Re: [PATCH 0/5] Add trace events for filesystem freeze/thaw events

[Dave Chinner <david@fromorbit.com>]

On Wed, Mar 02, 2016 at 09:47:44PM +0000, Al Viro wrote:
> On Wed, Mar 02, 2016 at 01:01:54PM -0600, Frank Sorenson wrote:
> Let's provide a sane stable ABI for the things it's going to be
> used for.  Because _this_ is very likely to end up with
> "something needs to keep track of those events boot-to-shutdown" ->
> "something in systemd guts will be keeping track of those and
> broadcasting the collected information on state over dbus for the
> rest of dbus-infested system to see" -> "touch any details and you
> break real userland code" -> "it's cast in stone forever".  With
> systemd folks not particularly happy about the details, but even
> less happy about the need to make the already grotty code parsing
> those depend on the kernel version.
> 
> And as much as I don't like Lennart et.al., in this case they would
> be perfectly justified.  Information in question is potentially interesting,
> due to the form it is presented in one really needs to keep track of all
> messages since boot to make any use of it and it's clearly a job for
> a long-running daemon to collect those - nothing else would be able to.
> And once such a daemon starts using that, its authors would have very good
> reasons to demand the fucking ABI to be fucking stable.  After all, they
> weren't the ones who came up with the nasty details we might want to change.
> 
> So let's get it right.  Preferably - without need for boot-to-shutdown
> tracking just to mirror the state.  What do we really want?
> 
> * an ioctl to query the state (frozen/freezing/not frozen) for something in
> util-linux to use?
> 
> * /proc/fs/freezing and /proc/fs/frozen, with ->s_id of affected filesystems
> or, pehaps, one file with (frozen|freezing) + ->s_id?
> 
> * ability to audit on state changes?  That'd need some thought re what to
> do when some joker freezes the fs syslogd is logging to...
> 
> * something else entirely?

Such as:

https://lkml.org/lkml/2015/6/16/456

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com