From:   Chuck Lever III <chuck.lever@oracle.com>
To:     Bruce Fields <bfields@fieldses.org>
CC:     Steve Dickson <steved@redhat.com>, Dai Ngo <dai.ngo@oracle.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH] nfsdcld: use WAL journal for faster commits
Thread-Topic: [PATCH] nfsdcld: use WAL journal for faster commits
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Date:   Fri, 3 Dec 2021 22:07:19 +0000
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> On Dec 3, 2021, at 4:55 PM, Bruce Fields <bfields@fieldses.org> wrote:
>=20
> From: "J. Bruce Fields" <bfields@redhat.com>
>=20
> Currently nfsdcld is doing three fdatasyncs for each upcall.  Based on
> SQLite documentation, WAL mode should also be safe, and I can confirm
> from an strace that it results in only one fdatasync each.
>=20
> This may be a bottleneck e.g. when lots of clients are being created or
> expired at once (e.g. on reboot).
>=20
> Not bothering with error checking, as this is just an optimization and
> nfsdcld will still function without.  (Might be better to log something
> on failure, though.)

I'm in full philosophical agreement for performance improvements
in this area. There are some caveats for WAL:

 - It requires SQLite v3.7.0 (2010). configure.ac may need to be
   updated.

 - All processes using the DB file have to be on the same system.
   Not sure if this matters for some DR scenarios that nfs-utils
   is supposed to support.

 - WAL mode is persistent; you could set this at database creation
   time and it should be sticky.

 - Documentation says "synchronous =3D FULL is the most commonly
   used synchronous setting when not in WAL mode." Why are both
   PRAGMAs necessary here?

I agree that nfsdcld functionality is not affected if the first
PRAGMA fails.


> Signed-off-by: J. Bruce Fields <bfields@redhat.com>
> ---
> utils/nfsdcld/sqlite.c | 3 +++
> 1 file changed, 3 insertions(+)
>=20
> diff --git a/utils/nfsdcld/sqlite.c b/utils/nfsdcld/sqlite.c
> index 03016fb95823..b248eeffa204 100644
> --- a/utils/nfsdcld/sqlite.c
> +++ b/utils/nfsdcld/sqlite.c
> @@ -826,6 +826,9 @@ sqlite_prepare_dbh(const char *topdir)
> 		goto out_close;
> 	}
>=20
> +	sqlite3_exec(dbh, "PRAGMA journal_mode =3D WAL;", NULL, NULL, NULL);
> +	sqlite3_exec(dbh, "PRAGMA synchronous =3D FULL;", NULL, NULL, NULL);
> +
> 	ret =3D sqlite_query_schema_version();
> 	switch (ret) {
> 	case CLD_SQLITE_LATEST_SCHEMA_VERSION:
> --=20
> 2.33.1
>=20

--
Chuck Lever