Message-ID: <20150417163914.GA28058@quack.suse.cz>
Date:	Fri, 17 Apr 2015 18:39:14 +0200
From:	Jan Kara <jack@...e.cz>
To:	Austin S Hemmelgarn <ahferroin7@...il.com>
Cc:	Jan Kara <jack@...e.cz>, John Spray <john.spray@...hat.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 12:29:07, Austin S Hemmelgarn wrote:
> On 2015-04-17 12:22, Jan Kara wrote:
> >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.
> >
> The use-case that immediately comes to mind for me would be diskless
> nodes with root-on-nfs needing to know if they can actually access
> the root filesystem.
  Well, most apps will access the root file system regardless of what we
send over netlink... So I don't see netlink events improving the situation
there too much. You could try to use it for something like failover but
even there I'm not too convinced - just doing some IO, waiting for timeout,
and failing over if IO doesn't complete works just fine for that these
days. That's why I was asking because I didn't see convincing usecase
myself...

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