Re: Benchmark does not show gains with DB on SSD

[Maged Mokhtar <mmokhtar@xxxxxxxxxxx>]

On 12/09/18 17:06, Ján Senko wrote:

We are benchmarking a test machine which has:

8 cores, 64GB RAM

12 * 12 TB HDD (SATA)

2 * 480 GB SSD (SATA)

1 * 240 GB SSD (NVME)

Ceph Mimic

Baseline benchmark for HDD only (Erasure Code 4+2)

Write 420 MB/s, 100 IOPS, 150ms latency

Read 1040 MB/s, 260 IOPS, 60ms latency

Now we moved WAL to the SSD (all 12 WALs on single SSD, default size (512MB)):

Write 640 MB/s, 160 IOPS, 100ms latency

Read identical as above.

Nice boost we thought, so we moved WAL+DB to the SSD (Assigned 30GB for DB)

All results are the same as above!

Q: This is suspicious, right? Why is the DB on SSD not helping with our benchmark? We use rados bench

We tried putting WAL on the NVME, and again, the results are the same as on SSD.

Same for WAL+DB on NVME

Again, the same speed. Any ideas why we don't gain speed by using faster HW here?

Jan

--

Jan Senko, Skype janos-
Phone in Switzerland: +41 774 144 602
Phone in Czech Republic: +420 777 843 818

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Hi Jan,
putting 12 wals + db on the same SSD is probably too much. try 4-5 may give better results.
the recommendation i have is to run a load monitoring tool (atop/sar/collectl) while you are bench-marking and look for % utilization for disks and cpus, this will let you know without guessing where the bottleneck is : could be your ssd or maybe your cpu are saturated.

/Maged

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