On 24/01/2012 14:06, Todd Goodman wrote:
> * Joakim Tjernlund<joakim.tjernlund@...> [120124 02:18]:
>> Ed W<lists@...> wrote on 2012/01/23 19:43:49:
> [ .. ]
>>> I'm doing something like this using aufs. The performance seems "not
>>> bad", but you get a couple MB or so memory hit. (I'm using squashfs as
>>> well, so unsure which causes the main memory increase).
>> hmm, not sure how aufs would work out. One would like to permanently delete the
> aufs is A Union FileSystem. You can have a RO "branch" (say SquashFS)
> and a r/w "branch" (say JFFS2 or UBIFS.) When you write it will write
> to the r/w branch and when you read it will read from the r/w branch if
> it exists, else the r/o.
> So, if you want to permanently delete the old software. Write it to the
> filesystem r/w filesystem (initially or after you have the aufs mounts
> mounted.) Then you can delete from the r/w filesystem and it's gone for
Agreed. Just to augment that explanation:
- The RO branch would be at the bottom (can be multiple of these)
- The RW branch goes over the top (*can* be multiple of these, but one
is more normal)
- You can effectively delete stuff from the RO branches because Aufs has
the concept of "white out" files. So for all intents and purposes the
top RW layer can create any end result you like, including that of
completely masking out some lower layer
- With some knowledge of how the whiteout files work you can also "undo"
changes to the RO files. Eg directly mounting the RW layer and erasing
all files (from the RW layer) leaves you back with just the stacked RO
layers again. I find this helpful for development where I can basically
work live on the last released build and then by inspection the RW layer
has all the changes needed to apply to the next RO layer release!
I believe recent kernels also have a much simpler "Overlay Filesystem"
that has fewer features. Also the big alternative to aufs is Unionfs 2
- most distros use aufs, but both seem viable?
Finally note that you don't need to use aufs for the entire filesystem.
A common setup might be to use a bunch of bind mounts where you know you
don't need overlay features, eg /usr might be a bunch of overlays, /home
might be a bind mount to writeable storage, /var might be a ram drive
which is initialised from some fixed template, etc? In my case I have
an overlay over most things, but /usr/lib/modules is a bind mount to a
RO filesystem (you can't write to it), /home is mounted to my writeable
storage (not layered), the main OS dirs are layered and /var is a mess...
> You still want to pick a r/w branch with a filesystem that handles power
> cuts well. You can continue to use JFFS2.
Thought: Is there any evidence that one modern filesystem is better than
another with regards to sudden power removal? You probably need to
speak to filesystem experts at this point and define the kind of thing
you are trying to protect against? Sounds like you have raw flash
storage here, so that constrains your choices somewhat?
Just note that with aufs you can use quite a few filesystems for the
different layers. So for example you could have a base RW layer which
is a DM snapshot, overlaid with a loopback mount to a DVD iso, overlay
that with a squashfs, and finally overlay an Ext4 RW mount... (And of
course each of the RO layers might be stored on varied filesystems
themselves - check distributions such as Slax which allow you to overlay
a squashfs that is itself inside some loopback mounted file...)
I believe for most cheapo consumer flash storage where the underlying
flash filesystem isn't exposed, its quite susceptable to *complete*
failure with sudden poweroff? The issue is the invisible, underlying
flash filesystem gets corrupted during a partial write and that can be
the end of your flash drive - you don't even get to see it again to
recover from it... I don't believe partitioning protects you from this,
but of course separating read/write concerns to physically separate
devices would help? I presume this isn't what you are using though?