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Re: [dm-devel] [Lsf-pc] [LSF/MM TOPIC] a few storage topics



> From: Andrea Arcangeli [mailto:aarcange redhat com]
> Sent: January 25, 2012 5:46 PM

....

> Way more important is to have feedback on the readahead hits and be
> sure when readahead is raised to the maximum the hit rate is near 100%
> and fallback to lower readaheads if we don't get that hit rate. But
> that's not a VM problem and it's a readahead issue only.
> 

A quick google showed up - http://kerneltrap.org/node/6642 

Interesting thread to follow. I haven't looked further as to what was
merged and what wasn't.

A quote from the patch - " It works by peeking into the file cache and
check if there are any history pages present or accessed."
Now I don't understand anything about this but I would think digging the
file-cache isn't needed(?). So, yes, a simple RA hit-rate feedback could
be fine.

And 'maybe' for adaptive RA just increase the RA-blocks by '1'(or some
N) over period of time. No more smartness. A simple 10 line function is
easy to debug/maintain. That is, a scaled-down version of
ramp-up/ramp-down. Don't go crazy by ramping-up/down after every RA(like
SCSI LLDD madness). Wait for some event to happen.

I can see where Andrew Morton's concerns could be(just my
interpretation). We may not want to end up like a protocol state machine
code: tcp slow-start, then increase , then congestion, then let's
back-off. hmmm, slow-start is a problem for my business logic, so let's
speed-up slow-start ;).


Chetan


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