[squid-users] cacheNumbObject unreasonably small with rock cache

[Dave Dykstra]

Yes that helps a lot, Alex.  I do see at the end of mgr:info:

Internal Data Structures:
	   576 StoreEntries
	   576 StoreEntries with MemObjects
	557676 Hot Object Cache Items
	584657 on-disk objects

We'll work on a pull request to change that to on-disk objects for rock cache.

Dave

On Wed, Apr 02, 2025 at 11:21:15AM -0400, Alex Rousskov wrote:
> On 2025-04-02 10:45, Dave Dykstra wrote:
> 
> > We're trying rock cache for the first time, on squid 6.13.  The
> > machine is quite large and heavily used, with 10 workers configured,
> > 140G of shared memory cache, and 500G of rock cache configured.
> > However, the cacheNumObject SNMP counter is staying quite steadily at
> > around only 570 even as the cache grows.  Currently du shows the rock
> > file taking 40G, and the mgr:storedir statistics show 448k entries.
> 
> I assume you are asking about an SNMP measurement that Squid MIB file 
> calls cacheNumObjCount (node 1.3.6.1.4.1.3495.1.3.1).
> 
> Today, that measurement reports the number of StoreEntry objects 
> currently in use. You should see similar stats on the "StoreEntry" line 
> in "Internal Data Structures" section of mgr:info cache manager report.
> 
> Roughly speaking:
> 
> * For UFS-based caches and non-SMP memory caches, that number is often 
> (but not always!) close to the number of cached objects because these 
> caches create a StoreEntry object for every object stored in the cache. 
> That is how these stores index cached content.
> 
> * For rock caches and SMP memory caches, that number is usually very 
> different from the number of cached objects because these caches do not 
> create StoreEntry objects when indexing cached content.
> 
> * StoreEntry objects are not limited to caches. Many transactions, 
> including many transactions that do not retrieve cache hits and do not 
> store cache misses create StoreEntry objects. In related Squid 
> terminology, "Store" is not just "cache", but a more general medium for 
> passing HTTP responses around.
> 
> I have not checked all the necessary details, but I suspect that SNMP 
> code needs to be fixed to report the same stats that are currently 
> available on "on-disk objects" line in "Internal Data Structures" 
> section of mgr:info cache manager report.
> 
> I hope that mgr:storedir statistics that you have mentioned also 
> reflects the actual disk cache usage, at least for rock caches.
> 
> 
> HTH,
> 
> Alex.
> 
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