Re: difference between -c and -p for send-receive?

[Andrei Borzenkov <arvidjaar@gmail.com>]

19.09.2017 03:41, Dave пишет:
> new subject for new question
> 
> On Mon, Sep 18, 2017 at 1:37 PM, Andrei Borzenkov <arvidjaar@gmail.com> wrote:
> 
>>>> What scenarios can lead to "ERROR: parent determination failed"?
>>>
>>> The man page for btrfs-send is reasonably clear on the requirements
>>> btrfs imposes. If you want to use incremental sends (i.e. the -c or -p
>>> options) then the specified snapshots must exist on both the source and
>>> destination. If you don't have a suitable existing snapshot then don't
>>> use -c or -p and just do a full send.
>>>
>>
>> Well, I do not immediately see why -c must imply incremental send. We
>> want to reduce amount of data that is transferred, so reuse data from
>> existing snapshots, but it is really orthogonal to whether we send full
>> subvolume or just changes since another snapshot.
>>
> 
> Starting months ago when I began using btrfs serious, I have been
> reading, rereading and trying to understand this:
> 
> FAQ - btrfs Wiki
> https://btrfs.wiki.kernel.org/index.php/FAQ#What_is_the_difference_between_-c_and_-p_in_send.3F
> 

This wiki entry is wrong (and as long as I can believe git, it has
always been wrong).

First, "btrfs send -c" does not start with blank subvolume; it starts
with "best parent" which is determined automatically. Actually if you
look at the help output in the very first version of send command:

        "By default, this will send the whole subvolume. To do",
        "an incremental send, one or multiple '-i <clone_source>'",
        "arguments have to be specified. A 'clone source' is",
        "a subvolume that is known to exist on the receiving",
        "side in exactly the same state as on the sending side.\n",
        "Normally, a good snapshot parent is searched automatically",
        "in the list of 'clone sources'. To override this, use",
        "'-p <parent>' to manually specify a snapshot parent.",

it explains fat better what -c and -p do (ignore -i, this is error that
was fixed later, it means -c).

Second, example in wiki simply does not work. All snapshots listed in -c
options and snapshot that we want to transfer must have the same parent
uuid, unless -p is explicitly provided. Example shows snapshots of two
different subvolumes. I could not make it work even if A and B
themselves are cloned from common subvolume.