Block I/O
=========
- | The difference (between character device and block device) comes
down to
| whether the device accesses data randomly --- in otherwords,
whether the
| device can seek to one position from another. - | The block is an abstraction of the filesystem --- filesystems can
be
| accessed only in multiples of a block.Although the physical device
is
| addressable at the sector level, the kernel performs all disk
operations in
| terms of blocks. - | the kernel (as with hardware and the sector) needs the block to be
a power
| of two.The kernel also requires that a block be no larger than the
page size. - | The purpose of a buffer head is to describe this mapping between
the
| on-disk block and the physical in-memory buffer (which is a
sequence of bytes
| on a specific page). - | the kernel does not issue block I/O requests to the disk in the
order they
| are received or as soon as they are received. - | Both the process scheduler and the I/O scheduler virtualize a
resource
| among multiple objects. - I/O schedulers perform two primary actions to minimize seeks: merging
and sorting. a) Merging is the coalescing of two or more requests
into one.
Consequently, merging requests reduces overhead and minimizes seeks.
| b) The entire request queue is kept sorted, sectorwise, so that all
seeking
| activity along the queue moves (as much as possible) sequentially over
the
| sectors of the hard disk.This is similar to the algorithm employed in
| elevators ------ try to move gracefully in a single direction.
#. Linus Elevator: The Linus Elevator I/O scheduler performs both
front and
back merging.
#. The Deadline I/O scheduler: ensure that write requests do not
starve
read requests.
#. The Anticipatory I/O scheduler aims to continue to provide
excellent
read latency, but also provide excellent global throughput.
#. The Complete Fair Queuing (CFQ) I/O scheduler is an I/O scheduler
designed for specialized workloads, but that in practice actually
provides
good performance across multiple workloads.It is now the default
I/O scheduler
in Linux(2.6).
#. the Noop I/O Scheduler truly is a noop, merely maintaining the
request queue in near-FIFO order, from which the block device
driver can pluck
requests.
