buffer page concepts in the page cache

Mulyadi Santosa mulyadi.santosa at gmail.com
Tue Jan 25 13:19:52 EST 2011

Hi Miguel...

Tough questions, let's see if I can made it :D

On Tue, Jan 25, 2011 at 19:56, Miguel Telleria de Esteban
<miguel at mtelleria.com> wrote:
> MY INTERPRETATION (please correct me if I am wrong)
> Q1  What is a "buffer page"?
> A "buffer page" is a "struct page" data describing a page allocated to
> hold one or more i/o blocks from disk.

I agree...in other word, they are pages that hold data when the I/O
are still in flight. But since it's part of page cache, they aren't
thrown away after the I/O is done...for few moment they are held in
RAM, in case they're subsequently read...thus, I/O frequency toward
physical discs are reduced

I think, we know call it page cache....

> Q2  Is the whole page cache content organized as buffer pages?
> YES, there is no other way to link memory-mapped disk i/o data to the
> struct page pointed by address_space radix-tree entries.

Not so sure, but it's something like that IMHO.

> ---
> Q3  block device buffer_pages vs file buffer_pages
> This I really don't understand.  From what UTLK page 614 says:
> *  File buffer_pages ONLY refer to non-contiguous (on disk layout) file
>   contents.
> *  blockdev buffer_pages refer to single-block or continuous (on disk
>   layout) portions of block.
> My question is:  what happens with non-fragmented medium size files
> that do not contain "disk holes" or non-adjancent block submissions?

Here's my understanding:
1. when you're dealing with file in raw, e.g using "dd" on /dev/sda1
or "dd" with direct I/O command, you use block buffer cache
2. when you deal with files using read()/write facility of filesystem
(thus via VFS), you use file page cache...

to experiment with it, simply start "top" and examine which field
increases when you do "dd", cat, etc....

I hope I help you instead confusing you :D


Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

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