Re: [PATCH 1/2] md bitmap bug fixes

Luca Berra [off-list ref] wrote:

If we want to do data-replication, access to the data-replicated device
should be controlled by the data replication process (*), md does not
guarantee this.

Well, if one writes to the md device, then md does guarantee this - but
I find it hard to parse the statement. Can you elaborate a little in
order to reduce my possible confusion?

(*) i.e. my requirements could be that having a replicated transaction
is more important that completing the transaction itself, so i might
want to return a disk error in case replica fails.

Oh - I see. "We did half off all the replications possible". That's an
interesting idea and it is trivial to modify md to return error if not
all the replications succeeded. The bitmap knows right now. No reason
not to call end_io(...,0) instead of end_io(...,1) if you want it that
way.

or to the contrary i might want data availability above all else, maybe
data does not change much.
or something in between, data availability above replication, but
data validity over availability. this is probably the most common
scenario, and the more difficult to implement correctly.

In any case it must be possible to control exactly which steps should be
automatically done in case of failure. and in case of rollback, with the
sane default would be "die rather than modify any data, in case of
doubt".

Well, if you want to be more exact about it, I am sure your wishes can
be accomodated. It's not a bad idea to be able to tailor the policy.

Peter