Splitting tar output, Resolved

Michael ODonnell michael.odonnell at comcast.net
Tue Aug 30 22:35:01 EDT 2005


>> You did, but it's no big deal.  If you're going to do
>> mea culpas about your email format I'd rather it was about
>> your MIME headers forcing a UTF-8 charset on us...            ;->
>
>    Switched to iso-8859-1.  Is that better?


Muy bueno!

	.
	.
	.
>It is a small gain, but I was eager to optimize.  And to increase my 
>understanding of Linux - which is progressing ever so slowly.
>
>    The script I will try is:
>
>cd /
>(tar -cf - ./ $bkoptions) | (tee $bkdir/monthly.tar | dd of=/dev/st0 
>ibs=512 obs=4096)
>
>I am presently timing the operation versus output block size.  It 
>probably needs to be increased to 16K if the drive accepts it.  The 
>tape drive uses up the 4K every 2ms at 2MB/s writing speed.
>
>    I will post the final results.

obs=4096 ?

Depending on how much memory you've got you should definitely
not be afraid to make that "obs=" value be quite large,
like 40960k.  Yes, folks, that's 40Mb.  I've written device
drivers for QIC-2 and SCSI streaming cartridge tapes from
scratch, so allow me to confirm what Jon Hall was saying:
ya keep those suckers fed!  That means your goal is to ALWAYS
have more data to cram down their throats than they can handle.
This is an important point; you can waste half the capacity of
streaming tapes by arranging for worst-case scenarios where
you write the smallest legal block size (512 bytes for QIC-2
and some SCSI streamers) and then allow the drive's underrun
logic to time out, forcing it to write a bunch of padding, stop,
reposition, get a running start, write a bunch of preamble junk,
write one more minimum size block, underrun, write more padding,
reposition, etc...

Therefore, complicating your backup situation by running
multiple concurrent processes may increase the liklihood
(depending on how much horsepower your system has) that you'll
starve the drive and waste time and space.  Better to generate
the on-disk image first and then stream it onto to the tape
in great big coninuous pieces using "dd" when it's complete.
 



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