[dm-devel] why "real" device and "snapshot-origin" differs after a snapshot is created

[lu ping]

Hi Folks,

I am a dm newbie and I am not sure if it's the right place to ask my
question. But I couldn't find any other related mail list...

After I create a LVM snapshot, there are four related devices. According to
the documentation, there is a "real" device which contains the original
mapping table of the source, while a "snapshot-origin" device gets the
original device number.

As to my understanding, when you read from "snapshot-origin", it will just
pass the read to the "real" device, while when you write to it, it does the
COW to the "COW device" and then again pass the writes to the "real" device.
But after I wrote to "snapshot-origin", I found it's content no longer the
same as the "real" device (from a "diff"). I must be missing something
important here. Can somebody explain to me why?

Thanks
Ping
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