Today's Agenda:
	Block devices
		Including mmap demo

Block vs. character device
	"random" access
		- random depends on your perspective
	Block device divided into sectors
		Sector vs. block

Using dd

/dev conventions, hda, sda, nvme, etc
	And symbolic links 

Blocks are represented as a buffer
	Size of a block:  At least a sector, no more than a memory page
	sector:  Hardware division, property of the hard drive
	block:  Software division, property of the operating system
	struct buffer_head defines one
	Flags for state

Interlude:  Memory-mapping files?
	Use mmap, munmap, msync, etc
	Demo
	Not for expanding files
		You can use write to append, and then unmap and mmap
	read/write/lseek sort of supports random access
		Not so natural as this
	This is *probably* faster for a lot of things

bio structure
	"Block I/O"

I/O schedulers
	Minimize head movement
	merging and sorting
	Independent of process scheduler
		- Process makes request
		- I/O scheduler gets around to it
	Merge if requests are for very close sectors
	Sort by proximity to head
		- Without reversing head direction if possible
		- Like an elevator
		- Sometimes called I/O elevator
	The Linus Elevator:
		- Default in 2.4, replaced in 2.6
		- New requests are checked against all others for merging
		- Merge onto front OR back depending
		- If no merging, added to sorted point in queue
		- If no good sorted point, add to the end of the queue
		- If tail is old, new requests go at the end regardless
			+ To prevent infinite starvation
	The Deadline I/O scheduler:
		- Writes starving reads
		- Requests have expiration time
			+ 500 ms for read, 5 seconds for write
		- Extra queue for each, sorted by time
		- If a request expires in the extra queue, it is serviced
		- Generally services requests by expiration time
	The Anticipatory I/O Scheduler
		- Heavy write load may perform badly on Deadline scheduler
		- Waits a few ms after performing read before resuming write
			+ In case the application needs something else
		- Keeps track of process access patterns to determine wait
	Completely Fair Queuing I/O Scheduler
		- Each process has a queue
		- Merging is performed, across queues (duplicates, references)
		- Round-robin with selection of a few items from each queue
		- Generally works well with non-pathological cases
		- Intended for multimedia originally
		- Default I/O scheduler as of book publication
	Noop I/O Scheduler
		- Merge requests, but don't do ANY sorting
		- Simpler than the others
		- Intended for flash memory only (no head position)