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Several bugs and issues have come into view when using RAM-only and COSS Squid that are relevant to this.
Most of it is related to the way storage rebuild currently depends on a global store_rebuilding flag to allow other things in Squid to proceed.
The first issue is showing all signs of this rebuild flag not even working, first storage _type_ to complete rebuild unsets it regardless of other types being incomplete (COSS vs AUFS reload races; COSS looses on high-end boxes).
Several of those things waiting for a rebuild only need _a_ cache for objects to start running. The RAM-cache would be completely sufficient for the first few minutes while disk storage indexes get loaded.