On 08/01/18 15:25, Gandalf Corvotempesta wrote:

2018-01-08 16:20 GMT+01:00 Wols Lists [off-list ref]:

quoted

Okay, you need spare ports to do a --replace so it's not always
possible, but it's always desirable. And with enough spare ports, you
can replace more than one drive at once, again with NO loss of redundancy.

I have 3 ports available, so I can replace 3 disks at once.
Any hint on the exact command so that I can safely test on a test server ?

Search the man page for --replace, but (off the top of my head) I think 
it's as simple as

mdadm /dev/md0 --add /dev/sdg --replace /dev/sdc

or

mdadm /dev/md0 --add /dev/sdg /dev/sdh /dev/sdi /dev/sdj --replace 
/dev/sdc /dev/sdd /dev/sde /dev/sdf

The man page says that the --replace option tells the raid code to 
failover the selected device as soon as a spare becomes available, so 
using --add --replace gives it the spare and tells it to use it.

With this number of devices to play with, I'd make sure to make a note 
of the original UUIDs and use those where possible - if linux plays with 
your sdx or you muddle your cables or whatever, then you could easily 
make an array-damaging typo ...

Cheers,
Wol