mirror of
https://git.postgresql.org/git/postgresql.git
synced 2024-12-15 08:20:16 +08:00
35 lines
1.9 KiB
Plaintext
35 lines
1.9 KiB
Plaintext
|
Fsync() patch (backend -F option)
|
||
|
|
=================================
|
||
|
|
|
||
|
|
Normally, the Postgres'95 backend makes sure that updates are actually
|
||
|
|
committed to disk by calling the standard function fsync() in
|
||
|
|
several places. Fsync() should guarantee that every modification to
|
||
|
|
a certain file is actually written to disk and will not hang around
|
||
|
|
in write caches anymore. This increases the chance that a database
|
||
|
|
will still be usable after a system crash by a large amount.
|
||
|
|
|
||
|
|
However, this operation severely slows down Postgres'95, because at all
|
||
|
|
those points it has to wait for the OS to flush the buffers. Especially
|
||
|
|
in one-shot operations, like creating a new database or loading lots
|
||
|
|
of data, you'll have a clear restart point if something goes wrong. That's
|
||
|
|
where the -F option kicks in: it simply disables the calls to fsync().
|
||
|
|
|
||
|
|
Without fsync(), the OS is allowed to do its best in buffering, sorting
|
||
|
|
and delaying writes, so this can be a _very_ big perfomance increase. However,
|
||
|
|
if the system crashes, large parts of the latest transactions will still hang
|
||
|
|
around in memory without having been committed to disk - lossage of data
|
||
|
|
is therefore almost certain to occur.
|
||
|
|
|
||
|
|
So it's a tradeoff between data integrity and speed. When initializing a
|
||
|
|
database, I'd use it - if the machine crashes, you simply remove the files
|
||
|
|
created and redo the operation. The same goes for bulk-loading data: on
|
||
|
|
a crash, you remove the database and restore the backup you made before
|
||
|
|
starting the bulk-load (you always make backups before bulk-loading,
|
||
|
|
don't you?).
|
||
|
|
|
||
|
|
Whether you want to use it in production, is up to you. If you trust your
|
||
|
|
operating system, your utility company, and your hardware, you might enable
|
||
|
|
it; however, keep in mind that you're running in an unsecure mode and that
|
||
|
|
performance gains will very much depend on access patterns (because it won't
|
||
|
|
help on reading data). I'd recommend against it.
|