Ceph RADOS linearizable?

[Ralph Boehme]

Hi David,

On 3/8/23 18:47, David Disseldorp wrote:
> This a question better suited to the Ceph development list, but I'll do
> my best to try to answer...

you're right. Sorry for being lazy and trying to shortcut... :)))

>> Can you confirm whether RADOS is indeed Linearizabile? I'm pretty sure
>> it is, but would like to be sure. :)
> 
> RADOS is a very broad interface when considering linearizability, but
> if you choose to focus on key/value storage accessed via the Ceph omap
> interface, then yes, my understanding is that OSD requests for a single
> object are processed in a way that provides atomic consistency from a
> RADOS client perspective.

well, atomicity is one point related to the single operations or 
transactions, consistency is a broader concept dealing with the ordering 
and relation between different operations.

With linearizable consistency, which is the strongest consistency you 
can get with single-key operations, you're guaranteed that operations 
appear in an order consistent with the real-time ordering of those 
operations. Which as another way to say that for

Time 1: Client 1: set A to X
Time 2: Client 2: get A -> ?

with linearizable consistency it's guaranteed that client 2 reads "X". 
Which is not the case with weaker consistency levels where the client is 
allowed to see the old value (whatever that was).

Thanks!
-slow

-- 
Ralph Boehme, Samba Team                 https://samba.org/
SerNet Samba Team Lead      https://sernet.de/en/team-samba
SAMBA+, Samba packages                  https://samba.plus/

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