linux-ext4 - Re: [RFC 1/4] fs: Add generic file system event notifications

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Date:	Fri, 17 Apr 2015 17:43:51 +0200
From:	Jan Kara <jack@...e.cz>
To:	John Spray <john.spray@...hat.com>
Cc:	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Beata Michalska <b.michalska@...sung.com>,
	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
	tytso@....edu, adilger.kernel@...ger.ca, hughd@...gle.com,
	lczerner@...hat.com, hch@...radead.org, linux-ext4@...r.kernel.org,
	linux-mm@...ck.org, kyungmin.park@...sung.com,
	kmpark@...radead.org,
	Linux Filesystem Mailing List <linux-fsdevel@...r.kernel.org>,
	linux-api@...r.kernel.org
Subject: Re: [RFC 1/4] fs: Add generic file system event notifications

On Fri 17-04-15 15:51:14, John Spray wrote:
> On 17/04/2015 14:23, Austin S Hemmelgarn wrote:
> >On 2015-04-17 09:04, Beata Michalska wrote:
> >>On 04/17/2015 01:31 PM, Jan Kara wrote:
> >>>On Wed 15-04-15 09:15:44, Beata Michalska wrote:
> >>>...
> >>>>+static const match_table_t fs_etypes = {
> >>>>+    { FS_EVENT_INFO,    "info"  },
> >>>>+    { FS_EVENT_WARN,    "warn"  },
> >>>>+    { FS_EVENT_THRESH,  "thr"   },
> >>>>+    { FS_EVENT_ERR,     "err"   },
> >>>>+    { 0, NULL },
> >>>>+};
> >>>   Why are there these generic message types? Threshold
> >>>messages make good
> >>>sense to me. But not so much the rest. If they don't have a
> >>>clear meaning,
> >>>it will be a mess. So I also agree with a message like -
> >>>"filesystem has
> >>>trouble, you should probably unmount and run fsck" - that's fine. But
> >>>generic "info" or "warning" doesn't really carry any meaning
> >>>on its own and
> >>>thus seems pretty useless to me. To explain a bit more, AFAIU this
> >>>shouldn't be a generic logging interface where something like severity
> >>>makes sense but rather a relatively specific interface notifying about
> >>>events in filesystem userspace should know about so I expect
> >>>relatively low
> >>>number of types of events, not tens or even hundreds...
> >>>
> >>>                                Honza
> >>
> >>Getting rid of those would simplify the configuration part, indeed.
> >>So we would be left with 'generic' and threshold events.
> >>I guess I've overdone this part.
> >
> >For some filesystems, it may make sense to differentiate between a
> >generic warning and an error.  For BTRFS and ZFS for example, if
> >there is a csum error on a block, this will get automatically
> >corrected in many configurations, and won't require anything like
> >fsck to be run, but monitoring applications will still probably
> >want to be notified.
> 
> Another key differentiation IMHO is between transient errors (like
> server is unavailable in a distributed filesystem) that will block
> the filesystem but might clear on their own, vs. permanent errors
> like unreadable drives that definitely will not clear until the
> administrator takes some action.  It's usually a reasonable
> approximation to call transient issues warnings, and permanent
> issues errors.
  So you can have events like FS_UNAVAILABLE and FS_AVAILABLE but what use
would this have? I wouldn't like the interface to be dumping ground for
random crap - we have dmesg for that :).

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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