What is the difference between buffer and cache memory in Linux

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To me it s not clear what s the difference between the two Linux memory concepts   buffer and cache  I ve read through this post and it seems to me  that the difference between them is the expiration policy   buffer s policy is first-in  first-out cache s policy is Least Recently Used   Am I right  In particular  I m looking at the two commands  free and vmstat james utopia    vmstat -S M procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu---- r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa 5  0      0    173     67    912    0    0    19    59   75 1087 24  4 71  1 james utopia    free -m              total       used       free     shared    buffers     cached Mem           2007       1834        172          0         67        914 -   buffers cache         853       1153 Swap          2859          0       2859

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It s not  quite  as simple as this  but it might help understand   Buffer is for storing file metadata  permissions  location  etc   Every memory page is kept track of here   Cache is for storing actual file contents

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Buffers are associated with a specific block device  and cover caching   of filesystem metadata as well as tracking in-flight pages  The cache   only contains parked file data  That is  the buffers remember what s   in directories  what file permissions are  and keep track of what   memory is being written from or read to for a particular block device    The cache only contains the contents of the files themselves    quote link

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Explained by Red Hat  Cache Pages  A cache is the part of the memory which transparently stores data so that future requests for that data can be served faster  This memory is utilized by the kernel to cache disk data and improve i o performance  The Linux kernel is built in such a way that it will use as much RAM as it can to cache information from your local and remote filesystems and disks  As the time passes over various reads and writes are performed on the system  kernel tries to keep data stored in the memory for the various processes which are running on the system or the data that of relevant processes which would be used in the near future  The cache is not reclaimed at the time when process get stop exit  however when the other processes requires more memory then the free available memory  kernel will run heuristics to reclaim the memory by storing the cache data and allocating that memory to new process  When any kind of file data is requested then the kernel will look for a copy of the part of the file the user is acting on  and  if no such copy exists  it will allocate one new page of cache memory and fill it with the appropriate contents read out from the disk  The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere in the disk  When some data is requested  the cache is first checked to see whether it contains that data  The data can be retrieved more quickly from the cache than from its source origin  SysV shared memory segments are also accounted as a cache  though they do not represent any data on the disks  One can check the size of the shared memory segments using ipcs -m command and checking the bytes column  Buffers  Buffers are the disk block representation of the data that is stored under the page caches  Buffers contains the metadata of the files data which resides under the page cache  Example  When there is a request of any data which is present in the page cache  first the kernel checks the data in the buffers which contain the metadata which points to the actual files data contained in the page caches  Once from the metadata the actual block address of the file is known  it is picked up by the kernel for processing

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Buffers  represent how much portion of RAM is dedicated to cache disk blocks   Cached  is similar like  Buffers   only this time it caches pages from file reading   quote from     https   web archive org web 20110207101856 http   www linuxforums org articles using-top-more-efficiently 89 html

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Quote from the book  Introduction to Information Retrieval  Cache     We want to keep as much data as possible in memory  especially those data that we need to access frequently  We call the technique of keeping frequently used disk data in main memory caching    Buffer     Operating systems generally read and write entire blocks  Thus  reading a single byte from disk can take as much time as reading the entire block  Block sizes of 8  16  32  and 64 kilobytes  KB  are common  We call the part of main memory where a block being read or written is stored a buffer

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Buffer is an area of memory used to temporarily store data while it s being moved from one place to another  Cache is a temporary storage area used to store frequently accessed data for rapid access  Once the data is stored in the cache  future use can be done by accessing the cached copy rather than re-fetching the original data  so that the average access time is shorter  Note  buffer and cache can be allocated on disk as well

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I think this page will help understanding the difference between buffer and cache deeply  http   www tldp org LDP sag html buffer-cache html  Reading from a disk is very slow compared to accessing  real  memory  In addition  it is common to read the same part of a disk several times during relatively short periods of time  For example  one might first read an e-mail message  then read the letter into an editor when replying to it  then make the mail program read it again when copying it to a folder  Or  consider how often the command ls might be run on a system with many users  By reading the information from disk only once and then keeping it in memory until no longer needed  one can speed up all but the first read  This is called disk buffering  and the memory used for the purpose is called the buffer cache   Since memory is  unfortunately  a finite  nay  scarce resource  the buffer cache usually cannot be big enough  it can t hold all the data one ever wants to use   When the cache fills up  the data that has been unused for the longest time is discarded and the memory thus freed is used for the new data   Disk buffering works for writes as well     On the one hand  data that is written is often soon read again  e g   a source code file is saved to a file  then read by the compiler   so putting data that is written in the cache is a good idea  On the other hand  by only putting the data into the cache  not writing it to disk at once  the program that writes runs quicker  The writes can then be done in the background  without slowing down the other programs

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Cache  This is a place acquired by kernel on physical RAM to store pages in caches  Now we need some sort of index to get the address of pages from caches  Here we need the buffer for page caches which keeps metadata of page cache

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buffer and cache    A buffer is something that has yet to be  written  to disk    A cache is something that has been  read  from the disk and stored for later use

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Buffer contains metadata which helps improve write performance  Cache contains the file content itself  sometimes yet to write to disk  which improves read performance

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Cited answer  for reference    Short answer  Cached is the size of the page cache  Buffers is the size of in-memory block I O buffers  Cached matters  Buffers is largely irrelevant  Long answer  Cached is the size of the Linux page cache  minus the memory in the swap cache  which is represented by SwapCached  thus the total page cache size is Cached   SwapCached   Linux performs all file I O through the page cache  Writes are implemented as simply marking as dirty the corresponding pages in the page cache  the flusher threads then periodically write back to disk any dirty pages  Reads are implemented by returning the data from the page cache  if the data is not yet in the cache  it is first populated  On a modern Linux system  Cached can easily be several gigabytes  It will shrink only in response to memory pressure  The system will purge the page cache along with swapping data out to disk to make available more memory as needed  Buffers are in-memory block I O buffers  They are relatively short-lived  Prior to Linux kernel version 2 4  Linux had separate page and buffer caches  Since 2 4  the page and buffer cache are unified and Buffers is raw disk blocks not represented in the page cache   i e   not file data  The Buffers metric is thus of minimal importance  On most systems  Buffers is often only tens of megabytes

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Seth Robertson s Link 2 said  quot For thorough understanding of those terms  refer to Linux kernel book like Linux Kernel Development by Robert M  Love  quot  I found some contents about  buffer  in the 2nd edition of the book   Although the physical device itself is addressable at the sector level  the kernel performs all disk operations in terms of blocks  When a block is stored in memory  say  after a read or pending a write   it is stored in a  buffer   Each  buffer  is associated with exactly one block  The  buffer  serves as the object that represents a disk block in memory  A  buffer  is the in-memory representation of a single physical disk block  Block I O operations manipulate a single disk block at a time  A common block I O operation is reading and writing inodes  The kernel provides the bread   function to perform a low-level read of a single block from disk  Via  buffers   disk blocks are mapped to their associated in-memory pages   quot

[linux] What is the difference between buffer and cache memory in Linux?

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