Roland Paterson-Jones wrote:
So, where is the overlay mapping stored? I believe that this is known
as the 'exception store', and I can see some stuff about it in the
source, but where does a persistent overlay map end up?
Also, how is the overlay (snapshot) device used? Is there a one-to-one
mapping of (logical device) block offsets onto the overlay (snapshot)
device? If so, where does the overlay map fit in?
OK, so nobody wants to talk to me :( I'll answer myself, just in case
anyone is interested.
It seems that the exception store is maintained in a header at the start
of the overlay device, and that CoW writes are done in chunks mapped
from the header. There's an in-memory cache of one 'map' area to avoid
going to the device for the mapping for every request. So, it seems to
be for now that:
1. There's no linearity between device offsets on the logical
snapshot device, and chunk offsets on the (real) overlay device. It
simply depends on access patterns. I'm guessing that this could cause
odd head thrashing in obscure cases.
2. To support the full logical device extent, the real overlay
device has to have enough space for the header plus the device extent.
How big is the header?
Lastly, how do I flush the overlay mapping? Suppose I have had quite
enough of the (modified) snapshot and I want to revert to the original
origin in a snapshot using the same overlay (snapshot) device. I
notice empirically that if I 'dmsetup remove' then 'dmsetup create ...
n ...', then 'dmsetup remove ...' once more, and finally 'dmsetup
create ... p ...', then I have a clean snapshot of the origin again.
I'm guessing there's a better way ;)
Writing over the header at the start of the overlay device does the
trick. I'm not sure that the above will work in all cases?\
I'm quite keen on a dm-cow plugin that maintains sector offsets (well
linearity at least) between the origin and the CoW overlay for
consistent performance. I'd be happy in this case to put the CoW mapping
on a separate (small) device, or to put it at the front of the overlay
and use a linear bump for all sectors.
The current snapshot facility seems oriented towards taking a small
snapshot of a big origin, where all the subsequent action is on the
origin. My use case is a sparse origin that I want to CoW over, where
all the action is in the CoW overlay.
The only previous CoW endeavour I can find is the dm-userspace one. I
don't really have a need to poke through into user-space tho.
Has anyone else persued a basic dm-cow like this. Otherwise I might just
have to write some code =8-O.
Regards
Roland
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