Very slow tape drive

[Kenneth D. Merry]

On Fri, Sep 05, 2003 at 11:57:36 +0100, Hywel Mallett wrote:
> I have a Tandberg SLR60 tape drive, connected to an Adaptec 29160
> controller. The drive can backup using dump at around 3MB/s, which is
> acceptable, if a bit slower than I was expecting.
> I have a problem though in that I have a large number tapes I need to
> read the data from. The tapes were written using cpio (on SCO
> OpenServer), and when trying to read from the tapes using cpio, I only
> obtain transfer rates of 150KB/s maximum.
> iostat shows that during both the cpio and the dump, sa0 is managing
> around ~290 transfers per second, but during the dump each transfer is
> 10KB, but during the cpio, each dump is 0.5KB.
> My basic question is do these figures sound normal? Is there a limit of
> 300 tps on a scsi bus?
> If so, how could I make the cpio faster? My thoughts included restoring
> from tape to a file, then using cpio on the file.

You can try using the --block-size option to cpio to specify what blocksize
cpio should use.  This should ideally use whatever blocksize was used to
write the tape originally.

That's probably why things are going so slowly.

Another option, as you pointed out, is to use dd to read the tape, using a
larger blocksize, and then use cpio on the output from dd.

With 512 byte blocks, your speed will be somewhat limited, since you can
only have one command outstanding to the tape drive at a time.  So you're
limited by the tape drive's latency, and you may also be causing the
tape to have to rewind and re-read blocks if it isn't able to stream.
(You'd proably be able to hear it doing that if that were the case.)

Ken
-- 
Kenneth Merry
ken at kdm.org