Coda File System

Re: Caching mechanism

From: <>
Date: Sun, 25 Nov 2012 18:18:45 +0100
Hello Dhaivat and Greg,

On Sat, Nov 24, 2012 at 07:34:45PM -0500, Greg Troxel wrote:
> 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

Right. Venus does not do anything like that. Unfortunately
some not-so-bright file managers do (e.g. under Windows
but Linux GUI file managers are known to do similar things).

This is a major PITA as the file manager tries to prepare the view for
the user in advance, no matter where the user would look from the current
point in the file system. I guess this is supposed to create a feeling
of the system being "fast". :-b

In reality this spectacularly fails on Coda as "opening" a file to peek
at it and even doing stat()s in advance can be a costly operation.

Look at codacon while trying to access a file in Windows - you will
see that Venus most probably will fetch attrs of all files in the same
directory and even in subdirectories (this can be a lot!). This takes
a lot of time and is not Venus fault or intention.

Prefetching without a detailed knowledge of the data expected
to be accessed in the future is a huge loss. The file system layer
does not prerequisites for doing a good guess. Even applications
usually do not know - otherwise they can easily let Venus prefetch
a file (open()-ing it in advance, asynchronously).

> 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.

My conclusion based on my experience (YMMV) is that lazy fetching
is a very efficient strategy. Given the "high cost" of accessing
a file without a reason, the less prefetching the better.

Application-level prefetching is a different story and _sometimes_
might make things better, but it is not a file system business.

Received on 2012-11-25 12:19:32