On 1 jan 2010, at 18.17, Bob Friesenhahn wrote: > On Fri, 1 Jan 2010, David Magda wrote: >> >> It doesn't exist currently because of the behind-the-scenes re-mapping >> that's being done by the SSD's firmware. >> >> While arbitrary to some extent, and "actual" LBA would presumably the number >> of a particular cell in the SSD. > > There seems to be some severe misunderstanding of that a SSD is. This severe > misunderstanding leads one to assume that a SSD has a "native" blocksize. > SSDs (as used in computer drives) are comprised of many tens of FLASH memory > chips which can be layed out and mapped in whatever fashion the designers > choose to do. They could be mapped sequentially, in parallel, a combination > of the two, or perhaps even change behavior depending on use. Individual > FLASH devices usually have a much smaller page size than 4K. A 4K write > would likely be striped across several/many FLASH devices.
Yes, but erases are always much larger, right? (With the flash chips of today, I am not sure why there aren't any flash chips with smaller erase page sizes yet.) > The construction of any given SSD is typically a closely-held trade secret > and the vendor will not reveal how it is designed. You would have to chip > away the epoxy yourself and reverse-engineer in order to gain some > understanding of how a given SSD operates and even then it would be mostly > guesswork. > > It would be wrong for anyone here, including someone who has participated in > the design of an SSD, to claim that they know how a "SSD" will behave unless > they have access to the design of that particular SSD. I certainly agree, but there still isn't much they can do about the WORM-like properties of flash chips, were reading is pretty fast, writing is not to bad, but erasing is very slow and must be done in pretty large pages which also means that active data probably have to be copied around before an erase. I believe this is why even fast flash SSD devices can take tenth or even hundreds of thousands of writes for a short burst, but then fall back to a few thousand writes/second sustained. /ragge _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
