copy full system from old disk to a new one

[Steve Ellis]

On Tue, Feb 19, 2013 at 3:52 PM, Gordan Bobic <gordan at bobich.net> wrote:

> On 02/19/2013 10:00 PM, Reindl Harald wrote:
>
>>
>>
> No, my experience does not go as far back 6 years for obvious reasons. My
> exprience with mechanical disks, however, goes as far back as 25 years, and
> I can promise you, they are every bit as unreliable as you fear the SSDs
> might be.
>
> So, my experience with mechanical disks dates back 25 years as well (my
first was a 5.25" HH 20M in a PC I bought in 1986), but I've had more
frightening experiences with SSDs (and yet I still use them) than I have
with conventional drives.  I've had 3 complete and total failures of
name-brand SSD (all from the same vendor, unfortunately) within the course
of 1 year, all drives were less than 1 year old, and were deployed in
fairly conventional desktop machines--one was a warranty replacement of an
earlier failure.  I've had unpleasant experiences with conventional disks
as well, but I don't believe I've ever had more than one conventional drive
fail so completely that _no_ data could be recovered--all of my SSD
failures were like that.

>
>  data without a raid are useless
>>
>
> My point was that even RAID is next to useless because it doesn't protect
> you against bit-rot.
>
> As we all know, both conventional drives (and I believe SSDs) use
extensive error detection/correction so that the drive will know if a block
is unreliable (most of the time the drive will manage to remap that block
elsewhere before it becomes unrecoverable)--individual drives only _very_
rarely manage to return the wrong data (I'm actually not sure I've _ever_
seen that happen).

The problem with RAID is when no one is looking to see if the RAID system
had to correct blocks--once you see more than a couple of RAID corrections
happen, it is time to replace a disk--if no one looks at the logs, then
eventually, there will be double (or in the case of RAID6, triple) failure,
and you will lose data.  A further problem with RAID is when some of the
blocks are never read.  Any reasonable RAID controller will not only make
the log of RAID corrections available (mine helpfully emails me when
corrections happen), but will also have the option of scanning the entire
RAID volume periodically to look for undetected individual block failures
(my system does this scan 2x per week).  I've never used software RAID, so
I don't know if these options are available (but I assume they are).  It
would be suicidal to rely on any RAID system that didn't offer both logs of
corrections as well as an easy way to scan every single block (including
unused blocks) looking for unnoticed issues.

>
>  and in case of
>> RAID you have to have at least one full backup
>> and so it does not care me if disks are dying
>>
>
> Depends how many versioned backups you have, I suppose. It is possible to
> not notice RAID silenced bit-rot for a long time, especially with a lot of
> data.
>
>
I have a 5x1TB RAID5 (plus 1 hot spare) system (I suppose this is no longer
considered a lot of data, but it was to me when I built it) that has
_never_ had an unrecoverable problem--and I've now replaced every drive at
least once (and I just started a migration to a 3x3TB RAID 5 w/ spare
before any more fail)--I built my system in late 2003 (with 250GB drives),
and the only time the RAID system has been down for more than a few minutes
is when I migrate either drives or controller (or when I upgrade fedora).

-se

--
They that can give up essential liberty to obtain a little temporary safety
deserve neither liberty nor safety.
                               --Benjamin Franklin, 1759
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.fedoraproject.org/pipermail/users/attachments/20130219/3af4d333/attachment.html>