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35 lines
1.9 KiB
Plaintext
35 lines
1.9 KiB
Plaintext
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Fsync() patch (backend -F option)
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=================================
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Normally, the Postgres'95 backend makes sure that updates are actually
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committed to disk by calling the standard function fsync() in
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several places. Fsync() should guarantee that every modification to
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a certain file is actually written to disk and will not hang around
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in write caches anymore. This increases the chance that a database
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will still be usable after a system crash by a large amount.
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However, this operation severely slows down Postgres'95, because at all
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those points it has to wait for the OS to flush the buffers. Especially
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in one-shot operations, like creating a new database or loading lots
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of data, you'll have a clear restart point if something goes wrong. That's
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where the -F option kicks in: it simply disables the calls to fsync().
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Without fsync(), the OS is allowed to do its best in buffering, sorting
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and delaying writes, so this can be a _very_ big perfomance increase. However,
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if the system crashes, large parts of the latest transactions will still hang
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around in memory without having been committed to disk - lossage of data
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is therefore almost certain to occur.
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So it's a tradeoff between data integrity and speed. When initializing a
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database, I'd use it - if the machine crashes, you simply remove the files
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created and redo the operation. The same goes for bulk-loading data: on
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a crash, you remove the database and restore the backup you made before
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starting the bulk-load (you always make backups before bulk-loading,
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don't you?).
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Whether you want to use it in production, is up to you. If you trust your
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operating system, your utility company, and your hardware, you might enable
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it; however, keep in mind that you're running in an unsecure mode and that
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performance gains will very much depend on access patterns (because it won't
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help on reading data). I'd recommend against it.
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