On Tue, Dec 11, 2012 at 5:17 PM, Evgeny Shishkin <itparan...@gmail.com>wrote:
> Actually most of low-end SSDs don't do write caching, they do not have > enough ram for that. > AIUI, *all* SSDs do write-caching of a sort: writes are actually flushed to the NAND media by erasing, and then overwriting the erased space, and erasing is done in fixed-size blocks, usually much larger than a filesystem's pages. The drive's controller accumulates writes in an on-board cache until it has an "erase block"'s worth of them, which are then flushed. From casual searching, a common erase block size is 256 kbytes, while filesystem-level pages are usually 4k. Most low-end (and even many mid-range) SSDs, including Sandforce-based drives, don't offer any form of protection (e.g., supercaps, as featured on the Intel 320 and 710-series drives) for the data in that write cache, however, which may be what you're thinking of. I wouldn't let one of those anywhere near one of my servers, unless it was a completely disposable, load-balanced slave, and probably not even then. rls -- :wq
