Re: Ill-informed musings on chard

In OpenSSI unstable data is commited to backing storage on chard mounts.

On Thu, Feb 4, 2010 at 5:14 AM, John Hughes <[email protected]> wrote:

> 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.
>
>
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