Re: Ill-informed musings on chard

...bah I was in a rush this morning.  So to answer your question yes the
"chard" mount option should effectively change all writes into synchronous
writes.

Theoretically "data=ordered" is good enough because CFS will retry failed
requests due to node failure.

Since we're on this topic I've started reviewing the CFS code to see whether
this is actually happening.

-Roger


On Thu, Feb 4, 2010 at 9:42 AM, Roger Tsang <[email protected]> wrote:

> 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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