Identify two mechanisms employed by the Linux block I/O subsystem that improve performance. Discuss how they improve performance and how they may, if ever, degrade performance.

Question

What will be an ideal response?

Answer 1

The kernel implements file caches and batched I/O operations using disk scheduling
to improve I/Operformance. File caches, which are implemented using the page cache, allow
the system to freely write to and read from pages in memory that store file data while a process
accesses them. The page cache improves performance by enabling multiple write
requests to accumulate, which can later be merged when flushed to disk. One limitation to
such an approach is the loss of data in the event of a power failure, given that file changes are
stored in volatile main memory until flushed to the disk. The use of batched I/O operations
using disk scheduling, implemented by sorting disk requests using the elevator algorithm,
improves performance by theoretically reducing seek times between reads or writes. Such a
technique is not always applicable or desirable, in the case of RAM disks or RAID devices.
In the former case, raw (direct) I/O should be used; in the latter case, I/O requests should be
passed directly to the RAID driver for sorting and processing.