Controls whether Squid should perform paranoid validation of cache entry
metadata integrity every time a cache entry is hit. This low-level
validation should always succeed. Each failed validation results in a
cache miss, a BUG line reported to cache.log, and the invalid entry
marked as unusable (and eventually purged from the cache).
Squid can only validate shared cache memory and rock cache_dir entries.
* Zero (default) value means that the validation is disabled.
* Positive values enable validation:
- values less than 1 day approximate the maximum time that Squid is allowed
to spend validating a single cache hit.
- values greater or equal to 1 day are considered as no limitation:
in this case all checks will be performed, regardless of how much time
they take.
Hits are usually stored using 16KB slots (for rock, the size is
configurable via cache_dir slot-size). Larger hits require scanning more
slots and, hence, take more time. When validation is enabled, at least one
slot is always validated, regardless of the configured time limit.
A worker process validating an entry cannot do anything else (i.e. the
validation is blocking). The validation overhead is environment dependent,
but developers have observed Squid spending 3-10 microseconds to check each
slot of a Rock or shared memory hit entry. If Squid cuts validation short
because it runs out of configured time, it treats the entry as valid.
When hit validation is enabled, its statistics is included in Cache
Manager mgr:counters, mgr:5min, and mgr:60min reports.