[ale] writing to tape

[Björn Gustafsson]

On Mon, Feb 1, 2010 at 9:06 AM, Geoffrey <lists at serioustechnology.com> wrote:
> Greg Freemyer wrote:
>> Bad news if this is dd.
>>
>> N+m records in
>>
>> N - blocks read succesfully
>> M - partial blocks read + plus failed blocks read
>>
>> Similar for output.
>>
>> My guess is your data source is not keeping up with your output.  I
>> think it is conv equal noerror,sync that causes dd not to wait on
>> partial reads and instead to zero out partial buffers.
>
> So does that mean my data on the tape is not valid???

That is a possibility.  It depends on the device and/or driver you are
using.  I have seen some scenarios where only the drive that wrote the
tape can read it when partial blocks are involved, because it means
that readability is dependent on the speed the tape goes by the head.
(Most high-capacity tape drives require the tape to stream at a
certain rate to be readable, which doesn't work if the data feed is of
an inconsistent length.)

Because the partial reads equal the partial writes, that means the
input doesn't have complete blocks based on the `dd' blocksize.  It
doesn't necessarily mean that a failure occurred, but you should
definitely test the media using dd | tar tf - (or equivalent commands)
to make sure.

You want to avoid conv=noerror, which means the opposite of what you'd
think: it means keep going if you hit an error during processing.  See
http://www.opengroup.org/onlinepubs/9699919799/utilities/dd.html

-- 
Björn Gustafsson