSL5026 Library connected to tru64 machine .
The tapes used are DLT tape IV (40GB/80GB) . Device used for backup is /dev/ntape/tape4_d1 (tape4_d1 is used for High compression ).


The problem is tape drive does not do hardware compression . Could anyone tell us why hardware compression working. I cannot store more than 40GB data in a tape.
Do we have to chenge some setting from tape drive side?



Details are given bellow.

Server Details : Compaq Tru64 UNIX V5.1A (Rev. 1885) ,

Patches installed on the system came from following patch kits:
- T64V51AB03AS0003-20020827 OSF520
- T64V51AB03AS0003-20020827 TCR520
- T64V51AB21AS0004-20030206 OSF520
- T64V51AB21AS0004-20030206 TCR520

Tape Drive Detail : Compaq Storageworks MSL5026 Library

Compression mode is ON for /dev/ntape/tape4_d1 as shown below in /etc/ddr.dbase.

/etc/ddr.dbase

SCSIDEVICE
#
# Matches DLT8000
#
#
Type = tape
Name = "COMPAQ" "DLT8000"
# with DLT entries 4 and 5
#
PARAMETERS:
TypeSubClass = tk
TagQueueDepth = 0
MaxTransferSize = 0x0ffffff # (16MB - 1)
ReadyTimeSeconds = 120 # seconds


DENSITY:
#
DensityNumber = 1
DensityCode = default
CompressionCode = 0x1
Buffered = 0x1


Regards,
Shashi

[Ivan Ferreira]

You can use LT&T to test the device, and remember, that sometimes there are files that won't be compressed anymore, because they are already in a compressed format. Have you tried with tape4c?

tape?c
By selecting this device special filename without any d0-d7 reference you will be selecting a density code of 00H, which will instruct the tape device to use the default density available for the tape type in the drive. Compression will be enabled for all I/O operations. It is important that you ensure the device supports compression before you use this device special filename.

[skiran]

Hello Ivan ...
Thanks for your quick responce .
Today I will start backup using dev/ntape/tape4c.

Almost 80% backup files are compressed dump files. Are these compressed files effect hardware compression of tape??

Thanks ,
Shashi

[Michael Schulte zur Sur]

Hi,

80% is a lot. The 2/1 compression factor is a good guess from experience. It can be more or less depending on the compressibility of the files. Since your files are to a high degree already compressed there will be little gain.

greetings,

Michael

[Ivan Ferreira]

I really think that these files won't be compressed anymore.