UBI

UBI

Unsorted Block Images

  • http://www.linux-mtd.infradead.org/doc/ubi.html
  • Volume management system on top of MTD devices (similar to what LVM provides for block devices)
  • Allows to create multiple logical volumes and spread writes across all physical blocks
  • Takes care of managing the erase blocks and wear leveling. Makes filesystems easier to implement
  • Wear leveling can operate on the whole storage, not only on individual partitions (strong advantage)
  • Volumes can be dynamically resized or, on the opposite, can be read-only (static)

UBI: internals

UBI is storing its metadata in-band

  • In each MTD erase block
    • One page is reserved to count the number of erase cycles
    • Another page is reserved to attach the erase block to a UBI volume
    • The remaining pages are used to store payload data

  • If the device supports subpage write, the EC and VID headers can be stored on the same page

  • UBI is responsible for distributing writes all over the flash device: the more space you assign to a partition attached to the UBI layer the more efficient the wear leveling will be

  • If you need partitioning, use UBI volumes not MTD partitions
  • Some partitions will still have to be MTD partitions: e.g. the bootloaders and bootloader environments
  • If you need extra MTD partitions, try to group them at the end or the beginning of the flash device

UBI layout: bad example

UBI layout: good example

Linux: UBI host tools

  • ubinize is the only host tool for the UBI layer
  • Creates a UBI image to be flashed on an MTD partition
  • Takes the following arguments:
    • -o Path to the output image file
    • -p The PEB size (MTD erase block size)
    • -m The minimum write unit size (e.g. MTD write size)
    • -s Subpage size, only needed if both your flash and your flashcontroller are supporting subpage writes
    • The last argument is a path to a UBI image description file (see next page for an example)
  • Example: ubinize -o ubi.img -p 16KiB -m 512 -s 256 cfg.ini

ubinize configuration file

Can contain several sections

  • Each section is describing a UBI volume
  • Example:

[kernel-volume] mode=ubi image=zImage vol_id=1 vol_type=static vol_name=kernel

[rootfs-volume] mode=ubi image=rootfs.squashfs vol_id=2 vol_type=static vol_name=rootfs

[data-volume] mode=ubi image=data.ubifs vol_id=3 vol_size=30MiB vol_type=dynamic vol_name=data vol_flags=autoresize

U-Boot: UBI tools

Grouped under the ubi command

  • ubi part Attach an MTD partition to the UBI layer
  • ubi info [layout] Display UBI device information (or volume information if the layout string is passed
  • ubi check Check if a volume exists
  • ubi readvol [] Read volume contents
  • U-Boot also provides tools to update the UBI device contents
  • Using them is highly discouraged (the U-Boot UBI implementation is not entirely stable, and using commands that do not touch the UBI metadata is safer)
    • ubi createvol [] []
    • ubi removevol
    • ubi writevol

Linux: UBI target tools

Tools used on the target to dynamically create and modify UBI elements

  • UBI device management:
    • ubiformat Format an MTD partition and preserve Erase Counter information if any
    • ubiattach -m /dev/ubi_ctrl Attach an MTD partition/device to the UBI layer, and create a UBI device
    • ubidetach -m /dev/ubi_ctrl Detach an MTD partition/device from the UBI layer, and remove the associated UBI device

UBI volume management:

  • ubimkvol /dev/ubi -N -s Create a new volume. Use -m in place of -s if you want to assign all the remaining space to this volume.
  • ubirmvol /dev/ubi -N Delete a UBI volume
  • ubiupdatevol /dev/ubi_ [-s ] Update volume contents
  • ubirsvol /dev/ubi -N -s Resize a UBI volume
  • ubirename /dev/ubi_ Rename a UBI volume

Linux tools: BusyBox UBI limitations

Beware that the implementation of UBI commands in BusyBox is still incomplete. For example:

  • ubirsvol doesn't support -N . You have to use specify the volume to resize by its id (-n num): ubirsvol /dev/ubi0 -n 4 -s 64 MiB
  • Same constraint for ubirmvol: ubirmvol /dev/ubi0 -n 4

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