File system caching configurations
The operating system, by default, caches file data that is read from and written to disk.
In some cases, caching at the file system level and in the buffer pools causes performance degradation because of the extra CPU cycles required for the double caching. To avoid this double caching, most file systems have a feature that disables caching at the file system level. This is generically referred to as non-buffered I/O. On UNIX, this feature is commonly known as Direct I/O (or DIO). On Windows, this is equivalent to opening the file with the FILE_FLAG_NO_BUFFERING flag. In addition, some file systems such as IBM® JFS2 or Symantec VERITAS VxFS also support enhanced Direct I/O, that is, the higher-performing Concurrent I/O (CIO) feature. The database manager supports this feature with the NO FILE SYSTEM CACHING table space clause. When this is set, the database manager automatically takes advantage of CIO on file systems where this feature exists. This feature might help to reduce the memory requirements of the file system cache, thus making more memory available for other uses.
- AIX JFS
- Linux for System z®
- All SMS temporary table space files
- Long Field (LF) and Large object (LOB) data files in SMS permanent table space files.
Supported configurations
Platforms | File system type and minimum level required | DIO or CIO requests submitted by the database manager when NO FILE SYSTEM CACHING is specified | Default behavior when neither NO FILE SYSTEM CACHING nor FILE SYSTEM CACHING is specified |
---|---|---|---|
AIX 7.1 and higher | Journal File System (JFS) | DIO | FILE SYSTEM CACHING (See Note 1.) |
AIX 7.1 and higher | General Parallel File System (GPFS) | DIO | NO FILE SYSTEM CACHING |
AIX 7.1 and higher | Enhanced Journal File System (JFS2) | CIO | NO FILE SYSTEM CACHING |
AIX 7.1 and higher | VERITAS Storage Foundation 6.0.5 and VERITAS InfoScale Foundation 7.0 (VxFS) | CIO | NO FILE SYSTEM CACHING |
Linux distributions
SLES 11 SP4 or higher, and RHEL 6.7 or higher (on these architectures: x86, x64) |
ext3, ext4, xfs | DIO | NO FILE SYSTEM CACHING |
Linux distributions
SLES 11 SP4 or higher, and RHEL 6.7 or higher (on these architectures: x86, x64) |
VERITAS Storage Foundation 6.0.5 and VERITAS InfoScale Foundation 7.0 (VxFS) | CIO | NO FILE SYSTEM CACHING |
Linux distributions
SLES 12 or higher, and RHEL 7.1 or higher (on this architecture: POWER® Linux Little Endian) |
ext3, ext4, xfs | DIO | NO FILE SYSTEM CACHING |
Linux distributions
SLES 12 or higher, and RHEL 7.1 or higher (on this architecture: zSeries) |
ext3, ext4 | DIO | FILE SYSTEM CACHING |
Windows | No specific requirement, works on all Db2 supported file systems | DIO | NO FILE SYSTEM CACHING |
- On AIX JFS, FILE SYSTEM CACHING is the default.
- The VERITAS Storage Foundation for the database manager might have different operating system prerequisites. The platforms listed previously are the supported platforms for the current release. Consult the VERITAS Storage Foundation for Db2 support for prerequisite information.
- If SFDB2 5.0 is used instead of the previously specified minimum levels, the SFDB2 5.0 MP1 RP1 release must be used. This release includes fixes that are specific to the 5.0 version.
- If you do not want the database manager to choose NO FILE SYSTEM CACHING for the default setting, specify FILE SYSTEM CACHING in the relevant SQL, commands, or APIs.
Examples
CREATE TABLESPACE table space name ...
CREATE TABLESPACE table space name ... NO FILE SYSTEM CACHING
ALTER TABLESPACE table space name ... NO FILE SYSTEM CACHING
ALTER TABLESPACE table space name ... FILE SYSTEM CACHING