[Cluster-devel] New cluster mirror log implementation (RFC)

[Jonathan E Brassow]

I've started on the rewrite for the cluster mirror log (can be found in 
CVS head in cluster/cmirror-kernel).  The purpose is to bring it inline 
with the new CMAN/OpenAIS framework.  Since this framework is now in 
user-space, the cluster mirror log server implementation will exist in 
user-space.

I've only worked on the client side (kernel) code so far.  I plan to 
use netlink to communicate requests from the client (kernel) to the 
server (user-space).  The server will be responsible for coordinating 
cluster events, responding to client request, handling disk commits, 
and notifying the kernel of any log device related failures.

The server will listen to requests from the client (netlink) and send 
them to the rest of the cluster via OpenAIS.  The results will be 
replicated on each machine (running the server) with the server that 
has the lowest cluster id responsible for committing the data to the 
log device.

There are 17 types of client functions - most of which will need some 
communication with the server.  The input and output data varies.  I 
don't want to create unique structures to pass between the client and 
server for each type of request, so right now I plan on passing generic 
data.  The server will interpret the data based on the request type.  
The part I don't like about this plan is it requires both the client 
and server to be able to interpret the data correctly without the help 
of a header file...  I can keep them in sync and check for version 
mismatches during the log creation phase easily enough, but perhaps 
there are better ideas.

Now that there are source files in-place, I'll try to submit patches to 
this list before committing them.  When I start work on the server 
side, I'll probably do an initial commit and post patches thereafter 
(unless people would prefer I submit the opening files here first).

  brassow

For those that don't know:  Logging implementations are modular and use 
the APIs found in linux/drivers/md/dm-log.h.  Primarily, the logging 
implementations are used by device-mapper mirroring.  Right now, there 
are only single machine logging implementations in the kernel.  Red Hat 
has a cluster logging implementation for RHEL4 (cvs co -r RHEL4 
cluster/cmirror-kernel), but since the upstream cluster infrastructure 
is based on userspace CMAN/OpenAIS, that implementation will likely not 
live outside of RHEL4.