postgresql/doc/TODO.detail/fsync

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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200011191923.OAA06790@candle.pha.pa.us>
Subject: Re: [HACKERS] WAL fsync scheduling
In-Reply-To: <002101c0525e$2d964480$b97a30d0@sectorbase.com> "from Vadim Mikheev
	at Nov 19, 2000 11:23:19 am"
To: Vadim Mikheev <vmikheev@sectorbase.com>
Date: Sun, 19 Nov 2000 14:23:06 -0500 (EST)
CC: Tom Samplonius <tom@sdf.com>, Alfred@candle.pha.pa.us,
        Perlstein <bright@wintelcom.net>, Larry@candle.pha.pa.us,
        Rosenman <ler@lerctr.org>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
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> > There are two parts to transaction commit.  The first is writing all
> > dirty buffers or log changes to the kernel, and second is fsync of the
>    ^^^^^^^^^^^^
> Backend doesn't write any dirty buffer to the kernel at commit time.

Yes, I suspected that.

> 
> > log file.
> 
> The first part is writing commit record into WAL buffers in shmem.
> This is what XLogInsert does.  After that XLogFlush is called to ensure
> that  entire commit record is on disk. XLogFlush does *both* write() and
> fsync() (single slock is used for both writing and fsyncing) if it needs to
> do it at all.

Yes, I realize there are new steps in WAL.

> 
> > I suggest having a per-backend shared memory byte that has the following
> > values:
> > 
> > START_LOG_WRITE
> > WAIT_ON_FSYNC
> > NOT_IN_COMMIT
> > backend_number_doing_fsync
> > 
> > I suggest that when each backend starts a commit, it sets its byte to
> > START_LOG_WRITE. 
>   ^^^^^^^^^^^^^^^^^^^^^^^
> Isn't START_COMMIT more meaningful?

Yes.

> 
> > When it gets ready to fsync, it checks all backends. 
>    ^^^^^^^^^^^^^^^^^^^^^^^^^^
> What do you mean by this? The moment just after XLogInsert?

Just before it calls fsync().

> 
> > If all are NOT_IN_COMMIT, it does fsync and continues.
> 
> 1st edition:
> > If one or more are in START_LOG_WRITE, it waits until no one is in
> > START_LOG_WRITE.  It then checks all WAIT_ON_FSYNC, and if it is the
> > lowest backend in WAIT_ON_FSYNC, marks all others with its backend
> > number, and does fsync.  It then clears all backends with its number to
> > NOT_IN_COMMIT.  Other backend will see they are not the lowest
> > WAIT_ON_FSYNC and will wait for their byte to be set to NOT_IN_COMMIT
> > so they can then continue, knowing their data was synced.
> 
> 2nd edition:
> > I have another idea.  If a backend gets to the point that it needs
> > fsync, and there is another backend in START_LOG_WRITE, it can go to an
> > interuptable sleep, knowing another backend will perform the fsync and
> > wake it up.  Therefore, there is no busy-wait or timed sleep.
> > 
> > Of course, a backend must set its status to WAIT_ON_FSYNC to avoid a
> > race condition.
> 
> The 2nd edition is much better. But I'm not sure do we really need in
> these per-backend bytes in shmem. Why not just have some counters?
> We can use a semaphore to wake-up all waiters at once.

Yes, that is much better and clearer.  My idea was just to say, "if no
one is entering commit phase, do the commit.  If someone else is coming,
sleep and wait for them to do the fsync and wake me up with a singal."  

> 
> > This allows a single backend not to sleep, and allows multiple backends
> > to bunch up only when they are all about to commit.
> > 
> > The reason backend numbers are written is so other backends entering the
> > commit code will not interfere with the backends performing fsync.
> 
> Being waked-up backend can check what's written/fsynced by calling XLogFlush.

Seems that may not be needed anymore with a counter.  The only issue is
that other backends may enter commit while fsync() is happening.  The
process that did the fsync must be sure to wake up only the backends
that were waiting for it, and not other backends that may be also be
doing fsync as a group while the first fsync was happening.  I leave
those details to people more experienced.  :-)

I am just glad people liked my idea.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 853-3000
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