Date:   Thu, 14 Nov 2019 13:28:05 +0000 (UTC)
From:   Sage Weil <sweil@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>, gfarnum@...hat.com
cc:     Luis Henriques <lhenriques@...e.com>,
        Ilya Dryomov <idryomov@...il.com>,
        "Yan, Zheng" <zyan@...hat.com>, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 0/4] ceph: safely use 'copy-from' Op on Octopus
 OSDs

On Thu, 14 Nov 2019, Jeff Layton wrote:
> On Thu, 2019-11-14 at 10:57 +0000, Luis Henriques wrote:
> > Hi!
> > 
> > So, after the feedback I got from v1 [1] I've sent out a pull-request
> > for the OSDs [2] which encodes require_osd_release into the OSDMap
> > client data.  This allows the client to figure out which ceph release
> > the OSDs cluster is running and decide whether or not it's safe to use
> > the copy-from Op for copy_file_range.
> > 
> > This new patchset I'm sending simply adds enough functionality to the
> > kernel client so that it can take advantage of this OSD patch:
> > 
> > 0001 - adds the ability to decode TYPE_MSGR2 addresses.  This is a
> >        required functionality for enabling SERVER_NAUTILUS in the
> >        client.  I hope I got the new format right, as I couldn't figure
> >        out what the hard-coded values (see comments) really mean.
> > 
> 
> nit: the first 3 patch subject lines should probably be prefixed with
> "libceph:"
> 
> > 0002 - allows the client to retrieve the new require_osd_release field
> >        from the OSDMap if available.  This patch also adds SERVER_MIMIC,
> >        SERVER_NAUTILUS and SERVER_OCTOPUS to the supported features,
> >        which TBH I'm not sure if that's a safe thing to do -- the only
> >        issue I've seen was that Nautilus requires the ability to decode
> >        TYPE_MSGR2 address, but I may have missed others.
> > 
> 
> Yes, this needs to be done with care. We have to ensure that the server
> side isn't assuming that the client supports something that it doesn't.
> I think that means just trawling through the code and verifying whether
> this is safe.
> 
> > 0003 - debug code to add require_osd_release to the osdmap debugfs file.
> > 
> > 0004 - adds the truncate_{seq,size} fields to the 'copy-from' operation
> >        if the OSDs are >= Octopus.
> > 
> > Also note that, as suggested by Ilya, I've dropped the patch that would
> > change the default mount options to 'copyfrom'.
> > 
> > These patches have been tested with the xfstests generic test suite, and
> > with a couple of other (local) tests that exercise the cephfs
> > copy_file_range syscall.  I didn't saw any issues, but as I said above,
> > I'm not really sure if adding the SERVER_* flags to the supported
> > features have other side effects.
> > 
> > [1] https://lore.kernel.org/lkml/20191108141555.31176-1-lhenriques@suse.com/
> > [2] https://github.com/ceph/ceph/pull/31611
> > 
> 
> I'm just getting caught up on the discussion here, but why was it
> decided to do it this way instead of just adding a new OSD
> "copy-from-no-truncseq" operation? Once you tried it once and an OSD
> didn't support it, you could just give up on using it any longer? That
> seems a lot simpler than trying to monkey with feature bits.

I don't remember the original discussion either, but in retrospect that 
does seem much simpler--especially since hte client is conditioning 
sending this based on the the require_osd_release.  It seems like passing 
a flag to the copy-from op would be more reasonable instead of conditional 
feature-based behavior.

Greg, do you remember why we ended up here?

sage

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