Re: Request: I/O request recording

[Andrew Morton <akpm@osdl.org>]

Bart Samwel <bart@samwel.tk> wrote:
>
> When I saw this thread I've fiddled for a bit with the block_dump
>  functionality that's in the laptop_mode patch. I wanted to see if it
>  could support a similar thing completely from user space (except for the
>  block_dump code, of course). I've written a small tool to generate a
>  complete file that lists tuples (sector, size, device) from the kernel
>  output in syslog; it parses all "READ block xxx" messages since the
>  last reboot. Putting this through sort -n -u delivers a nicely sorted
>  file, ready for optimized reading.
> 
>  Unfortunately I'm now stuck within the other part, which is reading the
>  pages back in memory at the next boot. It's not working, and I was 
>  hoping someone here could take a look and tell me what I'm doing wrong.

Linux caches disk data on a per-file basis.  So if you preload pagecache
via the /dev/hda1 "file", that is of no benefit to the /etc/passwd file. 
Each one has its own unique pagecache.  When reading pages for /etc/passwd
we don't go looking for the same disk blocks in the cache of /dev/hda1.

Which is why the userspace cache preloading needs to know the pathnames of
all the relevant files - it needs to open and read each one, applying
knowledge of disk layout while doing it.