Intel Corp (INTC)

[rudedog]

re: why conventional drive makers didn't just stuff a lot of RAM for a cache

For any serious enterprise use, the buffer on the disk is disabled for writes, because otherwise the data is vulnerable to corruption. So the primary benefit of on-disk cache is for read-ahead, or consolidating out of sequence reads if the controller does not do that already. As long as the cache has enough space for a couple of tracks, increasing it does not give much benefit.

Controllers like the HP SmartArray series use battery-backed RAM which allows the driver to cut the number of interrupts in half on a write, and does more effective consolidation of requests for large arrays of disks. For example, on a 14 disk array, with as much as 512MB of RAM, sustained I/O rates in the thousands per second are possible using standard magnetic media, and in the event of a power failure or other interruption, pending writes are maintained until the system is restored. You can even remove a controller and array from one system and put it in another, and the write will complete when the system is powered up, even before any OS loads.

The benefits of SSD are primarily when a single operation requires high I/O or bandwidth - such as database transaction logs, or initial loading of large files - in general, where the smaller number of threads or concurrent I/O load would not make an array-based design effective.