On Sun, 12 Oct 2014 20:26:05 -0500 Ram Ramesh [off-list ref] wrote:

I added a new disk and grew my raid6 md0 with the following command line

    sudo mdadm --grow --raid-devices=5
    --backup-file=/root/grow_md0.bkp_file --layout=normalise /dev/md0

I got a response that stated that mdadm needed to backup 384K of data. 
However, when I checked after about 5min there was no backup file. Is 
this expected?

Yes.

When increasing the size of an array, the backup file is needed  for a tiny
fraction of a second at the start.
When decreasing the size it is needed  for a tiny fraction if a second at the
end.
When the size isn't changed it is needed for the entire time.

... unless you have a new kernel and a new mdadm, and then the backup file
isn't needed.

NeilBrown

    386 > sudo ls /root
    <no files. so no output>
    387 > sudo ls /root/grow_md0.bkp_file
    ls: cannot access /root/grow_md0.bkp_file: No such file or directory

Ramesh

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