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

[ashok shankar das]

Douglas McClendon wrote:

> 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 problem is in symlinks(unionfs).  incase of devmaper this problem is 
not there. But there is another issue. The snapshot size. The method I 
follow is ofcourse Devmaper. But i tryed with fuse and funionfs but not 
tested vigourously.

> 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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-- 
Thanks
Ashok Shankar Das
RedHat, Pune
+91-9373695832