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

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Message-ID: <20150417162247.GB27500@quack.suse.cz>
Date:	Fri, 17 Apr 2015 18:22:47 +0200
From:	Jan Kara <jack@...e.cz>
To:	John Spray <john.spray@...hat.com>
Cc:	Jan Kara <jack@...e.cz>,
	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Beata Michalska <b.michalska@...sung.com>,
	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 17:08:10, John Spray wrote:
> 
> On 17/04/2015 16:43, Jan Kara wrote:
> >On Fri 17-04-15 15:51:14, John Spray wrote:
> >>On 17/04/2015 14:23, Austin S Hemmelgarn wrote:
> >>
> >>>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 :).
> In that case I'm confused -- why would ENOSPC be an appropriate use
> of this interface if the mount being entirely blocked would be
> inappropriate?  Isn't being unable to service any I/O a more
> fundamental and severe thing than being up and healthy but full?
> 
> Were you intending the interface to be exclusively for data
> integrity issues like checksum failures, rather than more general
> events about a mount that userspace would probably like to know
> about?
  Well, I'm not saying we cannot have those events for fs availability /
inavailability. I'm just saying I'd like to see some use for that first.
I don't want events to be added just because it's possible...

For ENOSPC we have thin provisioned storage and the userspace deamon
shuffling real storage underneath. So there I know the usecase.

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