Re: [PATCH RFC] vfs: add a O_NOMTIME flag

[Trond Myklebust]

On Fri, May 8, 2015 at 6:24 PM, Sage Weil <sage@xxxxxxxxxxxx> wrote:
> On Sat, 9 May 2015, Dave Chinner wrote:
>> On Thu, May 07, 2015 at 09:23:24PM -0400, Trond Myklebust wrote:
>> > On Thu, May 7, 2015 at 9:01 PM, Sage Weil <sage@xxxxxxxxxxxx> wrote:
>> > > On Thu, 7 May 2015, Zach Brown wrote:
>> > >> On Thu, May 07, 2015 at 10:26:17AM +1000, Dave Chinner wrote:
>> > >> > On Wed, May 06, 2015 at 03:00:12PM -0700, Zach Brown wrote:
>> > >> > > The criteria for using O_NOMTIME is the same as for using O_NOATIME:
>> > >> > > owning the file or having the CAP_FOWNER capability.  If we're not
>> > >> > > comfortable allowing owners to prevent mtime/ctime updates then we
>> > >> > > should add a tunable to allow O_NOMTIME.  Maybe a mount option?
>> > >> >
>> > >> > I dislike "turn off safety for performance" options because Joe
>> > >> > SpeedRacer will always select performance over safety.
>> > >>
>> > >> Well, for ceph there's no safety concern.  They never use cmtime in
>> > >> these files.
>> > >>
>> > >> So are you suggesting not implementing this and making them rework their
>> > >> IO paths to avoid the fs maintaining mtime so that we don't give Joe
>> > >> Speedracer more rope?  Or are we talking about adding some speed bumps
>> > >> that ceph can flip on that might give Joe Speedracer pause?
>> > >
>> > > I think this is the fundamental question: who do we give the ammunition
>> > > to, the user or app writer, or the sysadmin?
>> > >
>> > > One might argue that we gave the user a similar power with O_NOATIME (the
>> > > power to break applications that assume atime is accurate).  Here we give
>> > > developers/users the power to not update mtime and suffer the consequences
>> > > (like, obviously, breaking mtime-based backups).  It should be pretty
>> > > obvious to anyone using the flag what the consequences are.
>> > >
>> > > Note that we can suffer similar lapses in mtime with fdatasync followed by
>> > > a system crash.  And as Andy points out it's semi-broken for writable
>> > > mmap.  The crash case is obviously a slightly different thing, but the
>> > > idea that mtime can't always be trusted certainly isn't crazy talk.
>> > >
>> > > Or, we can be conservative and require a mount option so that the admin
>> > > has to explicitly allow behavior that might break some existing
>> > > assumptions about mtime/ctime ('-o user_noatime' I guess?).
>> > >
>> > > I'm happy either way, so long as in the end an unprivileged ceph daemon
>> > > avoids the useless work.  In our case we always own the entire mount/disk,
>> > > so a mount option is just fine.
>> > >
>> >
>> > So, what is the expectation here for filesystems that cannot support
>> > this flag? NFSv3 in particular would break pretty catastrophically if
>> > someone decided on a whim to turn off mtime: they will have turned off
>> > the client's ability to detect cache incoherencies.
>>
>> It's worse than that, now that I think about it. I think nomtime
>> will break nfsv4 as the I_VERSION check is done *after* the
>> NO[C]MTIME checks. e.g. the atomic change count used to detect file
>> changes is only updated during the mtime update on write() calls in
>> XFS. i.e. when the timestamp is changed, a transaction to change
>> mtime is run, and that transaction commit bumps the change count.
>>
>> So cutting out mtime updates at the VFS will prevent XFS and other
>> I_VERSION aware filesystems from updating the change count that
>> NFSv4 clients rely on to detect foreign data changes in a file.
>>
>> Not sure what to do here, because the current NOCMTIME
>> implementation intentionally cuts out the timestamp update because
>> it's usage is fully invisible IO. i.e. it is used by utilities like
>> xfs_fsr and HSMs to move data into and out of files without the
>> application being able to detect the data movement in any way. These
>> are not data modification operations, though - the file contents as
>> read by the application do not change despite the fact we are moving
>> data in and out of the file. In this case we don't want timestamps
>> or change counters to change on the data movement, so I think we've
>> actually got a difference in behaviour here between O_NOMTIME and
>> O_NOCMTIME, right?
>>
>> i.e. for nfsv4 sanity O_NOMTIME still needs to bump I_VERSION on
>> write, just not modify the timestamp? In which case, not modifying
>> the timestamps gains us nothing, because the inode is still dirtied?
>
> Right: if we dirty the inode we've defeated the purpose of the patch.
>
>> The list of caveats on O_NOMTIME seems to be growing...
>
> ...and remain consistent with our goals.  We couldn't care less if NFS or
> backup software or anything else doesn't notice these changes.  This is
> private data that is wholly managed by the ceph daemon.  The goal is to
> derive *some* value from the file system and avoid reimplementing it in
> userspace (without the bits we don't need).

That makes it completely non-generic though. By putting this in the
VFS, you are giving applications a loaded gun that is pointed straight
at the application user's head.

> I'm sure you realize what we're try to achieve is the same "invisible IO"
> that the XFS open by handle ioctls do by default.  Would you be more
> comfortable if this option where only available to the generic
> open_by_handle syscall, and not to open(2)?

It should be an ioctl(). It has no business being part of
open_by_handle either, since that is another generic interface.

Cheers
  Trond
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