Soft Updates Help

[Chuck Swiger]

On Dec 1, 2006, at 5:42 PM, Sean Murphy wrote:
> Thank you for your knowledge on this issue.   I have a few  
> questions that I need your help to clarify.

You are welcome, although knowledge can be a tricky thing.  :-)

> "Snapshots are taken via mksnap_ffs; some other tools like fsck or  
> dump also know how to create a snapshot."
>
>       OK, so if I understand this correctly I do not have to  
> initially take a snapshot and update this snapshot manually.  The  
> files system with soft updates does it correct?

If the system is rebooted with a filesystem which is marked unclean,  
and if soft updates is enabled, then yes, the background fsck process  
will create a snapshot automatically, and not need manual intervention.

> "Maybe.  I think that softupdates is a win in almost all  
> circumstances from the standpoint of data consistency, short of  
> fully syncronous data & metadata updates."
>
>       On this issue, if I do not have soft updates on does that  
> make it a fully synchronous file system?

No.  Take a look at the manpage for "mount", in particular the  
section on -o which discusses async, sync, and noasync.  The latter  
is the default:

"             noasync
                      Metadata I/O should be done synchronously,  
while data I/O
                      should be done asynchronously.  This is the  
default."

> If the background fsck can't handle the inconsistencies it will  
> report this in the /var/log/messages correct?  Then is the file  
> system unmounted because of the inconsistency to prevent data loss,  
> so I can run a manual fsck or does it stay mounted?

It should fall back to running a foreground fsck instead, which might  
indeed require manual intervention, I believe.

-- 
-Chuck