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Re: [Fedora-livecd-list] [PATCH] overlay/persistence second pass - for developer reference only



Jeremy Katz wrote:
On Mon, 2007-08-20 at 14:10 -0500, Douglas McClendon wrote:

Well, the way ubuntu is trying to do it of course, is with unionfs (since of course they use unionfs rather than dm-snapshot to get cow in the first place).

And as such, unionfs can provide just as persistful an implementation as the direction I've been going. In both cases you can think of the persistence as another embedded layer in the total root filesystem.

It's been quite a while since I looked at unionfs, but I vaguely
remember that it was more subtree overlays.  I guess you could perhaps
do a subtree of /.  But even so, I don't know that supporting multiple
ways of achieving the same goal is really where we want to go.  But it's
somewhat academic at the moment, so probably not much discussion needed.


I'm not sure if there is some meaning of subtree that is different than subdir. But the way most livecds work, is by having a big squashfs with your root filesystem (all of it, not seperated into subdirs or anything), and then having a tmpfs, and then using unionfs to make the tmpfs act as a layer over the squashfs, and then doing pivotroot to that single unionfs filesystem.

Kadischi used the method that was predominant before that unionfs method, which was to have many subdirs (/usr, /opt) be read only, and then have some subdirs (/tmp, /var, ...) be read only. Perhaps using bindmounting or symlinks to handle some specific sub-subdirs.

Back to unionfs- The major disadvantage of unionfs is that it is not 'perfect' as a real rootfs (why AFAIK fedora/rh refused to merge it). I.e. there are some known bugs, which knoppix and ubuntu just take as an acceptable tradeoff.

The major advantage of unionfs, for the specific persistence topic at hand, is that when you delete a file from the COW rootfs, in unionfs, the memory is actually freed. Whereas for the dm-snapshot implementation of persistence, that is not the case.

This may be acceptible. There may be workarounds for it (using shred to delete files into 0's, and then resparsifying the persistence overlay?)

Anyway...  yes, academic.

And unionfs can't get rebootless installation (bwa ha ha....)

-dmc



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