LWN article: Log-structured file systems: There’s one in every SSD
Yesterday I came across this excellent article on log-structured file systems and their implementation on SSD technologies. It is worth the read.
2. October 2009
Opinion: On pramfs and RAM based Linux file systems
A few days ago I received the latest issue of Linux Journal Magazine. I must admit that one of the sections I look forward to reading is diff -u. This section summarizes the latest updates and discussions of the Linux kernel development community. It becomes much easier to read a summary as opposed to signing up for the mailing list because you will just get bombarded with e-mails which can be overwhelming the majority of the time.
While reading I came across a Montavista developed project called pramfs. In summary pramfs is a non-volatile RAM based file system, similar to your ramfs and tmpfs with a few differences to distinguish it from the others and in turn adapted for an embedded environment. Two obvious differences are that it is persistent like a traditional disk-based file system and does not reside in volatile DRAM. Pramfs is not new. It was originally announced back in 2004. It is designed to be a simplified file system that does not carry the same weight of the journal-based file systems.
Apparently there had been some problems with the patch being merged into the Linux kernel for a number of reasons. (1) Montavista was attempting to patent some of the concepts and algorithms used in the file system (in 2004) and (2) even after the dropped the idea of patenting their code, there was some discussion on the redundancy of having yet another file system implemented into the Linux kernel (in 2009). What that means, is that the Linux kernel already has two commonly used RAM file systems and a large number of other file systems. So why was there a need to write another one? Why couldn’t Montavista patch already existing code? (3) It is also not a full featured file system in that it does not support symbolic links.
I agree with this logic. Please do not misunderstand me. Montavista is a very respectable company that has done an excellent job in supporting embedded Linux. I am also glad to see them contribute to the kernel and in turn the community. But truth be told, tmpfs was build on top of the ramfs code. Why couldn’t pramfs follow the same course of development. The GPL makes it easy to not have to re-invent the wheel.
The two most noteworthy goals achieved for pramfs (1) is to work with NVRAM and (2) provide and interface that does not utilize the kernel page caching mechanism. By utilizing the DIRECTIO flag available in the 2.6 kernel, Montavista claims that I/O performance is increased significantly to an already high performing interface. Pramfs also allows the user to specify regions of memory for file system usage.
mount -t pramfs -o physaddr=0x1e000000,init=0x2000000 none /mnt/pramfs |
With it working in non-volatile memory, the data contents will remain intact even after an expected/unexpected power cycle.
This concept got me thinking a bit. How difficult would it be to add some of these features in Ramfs? Ramfs offer some similar functionality as in it does not use the kernel’s page cache for file I/O. Tmpfs was designed to offer that functionality along with additional file system control and limitations. Ramfs also has a slightly similar general file system layout. Sure a few structures and routines need to be redefined but that isn’t a big deal in the grand scheme of things.
I mention this in the light of some of the latest headlines circulating through the internet regarding Linux Torvalds’ comments on the kernel being bloated. Does the kernel leave room for additional “bloat” or would it be wiser to add on top of current features/functionality? I would love to read some of your opinions.
For more blog posts relating to RAM-based file systems and RAM Disk device drivers, you can find them posted here, here and here.
