>
> What exactly is NetWorker doing when auto media verify is enabled?

First the man page definition...

     auto media verify   (read/write, yes/no, choice)
          If set to  yes,  NetWorker  verifies  data  written  to
          volumes  from  this  pool.   Data  is  verified  by re-
          positioning the volume to read a portion  of  the  data
          previously  written to the media and comparing the data
          read to the original data written.  If  the  data  read
          matches the data written, verification succeeds; other-
          wise it fails.  Media is  verified  whenever  a  volume
          becomes  full  while saving and it is necessary to con-
          tinue onto another volume, or when a volume  goes  idle
          because  all  save sets being written to the volume are
          complete. [...]

My take is that this is a defense against tape drives that over-buffer.
Basically it was (or is) possible to run into drives that have large
buffers for speed, might read 8K of data from the host, but then after
they acknowledge that the data was received, discover that there's only
5K of tape left.  Oops.

When the tape is marked full, Networker still has the last block of data
in memory, so it can go and ask the drive to replay its last block and
verify that it made the tape.  This doesn't take very long and is good
insurance.

If the verify works okay, the save stream continues on another volume.
If it fails, the ssid is failed and marked incomplete.

> Does it make sense to have this enabled for a clone pool?

Since the issue is with tapes and drives, I don't see why not.

> I have a pool that I do enable this for because it sounded like it might
> provide some additional checks, although it probably slows things down,
> too, I'm sure, and the checks may be cursory at best. I'm thinking I
> might want to turn it on for the clone as well. Guess it couldn't hurt?

I don't think it slows down anything except when drives stop.


--
Darren Dunham                                           ddunham AT taos DOT com
Senior Technical Consultant         TAOS            http://www.taos.com/
Got some Dr Pepper?                           San Francisco, CA bay area
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