[mythtv-users] impending failure

[Stephen Worthington]

On Sat, 27 Oct 2018 14:33:07 -0400, you wrote:

>On Wed, Oct 24, 2018 at 3:36 AM Swanseasurfing <swanseasurfing at gmail.com>
>wrote:
>
>> Concerning dd:
>> My experience is that this tool ignores errors. I copied data from a drive
>> that had defects and was so happy that dd did not report errors. But the
>> target did not contain all data that I expected... I was told better to use
>> ddrescue.
>>
>> I often use Clonezilla for Imaging purposes. Cloning an SSD drive to set
>> up a new frontend was comfortable.
>>
>> Regards
>>   Jens
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>
>
> Incredible joy! I made a live clonezilla DVD and started the process of
>cloning the about to fail drive, 36% into the process it failed, I
>continued to walk the process out and saw a grub failure then a shut down,
>which I asked it to do upon set-up. Out of curiosity I took a look at the
>new drive to see just what was there with gparted and found identical space
>used on both old and new disks, when I opened the drive with files I saw
>all of my recordings, so I installed it into the mythbox just for kicks
>telling myself that the grub failed so it won't boot here, but it did and I
>am left stunned and pleased at the same time.

I am not sure exactly what you did here - when Clonezilla said it had
a failure, was it just a single sector or block it was unable to read,
and it was then able to copy the rest of the disk?  If so, then your
new copy will likely have just one bad file somewhere.  But if
Clonezilla actually stopped at the point of failure, then it may well
have copied the file tables, so you can see all the file names in the
directories you looked at, but the contents of files themselves may
not have been copied.  You need to run fsck -f on all the partitions
on the drive you copied to make sure that the file structure is fine,
and you then need to check that the contents of the files are actually
OK.

When you are copying a failing drive, the best tool to use is ddrescue
(from the gddrescue package - do not use dd_rescue which is a
different program).  Clonezilla has only a few of the methods ddrescue
uses to recover bad data from a failing disk.  Ddrescue has various
modes, so it can copy an entire disk like Clonezilla.  But it is
usually better to use it to copy individual files to a new disk.  Then
when you get a failure, you know which file is damaged.  And you can
then re-run ddrescue with parameters that tell it to try to recover
the file more intensively, and after you have recovered as much as you
can that way, you can think about things like refrigerating your
failing drive to see if the recovery can work at a lower temperature.

The log files ddrescue creates tell it what data it has recovered and
what it still needs to recover, and it can use them on subsequent runs
to only try to recover the blocks or sectors it was previously unable
to read.  With drives that are gradually failing, trying again can
often lead to more sectors being able to be recovered, just due to
temperature changes or the heads flying slightly differently above the
track.  When using ddrescue, the man page is only an introduction on
how it works.  Use "info ddrescue" for full information.