The reason I kind of combined prefetching and caching was more because I
meant "prefetching or caching".

So, does Venus attempt to "predict" subsequent file open()s and prefetch
files? As far as I understand your email, the answer is no.

I am attempting to work on a system that tries to predict filesystem events
based on prior experience and then use that the prefetch. As you mentioned,
directory wide prefetching is impractical a lot of the time, so, something
that was more clever might be helpful. Any feedback on this would be great.

Thanks for the reply - it was very helpful.


On Sat, Nov 24, 2012 at 6:34 PM, Greg Troxel <gdt_at_ir.bbn.com> wrote:

>
> Dhaivat Pandya <dhaivatpandya_at_gmail.com> writes:
>
> > Does Venus attempt to do any sort of predictive prefetching/caching aside
> > from the user specified HDB entries?
>
> Prefetching and caching are quite different.  Venus will cache files
> that it reads, either due to VFS requests or hoarding.  I am unaware of
> any prefetching.  For example, one could have venus read (into the
> cache) files in a directory if the directory is read.  That doesn't
> sound like a good plan, because a directory of large files is something
> that might be listed but not entirely traversed.
>
> Were I to investigate prefetching, I'd add:
>
>   1) stat files in a directory when the directory is read
>
>   2) if any of those files are directories, fault them into the cache
>   (but don't 1 until there's a real VFS call)
>
> Before doing this, I'd want to have some sort of priority queueing by
> request uid or reason, so that prefetches don't compete with real
> requests.  And probably analyze some traces, although user behavior
> adapts to FS behavior, so the actual question of what's best is far
> trickier than it might at first seem.
>