Write-back caches
The CPUs and memory in your server are quite fast compared to its disk drives. Accordingly, making the rest of the system wait for the disks, particulary when things need to be written out, can drag overall performance down heavily. Systems that wait for the disks to complete their writes before moving into their next task are referred to as having a write-through cache. While the data may be stored temporarily in a memory cache, until it's made it all the way through to the physical disk, any write an application requested isn't considered complete.
The normal solution to making that faster is to introduce a different type of write cache between the program doing the writing and disks. A write-back cache is one where data is copied into memory, and then control returns to the application that requested the write. Those writes are then handled asynchronously, at some future time dictated by the design of the write-back cache. It can take minutes before the...
