Coda File System

Re: CODA kernel module limitations...

From: Roland Mainz <>
Date: Tue, 17 Oct 2000 18:39:51 +0200
Jan Harkes wrote:

> > Think about a ftp site (using CODA kernel-module and podfuk+ftpfs) which
> > has large INDEX files (~60MB). File manager likes to determinate the
> > datatype - reads the first two bytes - but triggers the download of
> > 60MB.
> > Quite efficient, isn't it ? ;-(((
> >
> > (Possible) solution:
> > Allow random-access (read/write etc.) to remote files.
> > What about an interface to the SYSV-VFS layer, e.g. moving more stuff
> > from kernel to userspace layer ?
> This would actually introduce far too much overhead in context switches
> between the application and the cache-manager. Besides, as far as Coda
> is concerned, it makes it impossible to guarantee consistency. However,
> there is another solution which we've been thinking of.
> When a large file is opened, CODA_OPEN could return early, f.i. when
> the first 8-64KB have been fetched. The kernel would get a `lease' on
> accessing these first pages (both read and write), while the cache
> manager pulls in the rest of the file.
> When the application accessing the file seeks or reads past it's `lease'
> it is blocked until the data is available, and the cachemanager has
> returned a new lease. However when the application is done and closes
> the file before everything has been fetched, the ongoing fetch can be
> aborted.
> In this model, we can keep streaming data into the container files as
> efficiently as possible, while at the same time allowing some early
> access to the containerfile. One of the big problems is that most
> applications don't handle read/write errors very well, so an interrupted
> transfer (disconnection) would lead to silently truncated files. Mostly
> due to user `error', when someone opens a file in an editor, makes some
> changes but doesn't notice the end of the file was lost and saves it back.

I was thinking about a completly different solution:
What about letting venus/podfuk decide at OPEN to download the complete
file to the local system (e.g. into the "cache") or handle each
operation (read/write/seek/locking etc.) manually and map them to the
remote file. This would give us the control whether file operations
should be executed on the "cached" file or on a remote file.
This would be a more universal approach as this also supports both your
and my idea, too (for example: venus decides at OPEN to access the
remote file readonly but triggers the download into the cache in
_parallel_. If download is complete it switches from remote to cached
And it would allow other projects to use the same interface, either
using cached operations, direct operations or both.



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Received on 2000-10-17 12:41:14