LTO-3 read performance issues

I have two LTO-3 (QUANTUM ULTRIUM 3) drives attached to a dual Adaptec U160 controller (one per SCSI host) on a Dell PE2850 running a RHEL4 based kernel (2.6.9 based).

I'm trying to read (with tar) LTO-3 tapes written on another system (possibly an SGI IRIX box), but I'm getting extremely variable read rates - from a few Kb/s to tens of Mb/s - while reading the same tape

After a bit of trial and error, it looks like the tapes have been written in variable block mode with a block size of 16Kb

To list the tapes, I need to set the block size to 0 (mt setblk 0) and run:

tar tvfb $TAPE 32

Running strace on the tar process shows that it does a number of read()'s then 'sticks' on a read() for a number of seconds, and then does a burst of read()'s - the number of reads it does in these bursts and the time if waits on a particular read vary.

My guess this is something to do the drive having to repositioning the tape between reads and breaking the tape streaming ...

I get the same issue on both drives with different tapes from the same source.

I am using the default st module options and not doing anything other than using 'mt setblk 0'.

Is there anything I can do to get a decent, sustained read rate from these tapes?

Thanks

James Pearson

The Quuntum LTO-3 sustained transfer rate is 68 MB/s. Compression does increase this. In order to avoid stopping the tape, your system should be able to able to process the data continuously at this rate. If this is not the case, you can't avoid the stops.

If your system can process the data fast enough and the tape blocks are of the same size, you can try using fixed block mode and buffered transfers. This allows the driver to use larger SCSI reads. You have to load the st module with parameter buffer_kbs=xxx, where xxx is, for instance, 1024 (1 MB buffer). You also have to disable direct i/o with the module parameter try_direct_io=0 (otherwise buffered transfers are disabled).

Kai

Thanks, I tried that, but it makes no difference. I also tried increasing the buffer_kbs size by doubling it a few times, but that made no difference either

I have now found out that the tapes were written on an SGI system - using a fixed block device and the default options to tar. However, I have to either use 'mt setblk 0' or 'mt setblk 16384' to read anything from these tapes.

Just to check that nothing is wrong with my system, I can read an LTO-3 tape previously written on my system, using the default options to tar and the st module - and I get over 40Mb/s on reads, compared with about 400Kb/s I'm getting with these problematic tapes ...

Is there anything else I can do to read these tapes at a decent speed?

Thanks

James Pearson