[SSI-devel] Ill-informed musings on chard

[John H. <john@Calva.COM>]

In order to do filesystem failover we need to mount filesystems with the 
"chard" flag, which ensures that on failover the backup node sees the 
same filesystem state as the last one the primary node saw.  This is 
necessary to avoid programs running on nodes other than the one that 
crashed seeing unexpected filesystem changes during the failover.

As I understand it on Linux the chard flag effectively changes all 
writes into synchronous writes.  Is this correct?

What effect would the "data=journal" ext3 mount option have?  In fact, 
isn't it needed?

As I remember the UnixWare cfs implementation would attempt to reduce 
the performance loss of "char" mounts by checkpointing writes to the 
filesystem backup node rather than forcing them to disk.

Maybe we could obtain the same effect by writing our own version of the 
Linux "jbd" (Journaling block device, http://kerneltrap.org/node/6741) 
which would checkpoint stuff to another node, instead of to disk.  (We'd 
still need to force real user requested sync data to disk though.)

Or maybe this is all nonsense.