Dear Jonathan Joseph,
one thing is not quite clear to me:
Is your RAID5- array "failed" (more than 1 disk is dead) or "degraded"
(1 disk dead)?
If the array is "degraded" you should be fine by just replacing the
faulty disk with a working one (same size or larger should not matter.
The array should be able to rebuild onto the new drive (if it doesn't =>
something else is wrong). There is no need for ddrescue in this situation.
In this case I would do a backup first anyway. Your array should still
be able to start and therefore the filesystem on it should be accessible
just fine. No data lost yet.
If the array has indeed "failed" you're looking at a rather complicated
situation as you basically have an even more ugly situation than a
failed raid-0 would be.
In that case just copying the sectors 1:1 onto a working drive will NOT
resolve your issue.
If the data on the array is important / valuable and you don't have a
backup of it I suggest that you hand this over to someone with some
experience in this field.
This is not meant as an offense, I just know how much we charge our
customers with their "we already tried to fix it before we asked for
help" kind of administrators - sometimes just to find out that their
data could have been salvaged if it had not been destroyed by improper
handling of the situation by their admin :-(
Regarding your actual question:
If the new 1.5TB-drive is exactly the same size as the failed drive, you
should be fine writing the data from the 2TB disk onto it with ddrescue
or even dd.
It will of course abort at a certain point because the new 1.5TB disk
will run "out of space" - but the remaining data from the 2TB drive does
not originate from your failed 1.5TB disk anyway, so this doesn't matter.
However as written above this is very likely not going to resolve the
issue with your array. Probably the meta information on the drives are
out of sync (if you really have a "failed" array). This can be fixed,
but it is very specific to what kind of raid you are using.
Kind Regards
Felix
On 17.10.2013 18:57, Jonathan Joseph wrote:
I have a failed disk in a failed raid-5 array that I am trying to
resurrect with ddrescue. The disk is 1.5TB, and I successfully did a
first pass ddrescue of it (using -f -n flags) to a 2TB drive that I
had available (3 errors reported). Since this is one disk of a raid,
there is no proper partition or file system information on it. You
can replace a disk in a raid-5 with a larger one and it will rebuild
OK, so I thought the 2TB disk would be fine, but it seems that that
just cloning one of the disks of the raid array onto a larger disk
will probably /not/ work because in a raid array of this sort, some
critical information must be stored at the last block of the disk
Microsoft Word - SNIA Technical Position - DDF v2.0.doc
The last block of the Anchor DDF Header MUST be the last addressable
logical block on a physical disk
Microsoft Word - SNIA Technical Position - DDF v2.0.doc
So I want the ddrescue copy to go onto an identical 1.5TB drive. I was
able to get a matching 1.5TB drive so I would like to do the ddrescue
onto that drive now. However, now that I have already done the
ddrescue once, and presumably have most of the data on the 2TB drive.
I want to access the original failed disk as little as possible to
prevent making it worse - and hopefully just access the original
failed disk to try and improve the error blocks as much as possible.
My question is can I now ddrescue from the 2TB drive to the fresh
1.5TB drive and have everything end up in the right place - including
having that "last block" from the original 1.5TB drive be in the
right place on the new 1.5TB drive? Or do I really need to redo the
ddrescue from the original failed 1.5TB disk to the new matching one?
Thanks.
-J
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