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Linux - Embedded & Single-board computer This forum is for the discussion of Linux on both embedded devices and single-board computers (such as the Raspberry Pi, BeagleBoard and PandaBoard). Discussions involving Arduino, plug computers and other micro-controller like devices are also welcome.

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Old 04-14-2009, 03:54 PM   #1
agkunkle
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File system selection for a volatile drive (ext2 or tmpfs?)


I have a new project that involves migrating an old software baseline to run embedded Linux v.2.6.28 on an Atmel AT91SAM9G20-based board.

One of the tasks we are starting with is determining a fast good file system candidate for an external memory buffer (located behind an FPGA off the main bus). We will eventually be writing a block device driver for it, so that will be the interface. Of course, this is all volatile memory, so journaling, etc, is unneeded. What are some good candidates for creating a file system on this area?

To get started, I am using a ramdisk device to simulate the forthcoming interface to the RAM buffer. I am playing around with ext2/ext3 to do some initial benchmarking and ext2 seems to be the winner right now in terms of raw speed. I am also testing formatting the ramdisk as a swap area, mounting a tmpfs, and then writing a large file to the tmpfs to start getting the swap cache busy. Does this seem like a reasonable setup? Are there settings in /proc/sys/vm that will help characterize the performance more accurately?

With that in place, the tmpfs file system does much better than ext2, but I want to make sure there are not other good candidates (xfs, jfs, reiserfs, etc?) that would be good to test out. Thanks in advance!
 
Old 04-18-2009, 12:19 PM   #2
ljkenny
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What kind of memory is it you're referring to?

At first glance, tmpfs or ramfs would be of the more viable options.
 
Old 04-18-2009, 12:35 PM   #3
agkunkle
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Its DDR2 memory (about 128MB of it).

Once we get around to developing the block driver for it, it will be accesed through something like /dev/externram.


How would ramfs be an option for this, given how the device won't be part of the actual "system" memory? We are really looking for the most lean FS for this, given the volatility.


Thanks for the reply.
 
Old 04-18-2009, 12:43 PM   #4
ljkenny
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I'm not entirely sure what it is you're trying to do.

So you have access to an FPGA via your processor, which in turn has access to ~128MB of it's own memory.

Something like this?:
http://www.alchemydevices.ru/images/...P110%20Fig.jpg

And you wish to run a file system in it?
Attached Images
File Type: jpg xEMP110%20Fig.jpg (96.5 KB, 0 views)
 
Old 04-18-2009, 12:55 PM   #5
ljkenny
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After further investigation, it appears that the Virtual File System (VFS) may well be the key.

Read THIS from section 5 onwards.

How you implement such a thing on an FPGA and it's associtated memory is beyond my current understanding, but it gives you somewhere to start looking.
 
Old 04-18-2009, 01:00 PM   #6
agkunkle
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Yes, that picture is a more-or-less accurate picture of our setup. Some configurable portion of the 128MB FPGA RAM will be exposed to the kernel through a block device driver.

So basically, imagine you have a fast, volatile "hard drive" you want to throw a file system on top of, what do you choose to maximize the performance, given we don't care about journaling, etc.?
 
  


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