I've noticed that our Blocks HTML output cache regularly needs to be refreshed, every day even when no one has been working on the Backend of Magento?
Why would this cache become invalid so frequently when no one has been updating CMS pages, static blocks, products etc?
What happens when the cache is invalid? Does Magento go to the file system rather than the cache when a cache is invalid?
We're using redis Backend cache, I'm thinking that perhaps redis does not have enough memory allocated to it.
Could this have something to do with this bug regarding Blocks HTML output cache on 1.9 https://www.magentocommerce.com/bug-tracking/issue/index/id/870