Re: growisofs should have a method for padding
On Friday 13 May 2005 14:34, scdbackup@gmx.net wrote:
> Hi,
>
> > I would have expected that what was true of CDs would be true with
> > DVDs.
>
> So let us assume for now that DVDs are still safe
> in that aspect. At least no real problems have
> been reported. (Nevertheless i pad 300 kB)
>
If you are interested in another two cents. I see the problem frequently
and in the beginning I thought it was defective hardware. Until I found
out about this kernel read-ahead issue.
On many servers now I use DVD+RW media for backup purposes just like the
old tape drives, sequential in format using the tar command and written
using sdd with 32k blocks. If this part fails it is almost always media
related, like the customer inserted new media and I didn't know it and it
wasn't formatted. This operation requires Andy's kernel patch.
After the backup process is complete a verification process is started. If
this error happens it will be near the end of the verification on the
media. It also has a side effect of taking the drive out of DMA mode and
after the failure the backups are a lot slower until the server is
rebooted. But the errors are reported like below.
Starting Backup . . .
/usr/bin/sdd: Read 3496920 records + 0 bytes (total of 1790423040 bytes =
1748460.00k).
/usr/bin/sdd: Wrote 54639 records + 12288 bytes (total of 1790423040 bytes
= 1748460.00k).
Starting Verification . . .
/bin/tar: /dev/scd0: Cannot read: Input/output error
/bin/tar: /dev/scd0: Cannot read: Input/output error
/bin/tar: /dev/scd0: Cannot read: Input/output error
/bin/tar: /dev/scd0: Cannot read: Input/output error
/bin/tar: /dev/scd0: Cannot read: Input/output error
/bin/tar: /dev/scd0: Cannot read: Input/output error
/bin/tar: /dev/scd0: Cannot read: Input/output error
/bin/tar: /dev/scd0: Cannot read: Input/output error
/bin/tar: /dev/scd0: Cannot read: Input/output error
/bin/tar: /dev/scd0: Cannot read: Input/output error
/bin/tar: /dev/scd0: Cannot read: Input/output error
/bin/tar: /dev/scd0: Cannot read: Input/output error
/bin/tar: Too many errors, quitting
/bin/tar: Error is not recoverable: exiting now
Backup operation completed.
The problem is more frequent on a new server where the customer is adding
data to the drives daily and rarely will be deleting anything. The backup
grows each night and more and more media is required to hold it. Once the
customer starts to delete information, the error becomes less frequent
because the media has stuff written past the new backup.
This problem might be worked around by writing zeroes to the entire media
before using it. At least it would provide something for it to read all
the way to the end of the media. Since this is not an ISO image it is
just raw data written to the media.
So far I have not had to use one these failures to reload the data back on
the server. But if I had to, I think that sdd with arguments instructing
how much data to read could be used again to just read the tar data back
to hard disk and then restore it normally. So I have not used this error
problem as a reason to stop using DVD+RW for backup purposes. The backup
itself is complete, it is only the verification process that will produce
this error.
The best resolution I can think of would be to turn off read-ahead
operations on optical media. Or at least a config option on certain
devices that could be set from rc.local to disable read-ahead caching.
Even just an echo command into the /proc system like the command to turn
on packet forwarding would be sufficient.
James
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