RAID and Subsystem Architectures
Disk Subsystem Caching
• Cache is fast memory, much lower access-time than disk
• Cache can speed up both READ and WRITE operations
• Recently used data is stored in cache
• Any READ request for this data is returned immediately
• No waiting for data from disk.
• We also read-ahead, in case adjacent data blocks are needed later
Memory (Fast) Physical Disks (Slow)
• Stores data before it gets sent to disk
• As soon as cache receives data, array responds to acknowledge data.
• No waiting for data to write to disk
The more cache memory the storage subsystem has, the more it can
store in cache, and the faster it moves.
If the server needs the same data again, it is already in the cache.
Thus, the server does not have to wait for the disk subsystem to move
the data up from the disks before it can perform another operation on it.
SAN storage subsystems may have a high-availability feature called
Mirrored cache. By using this feature, the data actually gets written to
two cache memory banks simultaneously inside the subsystem.
Mirrored cache protects the data in the event of a cache failure.