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From: Greg Stark <stark@mit.edu>
To: Benjamin LaHaise <bcrl@kvack.org>
Cc: Andres Freund <andres@anarazel.de>,
	Matthew Wilcox <matthew@wil.cx>, Andi Kleen <andi@firstfloor.org>,
	viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, robertmhaas@gmail.com,
	pgsql-hackers@postgresql.org
Subject: Re: Improve lseek scalability v3
Date: Fri, 16 Sep 2011 23:44:59 +0100	[thread overview]
Message-ID: <CAM-w4HNAJVRx5Vj87hXjL9JDjwbUoDiso_NZcfomk7wpd2zshw@mail.gmail.com> (raw)
In-Reply-To: <20110916200817.GD28519@kvack.org>

On Fri, Sep 16, 2011 at 9:08 PM, Benjamin LaHaise <bcrl@kvack.org> wrote:
> For such tables, can't Postgres track the size of the file internally?  I'm
> assuming it's keeping file descriptors open on the tables it manages, in
> which case when it writes to a file to extend it, the internally stored size
> could be updated.  Not making a syscall at all would scale far better than
> even a modified lseek() will perform.

There's no hardwired limit on how many tables you can have in a
database, it's not limited by the number of file descriptors. Postgres
would have to keep some kind of LRU for recently opened files and
their sizes or something like that. There would probably still be a
lot of lseeks/fstats going on.

Generally keeping a Postgres cached value for the size would then have
a reliability issue. It's much safer to have a single authoritative
value -- the actual length of the file -- than have the same value
stored in two locations and then need to worry about them getting out
of sync. If a write fails when extending the file due to a filesystem
running out of space then Postgres might not know how to update its
internal cached state accurately for example.

There's no question it could be done but it's not clear it would
necessarily be much faster than a lock-free lseek/fstat.

On Fri, Sep 16, 2011 at 6:27 PM, Andres Freund <andres@anarazel.de> wrote:
> It depends on where the information is used. For some of the uses it needs to
> be exact (the assumed size is rechecked after acquiring a lock preventing
> extension)

Fwiw this might give the wrong impression. I don't believe scans
acquire a lock preventing extension -- that is another process can be
concurrently extending the file at the same time as the scan is
proceeding. The scan only locks out truncation (vacuum). Any blocks
added by another process are ignored by the scan because they can only
contain records invisible to that transaction. This does depend on the
lseek/fstat being done after the transaction snapshot is taken which
is possibly "rechecking" the value taken by the query planner but
they're really two independent things.


-- 
greg

  parent reply	other threads:[~2011-09-16 22:45 UTC|newest]

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-09-15 23:06 Improve lseek scalability v3 Andi Kleen
2011-09-15 23:06 ` [PATCH 1/7] BTRFS: Fix lseek return value for error Andi Kleen
2011-09-15 23:47   ` Thomas Gleixner
2011-09-16 15:48   ` Christoph Hellwig
2011-09-16 16:38     ` Andi Kleen
2011-09-17  6:10     ` Jeff Liu
2011-09-17 23:03       ` Andreas Dilger
2011-09-18  1:46         ` Andi Kleen
2011-09-18  7:29           ` Jeff Liu
2011-09-18  8:42             ` Marco Stornelli
2011-09-18 10:33               ` Jeff liu
2011-09-18 10:33                 ` Jeff liu
2011-09-18 14:55                 ` Chris Mason
2011-09-18 14:55                   ` Chris Mason
2011-09-18 14:55                   ` Chris Mason
2011-09-19 17:52                   ` Andi Kleen
2011-09-19 19:30                     ` Chris Mason
2011-09-19 19:59                       ` Andi Kleen
2011-09-19 22:55                         ` Chris Mason
2011-09-15 23:06 ` [PATCH 2/7] VFS: Do (nearly) lockless generic_file_llseek Andi Kleen
2011-09-15 23:06 ` [PATCH 3/7] VFS: Make generic lseek lockless safe Andi Kleen
2011-09-15 23:06 ` [PATCH 4/7] VFS: Add generic_file_llseek_size Andi Kleen
2011-09-16 15:50   ` Christoph Hellwig
2011-09-15 23:06 ` [PATCH 5/7] LSEEK: EXT4: Replace cut'n'pasted llseek code with generic_file_llseek_size Andi Kleen
2011-09-15 23:06 ` [PATCH 6/7] LSEEK: NFS: Drop unnecessary locking in llseek Andi Kleen
2011-09-15 23:06 ` [PATCH 7/7] LSEEK: BTRFS: Avoid i_mutex for SEEK_{CUR,SET,END} Andi Kleen
2011-09-16 13:00 ` Improve lseek scalability v3 Matthew Wilcox
2011-09-16 13:19   ` Josef Bacik
2011-09-16 14:16   ` Andres Freund
2011-09-16 14:23     ` Andi Kleen
2011-09-16 14:41       ` Andres Freund
2011-09-16 15:36     ` Matthew Wilcox
2011-09-16 17:27       ` Andres Freund
2011-09-16 17:39         ` [HACKERS] " Alvaro Herrera
2011-09-16 17:39           ` Alvaro Herrera
2011-09-16 17:50           ` [HACKERS] " Andi Kleen
2011-09-16 20:08         ` Benjamin LaHaise
2011-09-16 21:02           ` Andres Freund
2011-09-16 21:05             ` [HACKERS] " Andres Freund
2011-09-16 22:44           ` Greg Stark [this message]
2011-09-19 12:31           ` Stephen Frost
2011-09-19 12:31             ` Stephen Frost
2011-09-19 13:25             ` [HACKERS] " Matthew Wilcox
2011-09-20  7:18               ` Marco Stornelli
2011-09-20  7:18                 ` Marco Stornelli
2011-09-19 13:30             ` Robert Haas
2011-09-16 14:26   ` Andres Freund
2011-10-01 20:46 ` Andres Freund
2011-10-01 20:49   ` [PATCH 1/2] LSEEK: BTRFS: Avoid i_mutex for SEEK_{CUR,SET,END} Andres Freund
2011-11-02  8:29     ` Christoph Hellwig
2011-11-05 15:27       ` Chris Mason
2012-03-07 17:16         ` Andres Freund
2011-10-01 20:50   ` [PATCH 2/2] btrfs: Don't have multiple paths to error out in btrfs_file_llseek Andres Freund
2011-10-02  5:28   ` Improve lseek scalability v3 Andi Kleen

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