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mdadm.8: add note information for raid0 growing operation 

When growing a raid0 device, if the new component disk size is not
big enough, the grow operation may fail due to lack of backup space.

The minimum backup space should be larger than:
 LCM(old, new) * chunk-size * 2

where LCM() is the least common multiple of the old and new count of
component disks, and "* 2" comes from the fact that mdadm refuses to
use more than half of a spare device for backup space.

There are users reporting such failure when they grew a raid0 array
with small component disk. Neil Brown points out this is not a bug
and how the failure comes. This patch adds note information into
mdadm(8) man page in the Notes part of GROW MODE section to explain
the minimum size requirement of new component disk size or external
backup size.

Reviewed-by: Petr Vorel <pvorel@suse.cz>
Cc: NeilBrown <neilb@suse.de>
Cc: Jes Sorensen <jsorensen@fb.com>
Cc: Paul Menzel <pmenzel@molgen.mpg.de>
Cc: Wols Lists <antlists@youngman.org.uk>
Cc: Nix <nix@esperi.org.uk>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>
This commit is contained in:
Coly Li 2020-02-24 12:34:09 +01:00 committed by Jes Sorensen
parent 1e93d0d159
commit 2551061c25
1 changed files with 6 additions and 0 deletions
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@ -2768,6 +2768,12 @@ option and it is transparent for assembly feature.
.IP \(bu 4
Roaming between Windows(R) and Linux systems for IMSM metadata is not
supported during grow process.
.IP \(bu 4
When growing a raid0 device, the new component disk size (or external
backup size) should be larger than LCM(old, new) * chunk-size * 2,
where LCM() is the least common multiple of the old and new count of
component disks, and "* 2" comes from the fact that mdadm refuses to
use more than half of a spare device for backup space.
.SS SIZE CHANGES
Normally when an array is built the "size" is taken from the smallest