Re: Wide variation in osd_mclock_max_capacity_iops_hdd

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Hi Vladimir,

Yes, device caching is disabled. The ODSs in question use a
> separate DB/WAL device on flash. Could that be the cause of
> IOPS over-estimation?
>

2000 IOPS definitely looks high for a HDD. The in-built osd bench tool
writes to the backing device of the osd to determine the IOPS capacity.
With DB/WAL configured, I wouldn't expect a wide variation.

To confirm the IOPS measurement, you can run the following commands:
$ ceph tell osd.N cache drop

followed by,

$ ceph tell osd.N bench 12288000 4096 4194304 100

Where N is the osd id.

You can run the above sequence a few times to confirm if there are any wide
variations and
if so, it probably warrants more investigation into why that's the case.

Also, may I know the following:
- what kind of HDDs are you using in your environment?
- Ceph version


Is it better for me to manually set
> osd_mclock_max_capacity_iops_hdd to a reasonable number or
> will things perform OK as long as IOPS are over-estimated
> consistently everywhere?
>

I think it's better to figure out why the IOPS are on the higher side first.

-Sridhar

<https://www.redhat.com>
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