![]() For SquashFS image the splitter also automatically adds rootfs_data to the list of the available mtd partitions, setting this partition's beginning to the first appropriate address after the SquashFS end and size to the remainder of the rootfs partition. For example if SquashFS image is found then the rootfs partition is added. The size and number of the “children” depends on what is detected. New partitions are usually some offset into the start of the original partition. Each partition type (usually determined from name) has its own list of parsers. The most common splitting here is kernel, followed by padding, followed by SquashFS root filesystem, followed by padding, followed by free space.ĭuring splitting, the kernel walks the erase blocks and detects magic bytes via parsers. The utility program mtd can be used to manage MTD devices.ĬONFIG_MTD_SPLIT_FIRMWARE can be used to control whether attempt is made on firmware partition. MTD devices support 3 main operations - read from some offset within an erase block, write to some offset within an erase-block, and erase a whole erase-block. In case of “raw flash”, the operating system should deal with such cases. In case of “FTL flash”, the controller should notice and avoid further access to bad erase-blocks. Erase-blocks do become worn out after some number of erase cycles – typically 100K-1G for SLC NAND and NOR flashes, and 1K-10K for MLC NAND flashes. The storage space of a MTD is split up into “erase-blocks”, of a size of e.g 64 KiB, 128 KiB or much more, which themselves are split up into “blocks”, which are more correctly named “pages”, of smaller sizes.Ī single “page” can be written to, but it cannot be overwritten, but instead the entire “erase block” that page is part of, has to be erased before it becomes possible to re-write its “pages”. When a block device is partitioned, the information is stored in the MBR or the GPT.įlash memory using MTD is different from this. Block devices support 2 main operations - read a whole block and write a whole block. In the very rare case this happens, the LBA hard disk controller takes care, that accesses to such a bad block are redirected to a replacement block. Blocks do not get corrupted during common operation, but only exceptionally. On a common block device such as a hard drive, the storage space is split up into “blocks”, which are also named “sectors”, of a size of 512 Bytes or 4096 Bytes. An MTD is different to a block device or a character device. The Linux kernel treats “raw/host-managed” flash memory (NOR and NAND alike) as an MTD (Memory Technology Device). "$x" done' - + Example 2: Hoo Too HT-TM02 #!/bin/sh # shows all overlay-whiteout symlinks # 2018: overlay-whiteouts are a character device on CC 'find /overlay -type c' seems to work # put me on the right track find /overlay -type c find /overlay -type l -exec sh -c \ In newer versions, the whiteout is created as a character device with 0/0 device number instead. A whiteout is a symlink to (overlay-whiteout) that mostly behaves like a file that doesn't exist. When the system is asked to delete a file that is in /rom, it instead creates a corresponding entry in /overlay, a whiteout. In this way /overlay overrides /rom and creates the effect of a writable / while much of the content is safely and efficiently stored in the read-only /rom. ![]() ![]() Whenever the system is asked to look for an existing file in /, it first looks in /overlay, and if not there, then in /rom. changed configuration files, additional packages installed with opkg, etc. It contains anything that was written to the router after installation, e.g. overlay is the writable part of the file system that gets merged with /rom to create a uniform /-tree. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |