[lug] XFS filesystem core code goes into AC series

[D. Stimits]

Nate Duehr wrote:

> >Data: Journaled does not compromise safety of metadata, only regular
> >file data for programs that do write-append (e.g. log files).  Data:
> >Jouranled is still safe with respect to the metadata itself.
>
> ...
> and open are damaged upon power loss, etc.  Okay.  I'm more interested 
> in...
> if the metadata's "okay" upon remounting after a power loss, etc... 
> does it
> just truncate those files and move on, or does it go back to the ol' 
> "needs
> manual intervention" mode, i.e. stares at you stupidly waiting for an 
> fsck?
> ...

Metadata describes what operations are not completed, and is used to 
replay the last disk transactions which have not yet been flushed to 
disk. What it does depends on what it can do, and how much meta data log 
is alive at the moment of power loss or other failure. Typically it will 
truncate files or parts of files based on the last seconds of writing 
during the power off. Otherwise the loss would possibly result in need 
to fsck to get to the same state, perhaps even with manual intervention. 
XFS has improved greatly on this, I don't know about how ext3 deals with 
sudden power loss (I haven't tested it). XFS can truncate the last one 
second or so of data if you cut power. Early on, it could truncate maybe 
  five seconds of operations when power was lost (this was way back, not 
in recent times). To guarantee that all data is never lost, you'd have 
to have a full journaling system, rather than a meta journaling system. 
Full journaling is an extreme performance killer. Ext3 supposedly 
supports full journaling if you enable it, but I've heard too many bug 
reports to trust it.

D. Stimits, stimits AT attbi DOT com