Re: [Xen-devel] Domain Save Image Format proposal (draft B)

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 11/02/14 16:38, Ian Jackson wrote:
> David Vrabel writes ("Domain Save Image Format proposal (draft B)"):
>> Records
>> =======
>>
>> A record has a record header, type specific data and a trailing
>> footer.  If body_length is not a multiple of 8, the body is padded
>> with zeroes to align the checksum field on an 8 octet boundary.
>>
>>      0     1     2     3     4     5     6     7 octet
>>     +-----------------------+-------------------------+
>>     | type                  | body_length             |
>>     +-----------+-----------+-------------------------+
>>     | options   | (reserved)                          |
>>     +-----------+-------------------------------------+
> ...
>> options      Bit 0: 0 - checksum invalid, 1 = checksum valid.
> There needs to be a flag saying what the receiver should do if it sees
> a record it doesn't understand.  There are two possible behaviours:
> ignore the record, and abandon the restore attempt.

No need.  Any unrecognised records can be safely ignored.  Any record
which couldn't be ignored would be required to bump the main stream
version number at which point the older reader would bail on that basis.

This would allow adding new backward compatible features without
breaking older systems, which is the way compatibility is maintained in
the current code.

~Andrew

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