Re: Request: I/O request recording

[Bart Samwel <bart@samwel.tk>]

Andrew Morton wrote:
> 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.

Hmmm, that explains why this didn't work. :( So if I wanted to do this 
completely from user space using only block_dump data I'd probably have 
to go through all files and find out if they had any blocks in common 
with my preload set -- presuming there is a way to find that out, which 
there probably isn't. That  makes this idea pretty much useless, I'm 
sorry to have bothered you with it.

-- Bart