On Fri, Dec 2, 2016 at 4:34 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> On Fri, Dec 2, 2016 at 3:05 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>> On Fri, Dec 2, 2016 at 10:30 AM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>>> Hi Miklos,
>>>
>>> Wanted to give you a heads up on upcoming announcement for
>>> overlayfs snapshots POC, just to give you a sense of what's been
>>> driving my recent overlayfs development efforts
>>> and also because I am eager to share the news ;-).
>>
>> Okay, so this is sort of reverse-overlay. For this to be useful,
>> however, there would have to be a way to make the snapshot *after*
>> /mnt is mounted, no?
>
Yes and no.
First, there could be use cases where the *after* part is not
critical. For example, setup a snapshot mount over directory foo before
an app is going to make changes to directory foo.
This could be useful maybe for the pre install package snapshot.
Second, I do intend to support taking a snapshots on live mount,
which would useful for more use cases. And also taking a new snapshot
and rotating the old one.
That is going to be a lot more challenging then the existing POC
and will require among other thing to support freeze of the snapshot
mount.
> Okay, so doing multiple snapshots on the fly would work something like this:
>
> - there are SNAP{1..N} and a MASTER branch
> - view of snapshot(i) is an overlay of (from bottom up) MASTER;
> SNAP{N}; ...; SNAP{i}
>
> Modification of master would always go to the latest snapshot branch (SNAP{N}).
>
Yes, something like that for API, my initial thought is:
mount -t snapshot -o upperdir=/lower none /mnt
...take snapshot 0:
mkdir -p 0/upper 0/work 0/snapshot
mount -t overlay -o lowerdir=/lower,upperdir=0/upper,workdir=0/work
none 0/snapshot
mount -t snapshot -o remount,snapshot=0/snapshot none /mnt
And something along the same lines for rotating to a new 1/snapshot.
Once the active snapshot is 1/snapshot, to access the content of snapshot 0,
need to remount it as follows:
umount 0/snapshot
mount -t overlay -o lowerdir=0/upper:1/snapshot none 0/snapshot
> The big change to overlayfs is to allow splitting the bottom layer.
>
In my view, it is less important to rotate the snapshot view without umount,
because I see the snapshots as a means for restore and not a live mount
that must be kept mounted at all times.
I do need, however, as you can see to allow a single level of stacking
for overlayfs
over overlayfs as the underlying lowermost ro layer.
That is enough to serve any number of readonly snapshots, e.g:
mount -t overlay -o lowerdir=0/upper,1/upper,2/upper:3/snapshot none 0/snapshot
To that end, I posted a fix for lockdep annotation of stacked
overlayfs instances.
Going to send a v2 soon.
Cheers,
Amir.
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