Dear Cephalopodians, for our MDS data, we are currently using 4 Bluestore-OSDs with classic SATA SSDs (Replica mode). In metadata stress testing, this easily becomes the bottleneck.
Also, the existing SSDs are too small (4 x 240 GB, 4 replicas) if the cluster
was to become full.
My question is: Is it more reasonable to buy small, but fast NVMe devices for a
WAL,
and larger (cheap) SSDs for the KeyValue-store, for example 8 SSDs to also have
more OSDs to share the load,
or would this not improve the I/O situation, and we should really buy large and
pricey NVMes?
(The main issue with that is that our current servers only have a single free
PCIe slot for that,
so scaling that is expensive.)
The answer strongly depends on how much I/O actually goes to WAL and how much
goes to the DB.
Is this the same I/O by design, or does the DB device need less actual IOPS
(i.e. only throughput) due to the deferred writing?
Anybody with experiences on this?
Cheers and many thanks,
Oliver
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