This read-only integer reports the total size, in bytes, of the file-system cache, combining system cache, driver, code, and paged-pool resident bytes into one figure. An admin uses this to see how much physical memory the OS has allocated to caching file data, which can shrink under memory pressure. For example, a shrinking cpqOsMemCacheBytes value alongside rising cpqOsMemPagesPerSec would indicate the OS is reclaiming cache memory to relieve overall memory pressure.