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Chapter 4

Global Improvements

in Firebird 2.1

Some global improvements and changes have been implemented in Firebird 2.1, as engine development moves towards the architectural changes planned for Firebird 3.

Note

Unless otherwise indicated, these improvements apply from v.2.1 forward.

Forced Writes on Linux Now Works!

A. Peshkov

For maximum database safety, we configure databases for synchronous writes, a.k.a. Forced Writes ON. This mode—strongly recommended for normal production usage—makes the write() system call return only after the physical write to disk is complete. In turn, it guarantees that, after a COMMIT, any data modified by the transaction is physically on the hard-drive, not waiting in the operating system's cache.

Its implementation on Linux was very simple - invoke fcntl(dbFile, F_SETFL, O_SYNC).

Yet databases on Linux were sometimes corrupted anyway.

Forensics

Speed tests on Linux showed that setting O_SYNC on a file has no effect at all on performance! Fine, fast operating system we may think? Alas, no, it's a documented bug in the Linux kernel!

According to the Linux manual, “On Linux this command (i.e. fcntl(fd, F_SETFL, flags)) can only change the O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags”. Though it is not documented in any place known to me, it turns out that an attempt to set any flag other than those listed in the manual (such as O_SYNC, for example) won't work but it does not cause fcntl() to return an error, either.

For Firebird and for InterBase versions since Day One, it means that Forced Writes has never worked on Linux. It certainly works on Windows. It seems likely that this is not a problem that affects other operating systems, although we cannot guarantee that. To make sure, you can check whether the implementation of fcntl() on your OS is capable of setting the O_SYNC flag.

The technique used currently, introduced in the Beta 2 release of Firebird 2.1, is to re-open the file. It should guarantee correct operation on any OS, provided the open() system call works correctly in this respect. It appears that no such problems are reported.

The Firebird developers have no idea why such a bug would remain unfixed almost two years after getting into

the Linux kernel's bug-tracker. Apparently, in Linux, a documented bug evolves into a feature...

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