Questions about SSDs, writing files

randian

One of the problems I have with flash thumb drives is that when you've been using them for a while, all the memory cells become "dirty". When that happens, writes to the drive become slow as molasses as the drive is forced to do reset-empty-cell-then-copy operations on every memory cell you write to. Since SSDs are also based on flash, do they also have this problem?

oakie

yes, but unlike USB drives, they have built-in memory controllers to direct data to cells in the most efficient way possible and slow the eventual degradation of individual cells called wear-leveling. plus, some operating systems or software built into the memory controller zeros out the memory to prevent this slowdown... it's called a TRIM command.



all current SSDs have memory controllers that enforce wear-leveling. Windows 7 supports TRIM. OSX does not support TRIM natively, but the Toshiba memory controllers used in their current MacBook Airs have a built-in trash control function that serves the same purpose as TRIM. all of these solutions prevent the type of slowdown you're referring to.

randian

I'm not talking about wear leveling, I'm talking about dirty cells. A flash cell starts out empty. Writing data to it is very, very fast. Writing new data, however, becomes slow because to do so you must first restore the cell to "empty" before writing new data to it. That is very slow. Disk controllers try to first direct your writes to pristine empty cells rather than do that. However, once all your cells have been made "dirty" you can't do that, even if they are nowhere near being worn out. One write is sufficient.

oakie

Quote:

Originally Posted by randian


I'm not talking about wear leveling, I'm talking about dirty cells. A flash cell starts out empty. Writing data to it is very, very fast. Writing new data, however, becomes slow because to do so you must first restore the cell to "empty" before writing new data to it. That is very slow. Disk controllers try to first direct your writes to pristine empty cells rather than do that. However, once all your cells have been made "dirty" you can't do that, even if they are nowhere near being worn out. One write is sufficient.

read my whole post. your concern is addressed by the TRIM command.

oakie

it's also referred to as "Garbage Collection"



but either way, as i said previously, TRIM is used to zero out cells that have been written to and flagged as "writeable," but not zeroed out, preventing the occurrence of having to write twice to a cell.