Buffer

Buffer cache

• When a process wants to access data from a file, the kernel brings the data into main memory, alters it and then request to save in the file system
• Example: copy cp one.c two.c
• To increase the response time and throughput, the kernel minimizes the frequency of disk access by keeping a pool of internal data buffer called buffer cache.
• Buffer cache contains the data in recently used disk blocks
• When reading data from disk, the kernel attempts to read from buffer cache.
• If data is already in the buffer cache, the kernel does not need to read from disk
• If data is not in the buffer cache, the kernel reads the data from disk and cache it

Buffer header

• A buffer consists of two parts:
  • A memory array
  • Buffer header
• Device num
  • Logical file system number
• Block num
  • Block number of the data on disk
• Status
  • Buffer is currently locked
  • Buffer contains valid data
  • Delayed-write
  • Kernel is reading/writing from/to disk
  • A process is currently waiting for the buffer to become free.
  • Kernel identifies the buffer content by examining device num and block num.