[ale] writing to tape

[Björn Gustafsson]

On Mon, Feb 1, 2010 at 6:50 AM, Geoffrey <lists at serioustechnology.com> wrote:
> When writing to tape, I see the following:
>
> 4204276+2204423 records in
> 4204276+2204423 records out
>
> What do those two numbers represent?

Those are the number of full+partial blocks being read and written by
the `dd' command.  Full block means a read/write of the blocksize
indicated in the bs= parameter.  Usually it's bad if you have lots of
partial blocks. Typically that means you are not using an appropriate
block size for your write media.  Historically that also meant very
poor write performance, though I don't know if that still holds with
modern tape devices.  On some devices I've also seen it affect
readability if you don't use an appropriate block size, but I imagine
that's a thing of the past nowadays.

If you're using the default block size of 512 bytes, the partial
records can indicate you have about 2.2m really small files in your
backup set, or that each file is just under 1.5KB on average.

Depending on your context, either using a block size that matches your
tape device or adding conv=sync can improve performance significantly
if you do indeed have additional mechanical overhead from all those
partial writes.

Since devices have changed so much since `dd' was invented, I can't
really say if those values mean anything at all any more.  (Plus I
don't have any tape devices handy to experiment on.)

-- 
Björn Gustafsson