Wednesday, February 9, 2011
Forced Writes are turned ON by default on Windows since the release of Firebird V1.0.
See the following item on the tracker: CORE-591
But Forced Writes were OFF on Linux even if they were turned ON, until an issue with Linux (Posix) was resolved in Firebird V2.1 Beta 2 and this fix has now also ben backported to Firebird V2.0.4.
See the following item on the tracker: CORE-1476
There was some discussion amongst the Firebird developers about the change (either to OFF on Win or to ON on Linux so we would have consistent defaults), but so far there has not been a final resolution since Windows and Linux handle I/O differently, and its perfectly valid to have different default settings. Before the fix in Firebird V2.1 Linux never did forced writes even when asked to, and pretty much everybody got used to the performance with Forced Writes off, when we turned them back on correctly the performance decrement was definitely going to be a problem.
Basically, Linux writes pages out of the page cache as often as possible, so the file on disk stays pretty much current, whilst Windows writes to disk only when it must, so the difference between the disk and cache pre Firebird V1.5 could be enormous - hours or days of changes if the database file was not closed correctly. So, although it may be preferable that all systems should run with forced writes on, for Firebird on Windows it was pretty critical.
Within Firebird V1.5 we attempted to make sure that the issue of Windows cache, the Firebird cache and what was on disk could not be vastly different even if Forced Writes were turn off by introducing the following two parameters in the Firebird configuration file. However the new parameters do not guarentee complete consistency between the cache and disk, for that you do need Forced Writes turned on.
# How often the pages are flushed on disk
# (for databases with ForcedWrites=Off only)
# Number of unflushed writes which will accumulate before they are
# flushed, at the next transaction commit. For non-Win32 ports,
# the default value is -1 (Disabled)
# Type: integer
#MaxUnflushedWrites = 100
# Number of seconds during which unflushed writes will accumulate
# before they are flushed, at the next transaction commit. For non-Win32
# ports, the default value is -1 (Disabled)
# Type: integer
#MaxUnflushedWriteTime = 5