http://www.linux-mtd.infradead.org/doc/ubi.html#L_subpage
NAND flash sub-pages
As it is said here, all UBI I/O should be done in fractions of min. I/O unit size, which is equivalent to NAND page size in case of NAND flash. However, some SLC NAND flashes allow for smaller I/O units, which are called sub-pages in MTD terminology. Not all NANDs have sub-pages.
- MLC NANDs do not have sub-pages, at least to the date of writing of this piece of documentation (April 2009).
- SLC NANDs usually do have sub-pages. E.g., 512-byte NAND pages usually consist of 2x256-byte sub-pages, and 2048-byte NAND pages consist of 4x512-byte sub-pages.
- SLC OneNAND chips with 2048 bytes NAND page size have 4x512-byte sub-pages.
If the NAND flash supports sub-pages, then what can be done is ECC codes can be calculated on per-sub-page basis, instead of per-NAND page basis. In this case it becomes possible to read and write sub-pages independently.
But obviously, even though the NAND chip may support sub-pages, the NAND controller may disallow them. Indeed, if the flash is managed by a controller which calculates ECC codes on per-NAND page basis, then it is impossible to do I/O in sub-page fractions. E.g. this is the case for the OLPC XO-1 laptop) - its NAND chip supports sub-pages, but the NAND controller does not.
Note, sub-page is an MTD term, but this is also referred to as "NOP" which stands for "number of partial programs". NOP1 NAND flashes have no sub-pages - UBI treats them as NANDS with sub-page size equivalent to NAND page size. NOP2 NAND flashes have 2 sub-pages (half a NAND page each), NOP4 flashes have 4 sub-pages (quarter of a NAND page each).
UBI utilizes sub-pages to lessen flash space overhead. The overhead is less if NAND flash supports sub-pages (see here). Indeed, let's consider a NAND flash with 128KiB eraseblocks and 2048-byte pages. If it does not have sub-pages, UBI puts the the VID header at physical offset 2048, so LEB size becomes 124KiB (128KiB minus one NAND page which stores the EC header and minus another NAND page which stores the VID header. In opposite, if the NAND flash does have sub-pages, UBI puts the VID header at physical offset 512 (the second sub-page), so LEB size becomes 126KiB (128KiB minus one NAND page which is used for storing both UBI headers). See this section for more information about where the UBI headers are stored.
Sub-pages are used by UBI only internally, and only for storing the headers. UBI API does not allow users doing I/O in sub-page units. One of the reasons for this is that sub-page writes may be slow. To write a sub-page, the driver may actually write whole NAND page, but put 0xFF bytes to the sub-pages which are not relevant to this operation. E.g., this means that writing 4 sub-pages may be 4 times slower than writing whole NAND page at once. Thus, UBI does use sub-pages for the headers, but this notion does not exist in the UBI API.