Summary- Exabyte dumps (Exabyte 8200-only 1.6G data)

From: William Unruh [Unruh] (
Date: Wed Jan 15 1992 - 19:19:42 CST

 got a number of replies to my query about not geting the 2.3GB on doing
 the dumps and dd to my exabyte. The general feeling was that I was not
 setting my block size correctly especially on dd ( I used the default)
 A few suggested that I use a multiple of 8K (ie 112 was mentioned) as
 being the most efficient.
 Michael van writes
> Try: dd of=/dev/nrst0 ibs=8k obs=32k

> The EXB8200 writes data with a physical blocksize of 1KB. Whenever
> you send it a block shorter than 1KB it will pad it (transparently)
> to fill a physical block. The EXB8500 supports stuffing of several
> logical blocks into a physical block but the EXB8200 does not.

> The 'dd' line above insures that you write 32KB blocks and read 8K
> blocks which should give the best performance. suggested that there is about 2Meg lost between
 each dump.
 There also seems to be a feeling that the bpi and length figures are
 only used by dump to figure out the length of tape that it has been
 using. The standard figures are 5400 bpi and 6000 ft, although one (
 which I have lost now) suggested 4100000bpi. Do these figures make any
 difference to anything?
 have yet to try these ideas on a new backup and will let you know how
 they work.

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