>>Date:    Tue, 30 Jul 1996 08:46:58 -0600
>From:    Van Stokes <VSTOKES AT VHA DOT COM>
>Subject: Q: Block Size, Tapes & Performance
>
>I am running AIX 4.1.4 on a J30.
>The tape drive is a 5.0GB Internal 8mm SCSI drive.
>
>When you do a change/show characteristics of a tape drive in
>SMIT you have the option to change the block size.
>
>First,
>The block size is in what? Bytes? Kbytes? Mbytes?
>512K Blocks?

This is done in bytes.....

>
>Second,
>What is a good performance block number to use? I have
>been told that zero (variable length) is SLOW and I tend to
>agree when it comes to reading the tape back into the
>system.
>I have been told that 1024 is a good number.
>Suggestions?
>
>*** Van ***
>vstokes AT vha DOT com

The block size is really dependent on the type of drive you have.  For
8mm drive 0 block size IS slow, as oppose to a DLT 0 block size is the faster
setting.

On the 8mm tape drive the block size should be in increments of 1024 (a
multiple of 1024) because that is the physical block size used.  Example if
you set the block size to 3000 bytes the drive will write a physical block
of 3072.....3000 bytes of data and 72 bytes of padding.  As to a suggested
size....I don't have an offical anwser for that.  But I understand that ADSM
write data to tape inincrements of 32 KB.  I have set the block size on my
8mm drive to match that (32 KB).


Chuck T.
(not ADSM support, just a user)