On Monday 05:13 PM 8/18/2008, Jeff Rogers wrote:
Simply put, fastpath caching is inherently broken because it's not
possible to guarantee that the file in question really should be served
from cache (again, short of a cache-defeating checksum).
The same can be said about nearly any caching system: it is unable to
handle changes in the data that happen outside of the cache's control or
knowledge. This is just the bargain you make when you use a cache.
I'd say "nearly any" is going too far, and in fact I'd say that for most
caching systems to fail to return the correct data is a serious bug. The
NFS example you bring up isn't really analogous since it's only about
attributes, which are frequently not a concern; were NFS to return
incorrect *data* for a file, that would be a serious bug. And in this
case we're talking about a web server that may silently return data that's
completely incorrect, which I'd say is very serious misbehavior.
The core problem here is that AOLserver is attempting to use the tuple of
(dev, inode, mtime, size) as a unique determiner of a file's identity, and
that's an inherently broken assumption--particularly so since the
granularity of mtime is one second and inodes are reused on many
filesystems (e.g. very common ones like ext3 and ufs).
I think you highlighting it is most of the fix. From there, get the
caveat inserted into the documentation and the knowledge into the
community so that the next person who runs into this problem will have an
easier, or at least less frustrating time solving it.
That'd be an improvement over the current situation, but it's still the
case that AOLserver as currently shipped has a file cache mechanism built
into it which 1) may return incorrect data and 2) is enabled by
default. Given the risk, I'd say fastpath caching should be disabled by
default rather than enabled.
- John
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