On 12/9/19 10:03 PM, Prasad Krishnan wrote:
On Tue, Dec 10, 2019 at 3:05 AM Casey Bodley <cbodley@xxxxxxxxxx
<mailto:cbodley@xxxxxxxxxx>> wrote:
The bucket index logs used for multisite replication are currently
stored in omap on the bucket index shards, along with the rest of the
bucket index entries. Storing them in the index was a natural choice,
because cls_rgw can write these log entries atomically when
completing a
bucket index transaction.
To replicate a bucket, other zones process each of its bucket index
shard logs independently, and store sync status markers with their
position in each shard. This tight coupling between the replication
strategy and the bucket's sharding scheme is the main challenge to
supporting bucket resharding in multisite, because shuffling these
log
entries to a new set of shards would invalidate the sync status
markers
stored in other zones.
Can we alternatively make the sync status markers resilient to changes
introduced by bucket resharding change? I don't have a specific method
in mind, but presuming that it should be relatively simple, risk-free and
backward compatible compared to making changes in the write path.
The omap keys of these bilog entries are essentially just encoding a
version number that's local to that index shard (see get_index_ver_key()
in cls_rgw.cc), so there's no defined ordering relationship between keys
in different shards. Other zones store an array of sync status markers
(ie omap keys) with their position in each log shard. If the bucket gets
resharded, the size of this sync status array no longer matches the new
bilog shards, with no way to associate its markers with the new sharding
scheme.
Thanks,
K.Prasad
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