File-system caches are a mechanism used by most of today's operating systems to avoid redundant disk accesses. In order to accomplish this, a part of the computer's physical memory (RAM) is used to buffer data that is read from and written to disk. This memory region is then called a "cache".

See "File systems" for introductory information about file systems.

Do not confuse the file-system caches with the other caches that exist on your system, such as the processor caches and maybe other hardware caches that are built into your hard disks. The concept of these caches is the same (avoid slow accesses by using fast memory if possible), but they are not described here.

If an application accesses disk data that is still residing in the file-system cache, &os2; does not have to read it from the disk, but can simply take it from the cache in memory, which is a lot faster.

On the other hand side, if an application is writing data to disk, &os2; can copy this data to the cache first and delay writing it to disk until the disk is ready for accepting it, or the system is idle. This is called "lazy writing" and can greatly improve your system's performance. On the other hand size, this increases the danger of data loss in case of power shortages or fatal system crashes, because data in the caches gets lost then. You should still have lazy writing enabled if you have a reasonably stable &os2; system and rarely get CHKDSK's at bootup.

&os2; implements separate caches for each file system that is installed. This means that all FAT volumes will use one cache, and all HPFS volumes a different one. These caches are separately configured with different &cfgsys; settings. You can edit these settings in the "&os2; Kernel" object. You will find the HPFS settings on the "Drivers" page in that object, since HPFS is an installable file system (IFS). By contrast, since FAT is built into the &os2; Kernel itself, it has its own page.

Please keep in mind that memory you reserve for file-system caches is no longer available for other purposes. If you reserve too much cache memory, this will have a negative impact on your system performance, because &os2; will spend more time on your swap file. As a result, you should reflect a bit on what settings you specify here.

In general, the more memory you have on your system, the more memory you should reserve for the caches. You should also take in account which file systems you use more frequently: for example, if you mainly use HPFS volumes, it's a waste to reserve much memory for FAT caches, and vice versa.

This is not so much an issue any more considering the amount of memory which is installed in today's computers. However, if you still only have 16 MB RAM or less, you shouldn't reserve more than 1024 KB cache for both file systems altogether.