Hi!
> However, where cramfs shines is: _no_ fragmentation. Forget about block
> device issues, it does data on 4 byte boundaries. That, together with
> basically having very minimalistic meta-data (who needs meta-data anyway,
> when it's all read-only: _just_ enough to find stuff and no more) is the
> biggest win.
>
> But you can't basically do these things if you want to be read-write. A
> truly log-based approach (ie not just meta-data journalling) might work
> out ok, actually, but most log-based stuff seem to want to have fairly
> large caches in order to work well. Which in embedded spaces isn't exactly
> a good idea.
>
> The large caches are needed because log-based filesystems very quickly
> tend to fragment files all over hell-and-gone. But if you're using
> flash memory in your embedded device, this is much less of an issue,
> since you aren't impacted by the seek times that you have when you have
> to move heads over spinning media.
>
> The real trick is being able to allocate on non-block boundaries, and
> dealing with fragmentation issues as you delete files and create
> irregularly shaped holes. Making a read/write filesystem that is
> optimized for the characteristics of flash memory would certainly be
> "interesting".
>
> One potential problem with log-based schemes is that they tend to
> rewrite many more blocks (for example, normally you have to rewrite
> every single directory up to the root every time you so much as touch an
> inode to update the atime). For flash memory, this is non-optimal since
> you have a limited number of write cycles. Although modern flash
> memories they've extended the number of write cycles significantly, it's
> still an issue.
>
> Of course, if you don't care about backing store, and want a pure
> memory-based compressed ram disk, life is much easier --- but writing to
> it is much less interesting, since it won't survive a reboot.
Tell me about surviving reboots... 80% linuxce machines have _no_
permanent storage. My velo 1 has 12Meg RAM, 8MB rom and no flash. This
is pretty typical. RAM has two baterries (main and backup), that's how
wince survives on it.
Pavel
-- I'm pavel@ucw.cz. "In my country we have almost anarchy and I don't care." Panos Katsaloulis describing me w.r.t. patents at discuss@linmodems.org- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.rutgers.edu Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Sun Jul 23 2000 - 21:00:19 EST