Ceph RADOS linearizable?
slow at samba.org
Wed Mar 8 18:18:54 UTC 2023
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).
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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