On Wed, Oct 23, 2024 at 5:44 AM Maged Mokhtar <mmokhtar@petasan.org> wrote:
This is tricky but i think you are correct, B and C will keep the new object copy and not revert to old object version (or not revert to object removal if it had not existed). To be 100% sure you have to dig into code to verify this.
My understanding..what is guaranteed is: 1) On success ack to client: obviously the last version of data written is what will be read back. 2) On either success or failure: data is consistent, if you repeat reads you will get the same data even if the acting set changes. 3) rados objects are transactional at the OSD level, they are either updated to new version or reverted to old version on failure at OSD level. Objects cannot be corrupt half way due to write failure.
I think 2) is good enough. Ideally in case of client ack failure you may want the old object version to be reverted back or the object removed in case it had not existed , this is the condition you are asking for B and C to revert.This would be a more complex requirement than 2) and may not be supported or practical to support. I think if clients want to guarantee data is reverted on failure, clients need to add transactional code themselves, like database journaling...etc. This is at least with block and filesystem clients. These clients would typically have such transactional code anyway to handle physical hardware failures..etc.
Even if rados did provide this extra transactional level and had B and C do revert back to old object version, this could still corrupt client data if client write spanned to 2 rados objects, one was success and one failed. Even though both rados objects are consistent, client data is corrupt as some of it is new and some old and so transactional code at the client will always be needed to revert data on failures.
All of this is transparent to RADOS users. Written objects will generally remain written, and librados will retry operations (and get the same response on retry as they should have gotten on first submit) without returning errors to the caller. The only way you can get the torn write you describe here is if the client had simultaneous outstanding writes and itself fails while they are in-progress — and you can see this same torn write on a local FS too if there is a crash while simultaneous writes are in progress. -Greg
/maged
On 23/10/2024 06:54, Vigneshwar S wrote:
Hi Frédéric,
5a section states that that the divergent events would be tracked and deleted. But in the scenario I’ve mentioned, both B and C will have same history.
So if the new primary is B, then when peering, it peers with C and looks at the history and thinks they’ve no divergent events and keep the object right?
Regards, Vigneshwar
On Wed, 23 Oct 2024 at 9:09 AM, Frédéric Nass < frederic.nass@univ-lorraine.fr> wrote:
Hi Vigneshwar,
You might want to check '5a' section from the peering process documentation [1].
Regards, Frédéric.
[1]
https://docs.ceph.com/en/reef/dev/peering/#description-of-the-peering-proces...
------------------------------ *De :* Vigneshwar S <svigneshj@gmail.com> *Envoyé :* mardi 22 octobre 2024 11:05 *À :* ceph-users@ceph.io *Objet :* [ceph-users] How Ceph cleans stale object on primary OSD failure?
Hi everyone, I'm studying Ceph replication and peering and have a doubt
in
how Ceph handles cleaning of stale objects on primary OSD failure.
Scenario:
Consider a placement group: PG1, whose acting set is [A, B, C] and A is the primary.
t1: Client writes an object to A t2: A initiates local write t3: A sends replication request to B t4: A sends replication request to C t5: A completes local write t6: A gets replication response from B t7: A gets replication response from C t8: A is crashed and haven't sent acknowledgement to the client
In case if A never comes back, the object written in B and C would be stale as it is not acknowledged to the client. How does Ceph cleans this stale object?
Also, it would be helpful if you can share any documentation or articles explaining Ceph replication and peering in low-level.
Regards, Vigneshwar. _______________________________________________ ceph-users mailing list -- ceph-users@ceph.io To unsubscribe send an email to ceph-users-leave@ceph.io
_______________________________________________ ceph-users mailing list -- ceph-users@ceph.io To unsubscribe send an email to ceph-users-leave@ceph.io
ceph-users mailing list -- ceph-users@ceph.io To unsubscribe send an email to ceph-users-leave@ceph.io