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

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Message-ID: <553134D3.9040001@gmail.com>
Date:	Fri, 17 Apr 2015 12:29:07 -0400
From:	Austin S Hemmelgarn <ahferroin7@...il.com>
To:	Jan Kara <jack@...e.cz>, John Spray <john.spray@...hat.com>
CC:	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 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.
>
> 								Honza
>
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.


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