On some kernels and/or filesystems, if there are files opened simultaneously for reading and writing, the buffer cache algorithms may cause the write I/O to swamp the read I/O, causing processes that are reading files to slow down because of buffer cache misses. The Linux 2.4 kernel, for example, suffers from this problem.

The mod_fsync module attempts to prevent such bottlenecks by forcibly flushing to disk the buffers used for files open for writing after a certain number of bytes have been written (for example, after 128 KB has been written to a file). This prevents the buffer cache from being dominated by data from files being written, freeing up space for data for files being read.

This module is contained in the mod_fsync.c file for ProFTPD 1.2.10rc1 and later, and is not compiled by default. Installation instructions are discussed here.

The most current version of mod_fsync can be found at:

  http://www.castaglia.org/proftpd/

Author

Please contact TJ Saunders <tj at castaglia.org> with any questions, concerns, or suggestions regarding this module.

Directives

• FsyncEngine
• FsyncLog
• FsyncThreshold

FsyncEngine

The FsyncEngine directive enables or disables the module's runtime sync engine. If it is set to off this module does no flushing of data outside of the normal I/O usage. Use this directive to disable the module.

FsyncLog

The FsyncLog directive is used to a specify a log file for mod_fsync reporting and debugging. The path parameter must be the full path to the file to use for logging. Note that this path must not be to a world-writeable directory and, unless AllowLogSymlinks is explicitly set to on (generally a bad idea), the path must not be a symbolic link.

FsyncThreshold

The FsyncThreshold directive configures a threshold value, in bytes. When this number of bytes has been written, mod_fsync will walk the list of files currently open for writing, and will forcibly flush the data for those files to disk.

Note: This directive is required for mod_fsync to operate.

Installation

  proftpd-dir/contrib/

  ./configure --with-modules=mod_fsync
  make
  make install

Usage

Example Configuration

  <IfModule mod_fsync.c>
    FsyncEngine on
    FsyncLog /etc/ftpd/var/fsync.log
    FsyncThreshold 131072 # 128KB
  </IfModule>