Re: FIFO

David Brown <[email protected]> wrote:

> > On Solaris you would be able to switch to a non-blocked mode by editing
> > the st.conf file.
>
> Linux's driver appears to allow the variable blocksize mode on DAT, but not
> AIT tape drives.  It seems odd to me, since I can 'mt setblk 10240' and get
> the same result.  It just makes it inconvenient because I have to know what
> the block size of the written tapes were when I wrote them.

This is bad and a known problem from the 1980s when SGI IRIX by default did this
instead of what was usual on UNIX before.


> In the days when a 40 MHz CPU was considered powerful, increasing the
> blocksize helped to keep the tape drive streaming.  Tape speeds have
> increased less than or about equal to an order of magnitude, while CPU and
> system speeds have increased several.  I guess an older machine might still
> benefit from larger blocks, but I'm guessing that star's fifo helps more
> than the larger blocks.

It seems that the tape drive itself is the limiting factor for the archiving 
speed. Note that every "block" has a "gap" and some other things in addition
and that even if the tape drive and the CPU do not limit the SCSI thoughput,
you send/read the net. block size but the tape drive writes/reads the gross
block size.

A block size of 126k always has been a good compromise between exchangeability 
and read/write speed. In any case, I would not recommend to use more than 1MB.



Jörg

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