Re: Q: How does raid1 sync if superblocks are dirty?

On Monday September 16, Kay.Potthoff@fci.com wrote:

Hi,

I have oberserved a little bit obscure behavior of my RAID setup if my
system 
comes up after a reset. I have setup a mirror and if I start the RAID after
a 
reset MD detects that the superblocks on the disks are not clean and starts
a 
resync.

If I check the output MD tells me that both disks of the mirror are
operational;
the same if I look at the output of /proc/mdstat: both disks are operational

"... [2/2] [UU] ...". But a resync is still in progress.

My question is: In which direction the resync is done; resp. what is the
sync algorithm in that case?

The raid1 driver arbitrarily chooses one drive and copies all data
from it onto all other drives (normally there is only one other).
While this is happening, all read requests are served off the chosen
drive.

The driver currently chooses the first drive as it's arbitrary choice,
and this is not likely to change.

NeilBrown