Re: Fwd: Re: Buffers VS Page Cache
On Sun, 2011-02-13 at 15:21 -0600, Boyd Stephen Smith Jr. wrote:
> In <[🔎] 4D58173B.7050005@gmail.com>, George Shuklin wrote:
> >What data stored in Buffers area, and what data a stored in Cached area?
> >And what difference between them?
> >
> >If you know at least one article in kernels sources/Documentation
> >folder, or anywhere in Internet, I'll read it gladly.
>
> http://www.linuxhowtos.org/System/Linux%20Memory%20Management.htm
>
> First hit when doing this search, here:
> http://www.google.com/search?q=what+is+the+difference+between+buffers+and+cache+in+linux+kernel+memory+reports&ie=UTF-8&oe=UTF-8
>
> It has a section called "The difference between buffers and cache".
>
> So, searching the web is a good start but, I can't say I know that there is
> specific documentation about this. My understanding is: "cache" is RAM that
> exactly mirrors a part of the block device, or at least a whole page in size.
> "cache" is used for satisfying read requests to that part of the block
> device, instead of doing real I/O. When a write occurs that that part of the
> disk the cache is invalidated and the RAM is scheduled for reuse. "cache"
> can be dropped at any time with just a performance hit. "buffers" can be any
> size (not necessarily a multiple of the page size) and are used mainly for
> write operations. Instead of waiting for I/O to finish before returning from
> a system call, data is stored in a "buffer". Once the "buffer" is confirmed
> as written to disk, the RAM is scheduled for reuse. "buffers" can't be
> dropped until their I/O completes. It seems that buffers can turn into cache
> and vice-versa for certain use cases, but I don't know if the kernel actually
> does that.
>
> Both types of memory are effectively free. Cache can be dropped willy-nilly.
> Buffers can be reclaimed by waiting for their I/O to complete. Both of these
> actions are done automatically by the kernel when application memory pressure
> increases.
That's a very helpful description; I never thought of dividing it as
read versus write. I do not know the details by any means and this may
be a holdover from my long ago CNE days but isn't there much more in
buffers than just disk writes, e.g., network I/O is also buffered in
memory I Thought. Thanks - John
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