File System Caches

Disk caching is the placing of frequently accessed data in a special buffer storage. It reduces access time and improves the performance of applications that rely heavily on hard disk data. It works with both HPFS and the FAT file system caches.

• To allow the operating system the greatest chance for finding data in the cache, both file systems incorporate asynchronous read ahead for sequential I/O. This read-ahead capability is always enabled in OS/2 2.0.

• To minimize the frequency with which the system ties up its resources writing cached data to the disk, both file systems also can take advantage of the lazy-writing feature (or write behind). Lazy writing provides a performance improvement when writing to the disk.

• To determine when I/O should be committed to the disk, both file systems use a set of parameters. See "OS/2 2.0 CONFIG.SYS Statements."

For applications that require absolute data integrity, files should be opened with WriteThru enabled. WriteThru ensures that disk write operations are committed to disk before the application continues. This is in direct contrast to lazy writing; lazy-written data can remain in the file-system cache for several seconds after the application has completed writing.

Disabling the lazy-writing feature of either file system does not provide the same capability as enabling WriteThru, and it decreases system performance. When it is not possible to open a file with WriteThru enabled, the shutdown procedure must be used to ensure that all data is written to disk.