Uriel Carrasquilla wrote:
> We were told to ignore them.
> We have been running with those messages for about 4 months now without any
> problems in 4xLPAR's.
I see that the message causes some confusion. So here's my short explanation
on barriers:
- a journaling filesystem like ext3 needs to make sure that things are stored
in a certain sequence on disk: first store info about the transaction to the
journal, then write the data itself, then maybe write competion notice to the
journal.
- the linux io stack is, from the filesystem perspective, an asynchronous thing
that reorders io requests:
- optimize for hard disk head movement
- io scheuduling algorithms trying to optimize either for "fair" use of
io bandwidth or for "no user-visible delays"
Journaling filesystems can either only submit the IO that they consider to be
safe to be reordered whatsoever at a time, then wait until it is completed,
and submit subsequent parts of the transactions later. Alternatively a
journaling
filesystem can include a barrier which prevents reordering of things "before"
with
things "after" the barrier. The journaling filesystem can detect if one
component
does not support barriers and fall back to the conservative strategy
automatically.
The dasd device driver does'nt support barriers at this time, thus the message
informs that the journaling filesystem uses the conservative fallback.
--
Carsten Otte
IBM Linux technology center
ARCH=s390
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