1. Linux Block I/O Layer Chris Gill, Brian Kocoloski
CSE 422S - Operating Systems Organization
Washington University in St. Louis
St. Louis, MO 63130

2. Character vs. Block Devices
Character Devices
Block Devices
SD card
keyboard
hard disk drive
serial port
USB drive
Data streams from a device sequentially
Data can be randomly accessed in an arbitrary order by software
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3. Character vs. Block Devices
Some input/output activities are stream-like
E.g., keyboard events are accessed one-at-a-time, at a fixed “position” (the head of the stream)
Such devices are called “character devices”
Others can access different locations (in a file)
E.g., data on a disk, in flash memory, etc.
Stored in sectors, accessed in blocks (of multiple sectors)
Such devices are called “block devices”
Block size is power-of-two multiple of sector size
May not be larger than physical memory page size
Further distinguished by how blocks are accessed
E.g., flash memory cards are truly random access
E.g., disks involve “seeking” from one position to another
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4. I/O scheduling for block devices
Performance comparison of most block devices with other hardware
Reading/writing a CPU register:
O(nanoseconds)
Access memory in CPU cache:
DRAM memory access:
O(microseconds)
Access to spinning hard drive:
O(milliseconds)
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5. Comparing orders of magnitude
Assume 1 nanosecond=1 minute
1 microsecond: almost 17 hours
1 millisecond: almost 2 years
Takeaway: block I/O operations are much slower than most other hardware the kernel accesses
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Sectors and Blocks
Hard disks typically have sectors, the size of which is specified by the device
File system code operates on units called blocks
Block size must be no smaller than sector size
Block size must be no larger than physical memory page size
Accessing disk sectors incurs several types of device-specific overhead
Seek time
Rotational latency
LKD pp. 291
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7. CSE 422S – Operating Systems Organization
I/O Scheduling
Way in in which I/O requests are handled matters
At least, requests for adjacent blocks should be merged (fewer accesses gives lower overhead)
Key ideas behind I/O scheduling:
Request merging
Request sorting
Read-vs-write priority
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Linux Elevator
Keeps sorted list of requests
i.e.; assume requests are issued for blocks 2, 5, 0, and 8
Maintains sorted order 0,2,5,8
Total seek time? =8
Total seek time if using FIFO?
=18
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9. Other I/O scheduling algs
deadline
Prioritize read over writes
Maintains three distinct request queues
Sorted: all requests, sorted by disk sector
Read FIFO: read requests, sorted by arrival time
Write FIFO: write requests, sorted by arrival time
LKD pp. 301
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10. Other I/O scheduling algs
anticipatory (as)
Builds on deadline
Tries to anticipate future read requests
After scheduling a read operation, wait for a little bit to see if another read comes
Assumes reads will come to nearby locations
Why?
LKD pp. 301
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11. Other I/O scheduling algs
Complete fair queuing (cfq)
Fundamentally different from other algs
Goal is per-process fairness
All processes have their own scheduling queues, sorted by disk sector
Noop
No read-write priority or sorting based on disk sectors
What is it used for? Block devices that can randomly access data just as easily as they can sequentially access data
Solid state drives
NVME flash drives
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12. Recap: I/O “Elevators”
For disks, sequencing of reads (especially) is also important
Like an “elevator” read position “seeks” along locations on disk
Deadline scheduler (deadline) avoids starvation but keeps throughput
Anticipatory scheduler (as) waits a little in case new request arrives
Completely fair I/O scheduler (cfq) uses per-process fair queuing (it’s also Linux’ default I/O scheduler)
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