Solid-state storage devices and the block layer
Posted Oct 5, 2010 20:38 UTC (Tue) by jzbiciak
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In reply to: Solid-state storage devices and the block layer
Parent article: Solid-state storage devices and the block layer
You can do random writes to random empty sectors. Again, that's nothing like how a hard disk works. I'm still strenuously disagreeing with your earlier statement that flash's properties make it more like a disk than like RAM. It's really an entirely different beast worthy of separate consideration, which is why I think wrapping it up in an SSD limits its potential.
With flash, you need entirely new strategies that apply neither to disks nor RAM to get the full benefit from the technology. Much of the effort spent on disks revolves (no pun intended) around eliminating seeks. No such effort is required with RAM or with flash. Flash does require you to think about how you pool your free sectors, though, and how you schedule writing versus erasing. I won't deny that. Rather, I say it only further invalidates your original conjecture that it makes flash more like disks. (I will agree it makes it less like RAM though.)
Because seeks are "free", I could totally see load balancing algorithms of the form "write this block to the youngest free sector on the first available flash device", so that a new write doesn't get held up by devices busy with block erases. That looks nothing like what you'd want to do with a disk. It takes advantage of the "free seek" property of the flash while helping to hide the block erase penalty it imposes. Neither property is a property of a disk drive. Of course, neither property is a property of RAM, either.
Am I splitting hairs over semantics here? Let me step back and summarize, and see if you agree: Raw flash's random access capability and relatively low access time can make it much more like RAM than disk, especially in terms of bandwidth and latency. Raw flash's limitations on writes, however, require the OS to have flash-specific write strategies. They prevent the OS from treating flash identically to RAM, and will require careful thought to be handled correctly. This is similar to how we had to put careful thought into disk scheduling algorithms, even if flash requires entirely different algorithms to address its unique properties.
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