Reply to post: 64 bits isn't close to enough

Hopping the flash stepping stones to DIMM future

Steve Chalmers

64 bits isn't close to enough

Today's high capacity SSD is about 16TB (2^44 bytes). Assuming perfect, dense use of address space with no set asides, a million of those drives would completely fill that address space. For a single system, again assuming perfect dense use of address space, I tend to agree that 64 bits will last a long time.

However, for shared memory semantic storage, it is not unreasonable to consider a modern Google class data center as having 100,000 servers and at least 200,000 disk drives. That means in a shared environment it is entirely possible to exhaust a 2^64 address space in a data center built by 2020. I haven't done the analysis myself, but would tend to point to at least a factor of 2^10 more (ie a 74-bit-ish address space) in any chip or fabric design intended for the 2020s.

Note that for fault containment reasons I doubt the entire storage fabric space will be mapped into any one CPU's address space: this address is not the size of a pointer or index register used by applications, it's the size of the address coming out of the MMU or similar mapping table between the CPU and the memory semantic fabric. (Some supercomputer app will prove me wrong here, but I still think there's no rush to re engineer CPUs the way we did from 32 to 64 bit pointers some decades ago.)


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