On my eZ installation I have separated cache directories for my user siteacces and my admin siteaccess.
Don't!
The caches are stored in the var-dir, and when you are publishing something from the admin interface, it does not clear it correctly if you are using different var dirs.
The rule is: If you are using the same database in two siteaccesses, the var-dirs must be the same.
Edit: Why would you like have the var-dirs separated?
I have one sitteaccess for the eZ admin interface, one siteaccess for the public, and one siteaccess for testing and doing part of developement.
So when I'm testing I have to clear cache from time to time (which can be very often), but I want only to clear the cache for the test siteaccess and not for the public. That's why my admin and test siteaccess have to have the same CacheDir folder which is different from cache dir for user siteaccess.
Is there any way to tell eZ for which siteaccess to clear the cache-block cache when publishing an object?
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Nothing is impossible. Not if you can imagine it!