[rescue] Block size and the single DD

Mike Meredith mike at blackhairy.demon.co.uk
Wed Feb 11 10:49:35 CST 2004

On Wed, 11 Feb 2004 10:52:29 -0500, Kevin Loch wrote:
> Sheldon T. Hall wrote:
> > 	rsh $MACHINE xfsdump -l 0 -F - $FS | dd of=$TAPE bs=$BS
> > 
> > I run that on the Sun with values like
> > 
> > 	MACHINE=blinky	# Indigo2 R4k
> > 	FS=/dev/root
> > 	TAPE=/dev/rmt/1cn	# DLT 2000xt on Sun SBUS card
> > 	BS=1024k

You might want to reduce the block size to 32Kbytes. The backup script I
used to use at work used that block size to write to an L280, and I'm
pretty sure I benchmarked a wide range of block sizes to check the time.

I seem to recall some people have trouble with blocking over rsh, so it
may be worth trying something like :-

	rsh $MACHINE 'xfsdump -l 0 -F - $FS | dd bs=$BS' | dd \
		of=$TAPE bs=$BS

(I haven't tested the above, so I've probably made a really dumb
mistake, but it does show what I mean).

Don't forget to sprinkle the chicken blood around.

> I thought the days of backing up directly to tape were long gone.

Not quite ... we're still waiting for an 8Tbyte array to use as a
holding disk. For some strange reason the managers seem to think that
backups without a holding disk are going well enough.

> disks and the dlt?  Also, is there anything that xfsdump gives
> you that you wouldn't get from gnu tar?

Plenty. GNU tar doesn't backup filesystems (which isn't just the files)
whereas xfsdump does (and doesn't have the limitations of ufsdump).
Picking a vendor-neutral tool to do backups with does make sense
sometimes, but there are tools that supposedly work better than GNU tar
--- such as star.

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