I have a requirement to create a new volume (it can be static) based on the size of the ubifs image (say rootfs.ubifs) which I am going to write into that volume. The aim is to create the volume with the minimum possible size required to write 'rootfs.ubifs' to that volume and boot the device from it.
Can somebody please help me in this regard?
The difference is the overhead of the UBI layer. This is documented as O in the web page or,
O - the overhead related to storing EC and VID headers in bytes, i.e. O = SP - SL.SP is a physical erase block size and SL is what UbiFs will get. Usually, it is the minimum page size times two. One for an EC and another for a VID; these are the two structures that UBI uses to manage the flash. Both are defined in ubi-media.h. EC is the ubi_ec_hdr structure and VID is the ubi_vid_hdr structure. The EC or erase count is written every time an erase block is erased and this is responsible for wear leveling.note The VID or volume id header allows UBI to support multiple volumes and provide the PEB to LEB (physical to logical erase block) management.
So for a 2k page NAND flash without sub-pages, it is 4k; if sub-pages are supported then it is possible to put both headers in the same page and only 2k is needed. If your flash page size differs, you just need to multiply by two without sub-pages and only add the page overhead if you have sub-pages. The overhead for NOR flash is 256 bytes as it doesn't have the idea of pages.
In order to create your rootfs.ubifs, you must have specified a logic erase block size (to mkfs.ubifs). The difference between logical erase block (LEB) and physical erase block (PEB) is just the overhead documented above. Multiply your rootfs.ubifs by PEB/LEB to get the minimum possible size for the UBI volume.
note: If an erase is interrupted (reset/power cycle) between the actual erase and the EC write, an average of all other erase blocks is used to set the erase count when UBI re-reads the ubi device.
