Re: fanotify: the fscking all notification system

[Bryan Donlan <bdonlan@gmail.com>]

On Tue, Jun 30, 2009 at 9:22 AM, <Valdis.Kletnieks@vt.edu> wrote:
> On Mon, 29 Jun 2009 16:08:45 EDT, Eric Paris said:
>
>> fanotify provides two things:
>> 1) a new notification system, sorta like inotify, only instead of an
>> arbitrary 'watch descriptor' which userspace has to know how to map back
>> to an object on the filesystem, fanotify provides an open read-only fd
>> back to the original object.  It should be noted that the set of
>> fanotify events is much smaller than the set of inotify events.
>>
>> 2) an access system in which processes may be blocked until the fanotify
>> userspace listener has decided if the operation should be allowed.
>
> I don't care much about virus scanners - but some of us with petabytes of
> disk space to manage could use tis for HSM applications.  The HSM daemon
> could fanotify on the file, notice that the file accessed referred to a
> special "I've been archived" stub/token, and put the file back before
> giving the go-ahead to the process.
>
> The only sticky question - does this happen early enough that the accessing
> process, when un-blocked, will continue through open() and get the *new*
> version of the newly restored file?
>

In that particular case, why not just make it a sparse file (so the
size fields in stat are correct), and put back the original data in
the very same inode?