UBIFS initial experiments

Introduction

In this page I document my steps in creating a release 8.2 image running on top of UBIFS for the purpose of initial experimentation with that file system and point out some of my initial findings and questions that have popped up. If you are just interested in downloading and running the final, image jump down to Resulting Image.

Flash Layout

Open Firmware does not support reading from UBIFS so to deal with this, the flash is partitioned into a 32MiB JFFS2 partition with the remainder of the space left over for use by UBI. OFW loads the kernel and initrd from the JFFS2 partition and the initrd handles mounting the UBIFS. Note that one flash erase block dedicated to the RedBoot partition table ("FIS directory" in RedBoot speak).

MTD Partition	Location
FIS directory.	0x00000000-0x00020000 (128KiB)
boot	0x00020000-0x02020000 (32Mib)
system	0x02020000-0x3ffc0000 (991.785 MiB)

JFFS2 Partition

The JFFS partition simply contains a boot/ directory with three files: olpc.fth OFW boot script, vmlinuz compressed kernel binary, and olpcrd.img ramdisk image.

olpc.fth

The olpc.fth script simply had the following modification applied to pass the proper boot parameters to the kernel and initrd:

@@ -77,7 +77,7 @@
    then

    " nand"  dn-buf count  sindex  0>=   if
-      " root=mtd0 rootfstype=jffs2"
+      " ubi.mtd=system root=ubi0:rootfs rootfstype=ubifs"
    else
       " root=LABEL=OLPCRoot rootfstype=ext3"
    then

The parameter "ubi.mtd=system" tells the UBI layer to attach to the named MTD device. This creates a new UBI device, ubi0 which contains the root filesystem volume, called "rootfs" The "root=ubi:rootfs" option tells the kernel to mount this volume. (This parameter is actually unused as the initrd handles the mounting of the root filesystem).

olpcrd

The olpcrd is identical to that in the 8.2 release except for the following change to initutil.py:

@@ -134,11 +134,12 @@
                     # when partitioned, expect bootpath like:
                     # /pci/nandflash@c:root,\boot\vmlinuz//jffs2-file-system:\boot\vmlinuz
                     if p is None: p = 0 # unpartitioned by default
-                    if type(p) is int:
-                        dev = 'mtd%d' % p
-                    else:
-                        dev = 'mtd:%s' % p
-                    extra = ['-t','jffs2']
+                    # if type(p) is int:
+                    #    dev = 'mtd%d' % p
+                    # else:
+                    #    dev = 'mtd:%s' % p
+                   dev = 'ubi0:rootfs'
+                   extra = ['-t','ubifs']
             else: # we're running under emulation
                 # these modules only needed if we're running in qemu
                 from stat import S_IFBLK

Note that this is not a permanent solution as we probably want to handle booting the same release on both a JFFS2 and UBIFS layout.

Kernel

The kernel used is available at here. It is composed of the OLPC kernel used for the 8.2 release merged with the linux-2.6.25 UBI backport tree. The kernel is built with UBI and UBIFS built in statically as we are going to be moving away from modules for required features to speed up boot time (see LWN article).

UBI Partition

The UBI system partition contains a single UBI volume named rootfs that covers the full partition minus overhead due to UBI overhead.

The UBIFS image is created via the following command:

/usr/local/bin/mkfs.ubifs -m 2KiB -e 124KiB -x lzo -c 7849 -d system/ -o system_ubifs.img

Where:

-m 2KiB

The minimum I/O size of the underlying UBI and MTD devices. In our case, we are running the flash with no sub-page writes, so this is a 2KiB page.

-e 124KiB

Erase Block Size: UBI requires 2 minimum I/O units out of each Physical Erase Block (PEB) for overhead: 1 for maintaining erase count information, and 1 for maintaining the Volume ID information. The PEB size for the XO flash is 128KiB, so this leads to each Logical Erase Block (LEB) having 124KiB available for data.

-x lzo

Use LZO compression

-c 7849

The maximum size, in LEBs, of this file system. See calculation below for how this number is determined.

-d system

Use the contents of the system/ directory to generate the initial file system image. In this case the system directory is composed of the contents of the 767 JFFS image.

-o system_ubifs.img

Output file

The output of the above command, system_ubifs.img is fed into the ubinize program to wrap it into a UBI image:

/usr/local/bin/ubinize -o system_ubi.img -m 2KiB -p 128KiB -s 2KiB ubinize.cfg

Where:

-o system_ubi.img

Output file

-m 2KiB

Minimum flash I/O size of 2KiB page

-s 2KiB

Mininum I/O size used for UBI headers. Since we do not do sub-page writes, this is the same as -m

ubinize.cfg

Configuration file

The configuration file contents:

# Section header
[rootfs]
# Volume mode (other option is static)
mode=ubi
# Source image
image=system_ubifs.img
# Volume ID in UBI image
vol_id=0
# Volume size
vol_size=973312KiB
# Allow for dynamic resize
vol_type=dynamic
# Volume name
vol_name=rootfs
# Autoresize volume at first mount
vol_flags=autoresize

The UBIFS image starts out at 266MiB and the "autoresize" flag tells the kernel to expand the volume (and the filesystem above) to fill up all 973312KiB of the usable flash.

Usable Size Calculation

As documented here, UBI reserves a certain amount of space for management and bad PEB handling operations. Specifically:

• 2 PEBs are used to store the UBI volume table
• 1 PEB is reserved for wear-leveling purposes;
• 1 PEB is reserved for the atomic LEB change operation;
• a % of PEBs is reserved for handling bad EBs. The default for NAND is 1%
• UBI stores the erase counter (EC) and volume ID (VID) headers at the beginning of each PEB. 1 min I/O unit is required for each of these.

Symbol	Meaning	Value for XO test case
SP	PEB Size	128KiB
SL	LEB Size	128KiB - 2 * 2KiB = 124 KiB
P	Total number of PEBs on the MTD device	991.625MiB / 128KiB = 7933
B	Number of PEBs reserved for bad PEB handling	79(1%)
O	The overhead related to storing EC and VID headers in bytes, i.e. O = SP - SL	4KiB

UBI Overhead = (B + 4) * SP + O * (P - B - 4) 
             = (79 + 4) * 128Kib + 4 KiB * (7933 - 79 - 4)
             = 42024 KiB 
             = 329.3125 PEBs (round to 329)

This leaves us with 7604 PEBs or 973312KiB available for user data.

Note that I used "-c 7849" in the above mkfs.ubifs command line to specify the maximum filesystem size, not "-c 7604" The reason for this is that mkfs.ubifs operates in terms of LEB size (124 KiB), not PEB size (128Kib). 973312KiB / 124 Kib = 7849. I found this very confusing and it took a few re-readings of the examples to grok it. Note that in reality this number can be > 973312KiB as it only tells UBI/UBIFS the maximum volume size. If the file system is installed on a UBI volume smaller than this value, UBI will simply expand it to fit the volume. To support UBI root on both 1GiB and 4GiB devices, we simply need to create one UBI image that will resize automatically to the MTD device size.