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Arcom Embedded Linux Technical Manual File system layout

File system layout

The exact layout of the file system on an AEL system depends on the target board. In

general, the Flash includes one or more smaller partitions containing the boot loader,

along with one large partition that covers the rest of the device and contains the root (/)

file system. The partition sizes are determined from RedBoot's Flash Image System

(FIS) and can be changed from RedBoot's command line.

In addition to the Flash file systems, a RAM-based file system is mounted on /var/tmp.

Journaling Flash file system

The Flash is formatted using the Journaling Flash File System (JFFS2). This places a

compressed file system onto the Flash transparently to the user. Key features of JFFS2

include:

Direct targeting of Flash devices. •

•

Robustness.

Consistency across power failure.

No integrity scan (fsck) is required at boot time after normal or abnormal shutdown.

Explicit wear levelling.

Transparent compression.

Flash partitions appear as pseudo-block devices with major number 31, which can be

mounted using JFFS2, as follows:

# mount –t jffs2 /dev/mtdblock1 mount-point

There are a maximum of 16 partitions. These are numbered 0 to 15, and correspond to

the block devices /dev/mtdblock0 through /dev/mtdblock15. In addition, each partition

has a character device, /dev/mtdN. This is used to access advanced features of the

Flash device, such as sector locking.

Any empty (erased) Flash partition can be mounted as a JFFS2 filesystem. No special

utility is required to format the device.

Simply erase the whole of the partition using

eraseall –j, and mount as normal.

The –j option causes an empty JFFS2 filesystem to

be created rather than completely erasing the flash device. Using this option can

optimize the first mount.

JFFS2 partitions do not require an integrity check (fsck) to be performed on startup,

after either normal or abnormal shutdown. The supplied /sbin/fsck.jffs2 is a dummy

which always succeeds and is present to simplify the boot scripts.

Although JFFS2 is a journaling file system, this does not preclude the loss of data. The

file system remains in a consistent state across power failure and is always mountable.

However, if the board is powered down during a write, the incomplete write is rolled-

back on the next boot. Any completed writes are not affected.

For more information about JFFS2, see sources.redhat.com/jffs2

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Eurotech ZEUS Manual de usuario Pagina 11