Many embedded systems have a block of non-volatile RAM seperate from normal system memory, i.e. of which the kernel maintains no memory page descriptors. For such systems it would be beneficial to mount a fast read/write filesystem over this "I/O memory", for storing frequently accessed data that must survive system reboots and power cycles. An example usage might be system logs under /var/log, or a user address book in a cell phone or PDA.

Currently Linux has no support for a persistent, non-volatile RAM-based filesystem, persistent meaning the filesystem survives a system reboot or power cycle intact. The existing RAM-based filesystems such as tmpfs and ramfs have no actual backing store but exist entirely in the page and buffer caches, hence the filesystem disappears after a system reboot or power cycle.

A relatively straight-forward solution is to write a simple block driver for the non-volatile RAM, and mount over it any disk-based filesystem such as ext2/ext3, reiserfs, etc.

But the disk-based fs over non-volatile RAM block driver approach has some drawbacks:

1. Disk-based filesystems such as ext2/ext3 were designed for optimum performance on spinning disk media, so they implement features such as block groups, which attempts to group inode data into a contiguous set of data blocks to minimize disk seeking when accessing files. For RAM there is no such concern; a file's data blocks can be scattered throughout the media with no access speed penalty at all. So block groups in a filesystem mounted over RAM just adds unnecessary complexity. A better approach is to use a filesystem specifically tailored to RAM media which does away with these disk-based features. This increases the efficient use of space on the media, i.e. more space is dedicated to actual file data storage and less to meta-data needed to maintain that file data.

2. If the backing-store RAM is comparable in access speed to system memory, there's really no point in caching the file I/O data in the page cache. Better to move file data directly between the user buffers and the backing store RAM, i.e. use direct I/O. This prevents the unnecessary populating of the page cache with dirty pages. However direct I/O has to be enabled at every file open. To enable direct I/O at all times for all regular files requires either that applications be modified to include the O_DIRECT flag on all file opens, or that a new filesystem be used that always performs direct I/O by default.

The Persistent/Protected RAM Special Filesystem (PRAMFS) is a full-featured read/write filesystem that has been designed to address these issues. PRAMFS is targeted to fast I/O memory, and if the memory is non-volatile, the filesystem will be persistent.

In PRAMFS, direct I/O is enabled across all files in the filesystem, in other words the O_DIRECT flag is forced on every open of a PRAMFS file. Also, file I/O in the PRAMFS is always synchronous. There is no need to block the current process while the transfer to/from the PRAMFS is in progress, since one of the requirements of the PRAMFS is that the filesystem exist in fast RAM. So file I/O in PRAMFS is always direct, synchronous, and never blocks.

The data organization in PRAMFS can be thought of as an extremely simplified version of ext2, such that the ratio of data to meta-data is very high.

PRAMFS is also write protected. The page table entries that map the backing-store RAM are normally marked read-only. Write operations into the filesystem temporarily mark the affected pages as writeable, the write operation is carried out with locks held, and then the pte is marked read-only again. This feature provides some protection against filesystem corruption caused by errant writes into the RAM due to kernel bugs for instance. In case there are systems where the write protection is not possible (for instance the RAM cannot be mapped with page tables), this feature can be disabled with the CONFIG_PRAMFS_NOWP config option.

In summary, PRAMFS is a light-weight, full-featured, and space-efficient special filesystem that is ideal for systems with a block of fast non-volatile RAM that need to access data on it using a standard filesytem interface.

Among the present and planned activities for PRAMFS are:

• Execute in Place (XIP) Support.