Working with the Unix OS

Buffer Cache
13. BUFFER CACHE
The Buffer Cache
When a process wants to access data from a file, the kernel brings the data into main memory where the process
can examine it, alters it and requests the data to be saved in the filesystem.
The kernel attempts to minimize the frequency of disk access by keeping a pool of internal data buffers, called the
buffer cache, which contains the data in recently used disk blocks.
When reading data, if data is already in cache (pre-cache), the kernel does not have to read from disk. Otherwise
the kernel reads the data from disk and caches it.
When writing data, data is written to cache to minimize disk writes (delay-write).
Buffer Headers
A buffer consists of two parts:
- the memory array that contains data from the disk and
- the buffer header that identifies the buffer.
The buffer is an in-memory copy of the disk block. A disk block can never map into more than one buffer at a time.
The buffer header contains:
- device number - logical filesystem number
- block number - from the disk
- status
- locked/busy
- valid data
- delayed-write
- currently read/write buffer to disk
- waiting for buffer to free
- pointer to data array for the buffer
- pointer to next buffer on hash queue
- pointer to previous buffer on hash queue
- pointer to next buffer on free list
- pointer to previous buffer on free list
Structure of the Buffer Pool
The kernel caches data according to least recently used: i.e. it cannot use the buffer until all other buffers have been
used more recently.
The free list is a circular list
of buffers linked both ways,
which uses a dummy buffer
header.
Figure 63. Free List of Buffers
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