Re: [PATCH 00/14] Pramfs: Persistent and protected ram filesystem

From: Marco
Date: Sun Jun 14 2009 - 12:09:17 EST

Jamie Lokier wrote:
> Marco wrote:
>> Simply because the ramdisk was not designed to work in a persistent
>> environment.
> One thing with persistent RAM disks is you _really_ want it to be
> robust if the system crashes for any reason while it is being
> modified. The last thing you want is to reboot, and find various
> directories containing configuration files or application files have
> been corrupted or disappeared as a side effect of writing something else.
> That's one of the advantages of using a log-structured filesystem such
> as Nilfs, JFFS2, Logfs, UBIFS, Btrfs, ext3, reiserfs, XFS or JFS on a
> ramdisk :-)
> Does PRAMFS have this kind of robustness?

There's the checksum, but the most important feature of this fs is the
write protection. 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 protection against
filesystem corruption caused by errant writes into the RAM due to
kernel bugs for instance. I provided a test module for this. When the
module is loaded tries to do a dirty write in the superblock, at this
point you should see an error on the write.

>> In addition this kind of filesystem has been designed to work not
>> only with classic ram. You can think at the situation where you have
>> got an external SRAM with a battery for example. With it you can
>> "remap" in an easy way the SRAM. Moreover there's the issue of
>> memory protection that this filesystem takes care. > Why is an
>> entire filesystem needed, instead of simply a block driver > if the
>> ramdisk driver cannot be used? > >From documentation: "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.
> All true, I agree. RAM-based databases use different structures to
> disk-based databases for the same reasons.
> Isn't there any good RAM-based filesystem already? Some of the flash
> filesystems and Nilfs seem promising, using fake MTD with a small
> erase size. All are robust on crashes.

Good question. The only similar thing that I know it's a patch called
pmem provided by WindRiver. It's the main reason that I led to develop
this kind of fs. In addition in my projects to have this feature has
always been very useful.

>> 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."
> There are other ways to include the O_DIRECT flag automatically. A
> generic mount option would be enough. I've seen other OSes with such
> an option. That code for that would be tiny.
> But standard O_DIRECT direct I/O doesn't work for all applications: it
> has to be aligned: device offset, application memory address and size
> all have to be aligned.
> (It would be a nice touch to produce a generic mount option
> o_direct_when_possible, which turns on direct I/O but permits
> unaligned I/O. That could be used with all applications.)
> As you say PRAMFS can work with special SRAMs needing memory
> protection (and maybe cache coherence?), if you mmap() a file does it
> need to use the page cache then? If so, do you have issues with
> coherency between mmap() and direct read/write?

See my response above about my concept of protection. However the mmap
it's a similar approach. I can "mmap" the SRAM and I can write into it
my data, but I think the possibility to have a fs it's great. We can use
the device as normal disk, i.e. we can use cp, mv and so on.

>> On this point I'd like to hear other embedded guys.
> As one, I'd like to say if it can checksum the RAM at boot as well,
> then I might like to use a small one in ordinary SRAM (at a fixed
> reserved address) for those occasions when a reboot happens
> (intentional or not) and I'd like to pass a little data to the next
> running kernel about why the reboot happened, without touching flash
> every time.
> -- Jamie

Yeah Jamie, the goal of this fs is exactly that!

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