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Subject: Re: [RFC PATCH] ceph: add remote object copy counter to fs client
From:   Jeff Layton <jlayton@kernel.org>
To:     Xiubo Li <xiubli@redhat.com>,
        Patrick Donnelly <pdonnell@redhat.com>
Cc:     =?ISO-8859-1?Q?Lu=EDs?= Henriques <lhenriques@suse.de>,
        Ilya Dryomov <idryomov@gmail.com>,
        Ceph Development <ceph-devel@vger.kernel.org>,
        linux-kernel@vger.kernel.org
Date:   Tue, 26 Oct 2021 07:40:51 -0400
In-Reply-To: <785d1435-4a2c-95aa-0573-2de54b4e7b6b@redhat.com>
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On Tue, 2021-10-26 at 11:05 +0800, Xiubo Li wrote:
> On 10/22/21 1:30 AM, Patrick Donnelly wrote:
> > On Thu, Oct 21, 2021 at 12:35 PM Jeff Layton <jlayton@kernel.org> wrote:
> > > On Thu, 2021-10-21 at 12:18 -0400, Patrick Donnelly wrote:
> > > > On Thu, Oct 21, 2021 at 11:44 AM Jeff Layton <jlayton@kernel.org> wrote:
> > > > > On Thu, 2021-10-21 at 09:52 -0400, Patrick Donnelly wrote:
> > > > > > On Wed, Oct 20, 2021 at 12:27 PM Jeff Layton <jlayton@kernel.org> wrote:
> > > > > > > On Wed, 2021-10-20 at 15:37 +0100, Lu?s Henriques wrote:
> > > > > > > > This counter will keep track of the number of remote object copies done on
> > > > > > > > copy_file_range syscalls.  This counter will be filesystem per-client, and
> > > > > > > > can be accessed from the client debugfs directory.
> > > > > > > > 
> > > > > > > > Cc: Patrick Donnelly <pdonnell@redhat.com>
> > > > > > > > Signed-off-by: Lu?s Henriques <lhenriques@suse.de>
> > > > > > > > ---
> > > > > > > > This is an RFC to reply to Patrick's request in [0].  Note that I'm not
> > > > > > > > 100% sure about the usefulness of this patch, or if this is the best way
> > > > > > > > to provide the functionality Patrick requested.  Anyway, this is just to
> > > > > > > > get some feedback, hence the RFC.
> > > > > > > > 
> > > > > > > > Cheers,
> > > > > > > > --
> > > > > > > > Lu?s
> > > > > > > > 
> > > > > > > > [0] https://github.com/ceph/ceph/pull/42720
> > > > > > > > 
> > > > > > > I think this would be better integrated into the stats infrastructure.
> > > > > > > 
> > > > > > > Maybe you could add a new set of "copy" stats to struct
> > > > > > > ceph_client_metric that tracks the total copy operations done, their
> > > > > > > size and latency (similar to read and write ops)?
> > > > > > I think it's a good idea to integrate this into "stats" but I think a
> > > > > > local debugfs file for some counters is still useful. The "stats"
> > > > > > module is immature at this time and I'd rather not build any qa tests
> > > > > > (yet) that rely on it.
> > > > > > 
> > > > > > Can we generalize this patch-set to a file named "op_counters" or
> > > > > > similar and additionally add other OSD ops performed by the kclient?
> > > > > > 
> > > > > 
> > > > > Tracking this sort of thing is the main purpose of the stats code. I'm
> > > > > really not keen on adding a whole separate set of files for reporting
> > > > > this.
> > > > Maybe I'm confused. Is there some "file" which is already used for
> > > > this type of debugging information? Or do you mean the code for
> > > > sending stats to the MDS to support cephfs-top?
> > > > 
> > > > > What's the specific problem with relying on the data in debugfs
> > > > > "metrics" file?
> > > > Maybe no problem? I wasn't aware of a "metrics" file.
> > > > 
> > > Yes. For instance:
> > > 
> > > # cat /sys/kernel/debug/ceph/*/metrics
> > > item                               total
> > > ------------------------------------------
> > > opened files  / total inodes       0 / 4
> > > pinned i_caps / total inodes       5 / 4
> > > opened inodes / total inodes       0 / 4
> > > 
> > > item          total       avg_lat(us)     min_lat(us)     max_lat(us)     stdev(us)
> > > -----------------------------------------------------------------------------------
> > > read          0           0               0               0               0
> > > write         5           914013          824797          1092343         103476
> > > metadata      79          12856           1572            114572          13262
> > > 
> > > item          total       avg_sz(bytes)   min_sz(bytes)   max_sz(bytes)  total_sz(bytes)
> > > ----------------------------------------------------------------------------------------
> > > read          0           0               0               0               0
> > > write         5           4194304         4194304         4194304         20971520
> > > 
> > > item          total           miss            hit
> > > -------------------------------------------------
> > > d_lease       11              0               29
> > > caps          5               68              10702
> > > 
> > > 
> > > I'm proposing that Luis add new lines for "copy" to go along with the
> > > "read" and "write" ones. The "total" counter should give you a count of
> > > the number of operations.
> > Okay that makes more sense!
> > 
> > Side note: I am a bit horrified by how computer-unfriendly that
> > table-formatted data is.
> 
> Any suggestion to improve this ?
> 
> How about just make the "metric" file writable like a switch ? And as 
> default it will show the data as above and if tools want the 
> computer-friendly format, just write none-zero to it, then show raw data 
> just like:
> 
> # cat /sys/kernel/debug/ceph/*/metrics
> opened_files:0
> pinned_i_caps:5
> opened_inodes:0
> total_inodes:4
> 
> read_latency:0,0,0,0,0
> write_latency:5,914013,824797,1092343,103476
> metadata_latency:79,12856,1572,114572,13262
> 
> read_size:0,0,0,0,0
> write_size:5,4194304,4194304,4194304,20971520
> 
> d_lease:11,0,29
> caps:5,68,10702
> 
> 

I'd rather not multiplex the output of this file based on some input.
That would also be rather hard to do -- write() and read() are two
different syscalls, so you'd need to track a bool (or something) across
them somehow.

Currently, I doubt there are many scripts in the field that scrape this
info and debugfs is specifically excluded from ABI concerns. If we want
to make it more machine-readable (which sounds like a good thing), then
I suggest we just change the output to something like what you have
above and not worry about preserving the "legacy" output.
-- 
Jeff Layton <jlayton@kernel.org>