PG Backend Proposal

[Loic Dachary <loic@dachary.org>]

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Hi Sam,

When the acting set changes order two chunks for the same object may co-exist in the same placement group. The key should therefore also contain the chunk number. 

That's probably the most sensible comment I have so far. This document is immensely useful (even in its current state) because it shows me your perspective on the implementation. 

I'm puzzled by:

CEPH_OSD_OP_DELETE: The possibility of rolling back a delete requires that we retain the deleted object until all replicas have persisted the deletion event. ErasureCoded backend will therefore need to store objects with the version at which they were created included in the key provided to the filestore. Old versions of an object can be pruned when all replicas have committed up to the log event deleting the object.

because I don't understand why the version would be necessary. I thought that deleting an erasure coded object could be even easier than erasing a replicated object because it cannot be resurrected if enough chunks are lots, therefore you don't need to wait for ack from all OSDs in the up set. I'm obviously missing something.

I failed to understand how important the pg logs were to maintaining the consistency of the PG. For some reason I thought about them only in terms of being a light weight version of the operation logs. Adding a payload to the pg_log_entry ( i.e. APPEND size or attribute ) is a new idea for me and I would have never thought or dared think the logs could be extended in such a way. Given the recent problems with logs writes having a high impact on performances ( I'm referring to what forced you to introduce code to reduce the amount of logs being written to only those that have been changed instead of the complete logs ) I thought about the pg logs as something immutable.

I'm still trying to figure out how PGBackend::perform_write / read / try_rollback would fit in the current backfilling / write / read / scrubbing ... code path. 

https://github.com/athanatos/ceph/blob/ba5c97eda4fe72a25831031a2cffb226fed8d9b7/doc/dev/osd_internals/erasure_coding.rst
https://github.com/athanatos/ceph/blob/ba5c97eda4fe72a25831031a2cffb226fed8d9b7/src/osd/PGBackend.h

Cheers

-- 
Loïc Dachary, Artisan Logiciel Libre
All that is necessary for the triumph of evil is that good people do nothing.


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