Log-structured file systems: There's one in every SSD

Posted Sep 21, 2009 9:10 UTC (Mon) by nix (subscriber, #2304)
In reply to: Log-structured file systems: There's one in every SSD by dlang
Parent article: Log-structured file systems: There's one in every SSD

on modern systems, it's impossible to directly access a particular byte of ram. the memory chips actually act more like tape drives, it takes a significant amount of time to get to the start position, then it's cheap to do sequential read/writes from that point forward. your cpu uses this to treat your ram as if it was a tape device with blocks the size of your cpu cache lines (64-256 bytes each)

That's almost entirely inaccurate, I'm afraid. Ulrich Drepper's article on memory puts it better, in section 2.2.1.

The memory is necessarily read in units of cachelines, and it takes a significant amount of time to load uncached data from main memory, and of course it takes time to latch RAS and CAS, but main memory itself has a jagged access pattern, with additional delays from precharging and so on whenever RAS has to change.

But that doesn't make it like a tape drive, it's still random-access: it takes the same time to jump one row forwards as to jump fifty backwards. It's just that the units of this random access are very strange, given that they're dependent on the physical layout of memory in the machine (not machine words and possibly not cachelines), and are shielded from you by multiple layers of caching.