Re: Rewrite md raid1 member

[Chris Dunlop <chris@onthe.net.au>]

On Fri, Aug 19, 2016 at 12:52:21PM +0100, Wols Lists wrote:
> On 18/08/16 05:01, Chris Dunlop wrote:
>> I'm interested to see if there's a way of essentially doing the above on a
>> live system, assuming there's appropriate care taken to not trash any
>> existing data (including superblocks).
>> 
>> I.e. is it *theoretically* possible to write the same data back to the whole
>> disk safely. E.g. using 'dd' from/to the same disk is almost there, but, as
>> described, there's a window of opportunity where you could get stale data on
>> the disk and a raid repair could then copy that stale data to the good disk.
> 
> There is something called "scrub". My superficial knowledge of raid
> doesn't let me know what it is, but as far as I can make out it forces a
> whole-disk-write or somesuch. Explicitly to flush out such problems. If
> someone else can tell you how to scrub your disks, I'd try that.

A scrub will read the RAID members to check that both sides match (raid 1,
10), or that the checksum is correct (raid 4,5,6).

To initiate a scrub of md0:

echo repair > /sys/block/md0/md/sync_action

You can watch it using /proc/mdstat, e.g.:

watch cat /proc/mdstat

It won't write anything if it doesn't detect any errors.

In my case, I want it to write everything.

If I do my 'dd' to write everything as previously described, with the window
of opportunity for stale data to end up on the written disk, one option
would to run a scrub / repair to check the data is the same - but if I'm
unlucky with my dd and the data isn't the same for some sector[s], I want to
ensure the correct data is copied over the stale data and not the other way
around, e.g. to specify "in the event of a mismatch, use the data from sda
and overwrite the data on sdb".

Unfortunately I don't know how that can be done.

Does anyone know?

Cheers,

Chris