Re: Storage LRU Expiration Age -- what does it mean and is it accurate?

On Wed, 9 Aug 2000, Shawn Barnhart wrote:

> What does the value "Storage LRU Expiration Age" represent (from
> cachemgr ?
>
> In my mind, it represents the age of the oldest object in the cache
> based upon current cache utilization. One thing that confuses me is
> that this value bounces a lot for me. I usually see it at .9 days, but
> I just checked it and with a five minute average of 11 http
> requests/second it just showed me a Storage LRU Expiration Age of 40
> days! It's not unusual to see it at .9 days and check it again 10
> minutes later and see it at 5 days.
>
> In my mind that means that this counter is either totally inaccurate or
> is based on such a short snapshot of cache activity that it might as
> well be totally inaccurate. Am I off base here?
>
> My cache size is pretty small -- 750MB -- due to lack of RAM. I'm
> planning on upgrading the hardware to much more ram and a seperate,
> softupdate-mounted disk for the cache. Would a small disk cache size
> have anything to do with this value? If the figure was at all accurate,
> I would expect that the LRU time would stay permanently at a low figure
> during the day.

I think the small cache size could cause the behaviour that you see.

  12.12. What is the ``Storage LRU Expiration Age?''

  For example:

          Storage LRU Expiration Age: 4.31 days

  The LRU expiration age is a dynamically-calculated value. Any objects
  which have not been accessed for this amount of time will be removed
  from the cache to make room for new, incoming objects. Another way of
  looking at this is that it would take your cache approximately this
  many days to go from empty to full at your current traffic levels.

  As your cache becomes more busy, the LRU age becomes lower so that
  more objects will be removed to make room for the new ones. Ideally,
  your cache will have an LRU age value in the range of at least 3 days.
  If the LRU age is lower than 3 days, then your cache is probably not
  big enough to handle the volume of requests it receives. By adding
  more disk space you could increase your cache hit ratio.

  The configuration parameter reference_age places an upper limit on
  your cache's LRU expiration age.

Duane W.
Received on Wed Aug 09 2000 - 23:35:03 MDT