Maximum block size on scsi tape?

[Mark Powell]

Hi,
   I'm having trouble with large block sizes.
   FreeBSD 7.0-BETA2 on amd64:

ahc0: <Adaptec (Compaq OEM) 3960D Ultra160 SCSI adapter> port 0xd000-0xd0ff mem 0xf5000000-0xf5000fff irq 20 at device 0.0 on pci7
ahc0: [ITHREAD]
aic7899: Ultra160 Wide Channel A, SCSI Id=7, 32/253 SCBs
ahc1: <Adaptec (Compaq OEM) 3960D Ultra160 SCSI adapter> port 0xd100-0xd1ff mem 0xf5001000-0xf5001fff irq 19 at device 0.1 on pci7
ahc1: [ITHREAD]
aic7899: Ultra160 Wide Channel B, SCSI Id=7, 32/253 SCBs
...
sa0 at ahc1 bus 0 target 6 lun 0
sa0: <IBM ULTRIUM-TD2 67U1> Removable Sequential Access SCSI-3 device
sa0: 160.000MB/s transfers (80.000MHz DT, offset 31, 16bit)

   In variable block-size mode, it seems to have a 64KB maximum:

# mt blocksize 0
# sdd if=/dev/random of=/dev/sa0 bs=32k count=1
sdd: Read  1 records + 0 bytes (total of 32768 bytes = 32.00k).
sdd: Wrote 1 records + 0 bytes (total of 32768 bytes = 32.00k).

Try to read it back without specifying a buffer size:

# sdd if=/dev/sa0 -onull
(sa0:ahc1:0:6:0): 32768-byte tape record bigger than supplied buffer
sdd: Input/output error. Error reading '/dev/sa0'.
sdd: Read  0 records + 0 bytes (total of 0 bytes = 0.00k).
sdd: Wrote 0 records + 0 bytes (total of 0 bytes = 0.00k).

As expect we are warned that the blocksize is 32k. Do that with 64k and we 
get expected behaviour too:

# sdd if=/dev/random of=/dev/sa0 bs=64k count=1
sdd: Read  1 records + 0 bytes (total of 65536 bytes = 64.00k).
sdd: Wrote 1 records + 0 bytes (total of 65536 bytes = 64.00k).
# sdd if=/dev/sa0 -onull
(sa0:ahc1:0:6:0): 65536-byte tape record bigger than supplied buffer
sdd: Input/output error. Error reading '/dev/sa0'.
sdd: Read  0 records + 0 bytes (total of 0 bytes = 0.00k).
sdd: Wrote 0 records + 0 bytes (total of 0 bytes = 0.00k).

But go beyond 64k and the blocksize seems to be limited:

# sdd if=/dev/random of=/dev/sa0 bs=65k count=1
sdd: Read  1 records + 0 bytes (total of 66560 bytes = 65.00k).
sdd: Wrote 1 records + 0 bytes (total of 66560 bytes = 65.00k).
# sdd if=/dev/sa0 -onull
(sa0:ahc1:0:6:0): 65536-byte tape record bigger than supplied buffer
sdd: Input/output error. Error reading '/dev/sa0'.
sdd: Read  0 records + 0 bytes (total of 0 bytes = 0.00k).
sdd: Wrote 0 records + 0 bytes (total of 0 bytes = 0.00k).

What's going on here? Why is blocksize getting limited to 64k?
   Isn't that rather small these days? An LTO-2 drive can support block 
sizes in the megabytes. Or is this just a problem in variable block mode?
   Yes, I don't understand how sa(4) really works :)
   Many thanks.

-- 
Mark Powell - UNIX System Administrator - The University of Salford
Information Services Division, Clifford Whitworth Building,
Salford University, Manchester, M5 4WT, UK.
Tel: +44 161 295 6843  Fax: +44 161 295 5888  www.pgp.com for PGP key