On 3/16/2020 3:25 PM, vitalif@xxxxxxxxxx wrote:
Hi Victor,
1) RocksDB doesn't put L4 on the fast device if it's less than ~ 286
GB, so no. But, anyway, there's usually no L4, so 30 GB is usually
sufficient. I had ~17 GB block.dbs even for 8 TB hard drives used for
RBD... RGW probably uses slightly more if stored objects are small...
but you're still unlikely to overflow a 30 GB partition with 2 TB OSDs.
As already mentioned by Janne and given plenty of available space at
NVMes I would recommend to have some spare space above 30GB at DB. In my
lab I observed up to 100% transient excess under peak load when running
some (pretty artificial!) benchmarks.
IMO 64GB is the perfect size for combined WAL/DB volume for almost any
OSD except ones that have huge main device behind and handle heavy RGW
load. 300+ GB is required in that case.
2) WAL is the on-disk mirror of RocksDB memtables, its size is defined
by bluestore_rocksdb_options. Default is max_write_buffer_number=4,
write_buffer_size=256MB, thus 1GB. You don't even need to split wal
and db partitions if they're on the same device. Calculations are
here: https://yourcmc.ru/wiki/Ceph_performance#About_block.db_sizing
Hi,
Vitaliy - Sure, I can use those absolute values (30GB for DB, 2GB for
WAL) you suggested.
Currently - Proxmox is defaulting to a 178.85 GB partition for the
DB/WAL. (It seems to put the DB and WAL on the same partition).
Using your calculations, with 6 x OSDs per host - that means a total
of 180GB for DB, 12GB for WAL = 192GB in total. (Optane drive is 960GB
in capacity).
Question 1 - Are there any advantages to using larger DB partition
than 30GB, or larger WAL than 2GB? (Just thinking how to best use the
entire Optane drive if possible).
Question 2 - How do I check the WAL size in Ceph? (Proxmox seems to be
putting the WAL on the same partition as the DB, but I don't know
where its size is specified).
Thanks,
Victor
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