On Mon, 2011-09-19 at 18:00 +0200, Lennart Poettering wrote:
a) tracker uses inotify recursively and creates a massive number of
watches due to that. That is both ugly and doesn't scale. Tracker
apparently tries to not take up the full pool of inotfy handles the
system provides, but that won't help if you have more than one user on
the system. The solution here should probably be fanotify, which allows
proper recursive file system watches. So far fanotify has been
accessible to root only, which is presumably why tracker doesn't use
it. However, the solution here cannot be to work around that fact by
using inotify, but must be to invest the necessary kernel work to make
fanotify useful from unprivileged processes.
The current recursive inotify watches are definitely far from ideal. We
were looking forward to fanotify entering mainline, however, last time I
checked it was still missing too much functionality to even match
inotify for our purposes. In addition to permission issues it did not
support notification of directory events at all. If I remember
correctly, this was all planned for future iterations but not considered
a priority use case for fanotify. Does anyone know Eric's current plans?
The max_user_watches limit is per inotify instance and each user can
have max_user_instances inotify instances, as far as I know. I don't see
how this would be an issue on multi-user systems. The default limit
should be sufficient for a large part of the user base with the default
tracker configuration. However, it might make sense to increase the
default on distributions using tracker to be on the safe side.
b) We still don't have a way to detect offline modification of
directories. That means detecting changes to the home directory made
offline is very expensive. btrfs now has hooks to improve the situation,
but ext4 still hasn't. Does tracker at least use the btrfs hooks? (btrfs
provides a log of changes to userspace, which can be used for that. Another
solution are recursive directory change timestamps).
No, tracker does not have any btrfs-specific code at the moment. I'm
still waiting for a btrfs fsck release to start using btrfs myself, but
I'm looking forward to learning more about the btrfs facilities and how
we could use them to improve tracker.
I'd really prefer if we could fix these fundamental issues before
enable tracker. To me it appears here as if we are trying to make the
second step before the first.
I think you were in the room when we discussed these issues in Gran
Canaria. At that point the plan was to wait for fanotify and try to
convince filesystem developers of recursive directory timestamps. If I
remember correctly, Matthew Garrett volunteered to talk to other kernel
developers as a next step. Unfortunately, I don't think we had any
follow-up discussions about recursive directory timestamps.
fanotify was delayed quite a bit, and we were told that there was
nothing we can do to help and it was way too early to start
experimenting with it for tracker. Now that it is in mainline, it would
probably be easier to help out, but given that it took a long time for a
kernel developer to get a subset of the planned features into mainline,
I didn't attempt to work on the missing features myself so far.
[ And there are acouple of other things I'd like to see changed.
example, I am pretty sure that tracker's open() calls to files should
not be considered accesses in regards to access time. O_NOATIME should
be used here, which would reduce the amount of disk writes
We already use O_NOATIME in a few extractors, however, certain libraries
don't make this very easy. Not even GIO allows to open a file for
reading with O_NOATIME set, as far as I can tell. Also, I don't expect
the amount of atime-related writes to be very high with relatime, but I
haven't measured this and could be mistaken.
substantially. Also, tracker appears to BSD lock all files it
accesses. That looks quite borked. Which other tool is it synchronizing
against here? This looks unsecure to do (because the files are often
accessible to others), and since these locks are advisory only there
needs to be a strict protocol followed by everybody else accessing these
files, which I guarantee you there isn't since these are basically all
the user's files. Moreover it appears tracker is mixing BSD and POSIX
locks, which is dangerous due to ABBA, in particular when used on NFS
directories, which will just end up in total chaos since Linux is so
stupid to "upgrade" BSD locks on NFS shares to POSIX locks on the
way. In any case you should NEVER EVER use POSIX locking, since it is
compltely borked anyway. The locking must go. I also see a massive
amount of futex calls in strace, i.e. probably some mutexes thrown in
the mix to make the locking problems even more interesting, which makes
my fingernails roll up, since they apparently are congested all the
Can you please share your findings in a bug report? As far as I know,
tracker itself doesn't use BSD or POSIX locks at all. SQLite is using
file locks, but if there are issues with how SQLite is using locks, this
should probably be discussed with SQLite upstream.
As a side-note, I myself would like to see more radical changes in how
user files will be stored in the future. Ideally, we would stop storing
them in traditional directory hierarchies. Among other things, this
would completely avoid the need for recursive directory monitoring,
recursive directory timestamps, and crawling on startup. On the
downside, this would require changes in many applications, although FUSE
could certainly help providing a compatibility layer. If anyone is
interested in discussing or working on this, let me know.