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@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 -- regards, Mulyadi Santosa Freelance Linux trainer and consultant blog: the-hydra.blogspot.com training: mulyaditraining.blogspot.com