Gentoo Archives: gentoo-embedded

From: Joakim Tjernlund <joakim.tjernlund@×××××××××.se>
To: gentoo-embedded@l.g.o
Subject: Re: [gentoo-embedded] quickpkg and PKG_INSTALL_MASK
Date: Thu, 26 Jan 2012 12:10:00
In Reply to: Re: [gentoo-embedded] quickpkg and PKG_INSTALL_MASK by Ed W
Ed W <lists@××××××××××.com> wrote on 2012/01/25 14:21:25:

> > On 24/01/2012 14:06, Todd Goodman wrote: > > * Joakim Tjernlund<joakim.tjernlund@×××××××××.se> [120124 02:18]: > >> Ed W<lists@××××××××××.com> 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 > > good. > > Agreed. Just to augment that explanation:
Sorry for the late reply, got sidetracked with high prio stuff.
> > - 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!
When you update the RO layer you will be back to a single copy, the previous release which is only in the overlay is gone, right? It also implies 2 different SW update methods, one for updating the overlay copy and one for updating the RO layer. For development it works well though.
> > 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?
Dunno, I haven't tried any overlay FS yet.
> > > 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?
AFAIK, only FSes which are designed for flash(JFFS2, UBIFS, YAFFS etc. ) are safe w.r.t power cuts.
> > 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...)
hmm, DM snapshots might be something. Have to look closer on that.
> > > > > 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?
We are not using NAND flash yet but our next product will. I do have the impression that any block emulating device such as SSD are unreliable w.r.t power cuts. I would love to be proven wrong though :) Jocke


Subject Author
Re: [gentoo-embedded] quickpkg and PKG_INSTALL_MASK Todd Goodman <tsg@×××××××××.net>
Re: [gentoo-embedded] quickpkg and PKG_INSTALL_MASK Peter Stuge <peter@×××××.se>