Block I/O In Linux

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Block I/O In Linux

• Q36921115
• ???

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Block Device

• Block devices are devices in which data is
  organized as an array of fixed size chunks called
  'blocks'
• Data is read/written in terms of blocks
• Block is the basic unit of I/O operations.

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Devices Are Accessed In Three Ways

• The kernel needs to read the block.
• -example A file system needs to read the
  super block from the disk
• Read/Write to the device file
• - read/write system call on the device file
• The device file is mapped into memory
• - page fault or page flush on the mapped
  memory

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File Operation on Block Devices
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File Operation on Block Devices

• Because most of the behavior of file operations
  are the same ,linux provides default
  file_operations functions for common block
  devices.
• Different from character device drivers which
  implement file_operations, block device drivers
  implement block_device_operations

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Generic Block Device Layer

• Provides common functionality for all block
  devices in Linux
• Uniform interface (to file system)
• e.g. bread( ) block_prepare_write( )
• block_read_full_page( ) ll_rw_block( )
• buffer management and disk caching
• Block I/O requests scheduling
• Generates and queues actual I/O requests in a
  request queue (per device)
• Individual device driver services this queue
  (likely interrupt driven)

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Read Operation In Block Device
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Read Operation In Block Device

• submit_bh( )
• -make I/O request invoke generic_make_request
  ( )
• block_read_full_page( )
• - check whether the buffers in the page are up
  to date or not and invoke submit_bh( )
• bread( )
• - reads a specified block and returns the
  buffer head.

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Read Operation In Block Device

• ll_rw_block( )
• - read check whether the requested block in
  the
• buffer is up to date or not.
• - write check whether the requested block
• in the buffer is dirty or not.
• - invoke submit_bh( )

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Write Operation In Block Deviceprepare_write
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Write Operation In Block Devicecommit_write
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Write Operation In Block Device

• prepare_write
• - in fact, it reads block data from disk.
• - check whether the buffer in the memory is
  mapped to block in the disk. If not, allocate a
  new disk block for the file.
• - if write operation doesnt write the whole
  buffer, it invoke ll_rw_block to read content
  from disk.
• commit_write
• - set buffer head uptodate and dirty.
• - check whether the write operation enlarge the
  file size, update i_size in inode object
• - invoke balance_dirty( ) to keep dirty buffers
  in the system bound.

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I/O Request

• __make_request()
• - create bounce buffer ,if the buffer is in
  high memory
• - inserts the buffer in the request queue
• - plug and unplug
• - enlarger an existing request
• elevator algorithm

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I/O Request
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I/O Request
__make_request() inserts the buffer in the
request queue
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I/O Request

• Request queue
• Data structure in include/linux/blkdev.h
• Queue header type request_queue_t typedef
  structure request_queue request_queue_t
• queue_head double linked list of pending
  requests
• request_fn pointer to request service routine
• Queue element struct request
• cmd read or write
• Number of request sectors, segments
• bh, bhtail a list of buffer header
• Memory area (for I/O transfer)

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I/O Request
I/O request processing Plug and Unplug
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Block Device Drvier

• Implement device operation functions
• - open, release, ioctl, check_media_change,
• revalidate
• Implement a request service routine
• - request_fn_proc
• Write interrupt handler and related tasklets