Thank you, Jeff, --
answer is comprehensive.
On Sat, Feb 04, 2006 at 07:26:57PM -0500, Josef Sipek wrote:
> 1) Don't touch the lower filesystems (prefered)
I see,
unionfs specification assumes writable-only union.
In such case, term "unionfs" is not accurately choosen,
it's too common for this project.
For instance, in SQL world, the term UNION is certain,
and literally mean union, without any assumption about
update permissions.
Extreme example is whole web:
it's the union, and mostly read-only, --
reader must not submit the news article first
just for read it ;)
> What do you mean by _properly_?
I mean keep dentry cache [almost] coherent
with physical filesystem metadata.
Apache designed to deal gently with content,
and read-only mounting of documents root is preferred use.
When we mount content to apache box from NFS server,
we just do it in read-only mode.
After that, web server will be secured against web clients attacks.
NFS client has the options to supress or reduce caching aggression:
-------- man 5 nfs -----------------------------------------------
acregmin=n The minimum time in seconds that attributes of a
regular file should be cached before requesting
fresh information from a server.
The default is 3 seconds.
...
noac Disable all forms of attribute caching entirely. ...
------------------------------------------------------------------
> There is really no sane way around it (at least for now).
Sure, timeouts tuning may sound insane,
but people agree to deal with this headache,
mostly because performance requirements are different
for sandbox and live content.
Other ways, such as updates advertisements, "redo-log", etc.
architectures would be more effective in performance,
but could be also too complicated in maintenance.
Back again,
> 1) Don't touch the lower filesystems (prefered)
With unionfs semantics and read-only union,
sandbox mounting is very easy, yet very powerful feature.
Updates are performed in delta branch, outside of apache box,
however sandbox' apache sees updates in the union
of live (bottom branch) and delta (top branch) content.
I understand that unionfs project's goals are different
then [simpler] read-only union support.
This is just not clearly expressed in manual, don't you agree?
--w
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