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

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Message-id: <55313401.5080008@samsung.com>
Date:	Fri, 17 Apr 2015 18:25:37 +0200
From:	Beata Michalska <b.michalska@...sung.com>
To:	John Spray <john.spray@...hat.com>
Cc:	Jan Kara <jack@...e.cz>,
	Austin S Hemmelgarn <ahferroin7@...il.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 04/17/2015 06:08 PM, 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?
> 
> John
> 

I think we should support both and leave the decision on what
is to be reported or not to particular file systems keeping it
to a reasonable extent, of course. The interface should hand it over
to user space - acting as a go-between. I would though avoid
any filesystem specific events (when it comes to specifying those),
keeping it as generic as possible.


BR
Beata
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