On Tue, Mar 16, 2021 at 9:39 PM Manuel Riel <manu@xxxxxxxxxxxxx> wrote: > > To follow up on a previous discussion[1] about stuck RAIDs, I'd like to propose adding a warning > about this to the relevant docs. Specifically users shouldn't add other MD arrays as journal device. > > Ideally mdadm would check for this, but having it in the docs is useful too. > > 1: https://lore.kernel.org/linux-btrfs/d3fced3f-6c2b-5ffa-fd24-b24ec6e7d4be@xxxxxxxxxxxx/ > > --- > > diff --git a/Documentation/driver-api/md/raid5-cache.rst b/Documentation/driver-api/md/raid5-cache.rst > index d7a15f44a..128044018 100644 > --- a/Documentation/driver-api/md/raid5-cache.rst > +++ b/Documentation/driver-api/md/raid5-cache.rst > @@ -17,7 +17,10 @@ And switch it back to write-through mode by:: > echo "write-through" > /sys/block/md0/md/journal_mode > > In both modes, all writes to the array will hit cache disk first. This means > -the cache disk must be fast and sustainable. > +the cache disk must be fast and sustainable. The cache disk also can't be > +another MD RAID array, since such a nested setup can cause problems when > +assembling an array or lead to the primary array getting stuck during > +operation. Sorry for being late on this issue. Manuel and Vojtech, are we confident that this issue only happens when we use another md array as the journal device? Thanks, Song > > write-through mode > ==================
