Is there any chance for the hybrid cache to get phantom entry?

[MrCroxx]

Hi, I'm new to CacheLib and using it to build my app. And I'm interested in reading CacheLib's source code.

I found the BlockCache (large entry cache) of the hybrid cache engine navy doesn't persist tombstones when removing entries. Is there any chance for the hybrid cache get a phantom entry after recovery?

[haowu14]

What's accessible in BlockCache is only those entries in the BlockCache Index. So as long as an entry is removed from the index, it won't be accessible again.

The tombstone in NvmCache code is used for synchronization for inflight lookups/puts/deletes only. Before a proper shut down all inflight NVM operations will be drained, so it doesn't need to be persisted.

[haowu14]

Yes. They were persisted in the cache directory. On startup, the recover() call read from the thrift file and got a list of buckets: https://github.com/facebook/CacheLib/blob/main/cachelib/navy/block_cache/Index.cpp#L186

[MrCroxx]

IIUC, the indices don't persist itself or log its mutatuon per kv. Is there a chance that after a deletion but before the new indices are persisted the server crashes and recovers? In this case, will a deleted entry comes back to life?

[haowu14]

I guess the term persisted is misleading. CacheLib is not a persisted storage in the sense that it won't recover from power loss or crashes.

If the process does not get proper shut down (didn't call CacheAllocator::shutDown() , the cache will be dropped.

[MrCroxx]

Oh, I see. Then it makes sense.

[MrCroxx]

Thanks for your patience. Have a nice day. 🙏