Re: Ceph Luminous RocksDB vs WalDB?

[Igor Fedotov <ifedotov@xxxxxxx>]

Hi Pardhiv,

there is no WalDB in Ceph.

It's WAL (Write Ahead Log) that is a way to ensure write safety in RocksDB. In other words - that's just a RocksDB subsystem which can use separate volume though.

In general For BlueStore/BlueFS one can either allocate separate volumes for WAL and DB or have them on the same volume. The latter is the common option.

The separated layout makes sense when you have tiny but super-fast device (for WAL) and less effective (but still fast) larger drive for DB. Not to mention the third one for user data....

E.g. HDD (user data) + SDD (DB) + NVME  (WAL) is such a layout.

So for you case IMO it's optimal to have merged WAL+DB at NVME and data at SSD. Hence no need for separate WAL volume.

Regards,

Igor

On 6/26/2018 10:22 PM, Pardhiv Karri wrote:

Hi,

I am playing with Ceph Luminous and getting confused information around usage of WalDB vs RocksDB.

I have 2TB NVMe drive which I want to use for Wal/Rocks DB and have 5 2TB SSD's for OSD. 

I am planning to create 5 30GB partitions for RocksDB on NVMe drive, do I need to create partitions of WalDB also on NVMe drive or does RocksDB does same work as WalDB plus having metadata on it? 

So my question is do I really need to use WalDB along with RocksDB or having RocksDB only is fine?

Thanks,

Pardhiv K

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