Re: Wide variation in osd_mclock_max_capacity_iops_hdd

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SATA/SAS volatile drive cache comes up often - have we considered having the OSD startup process disable it by default? Or would that violate the principle of least astonishment, especially since drive behavior varies considerably?

> On Sep 7, 2022, at 05:58, Sridhar Seshasayee <sseshasa@xxxxxxxxxx> wrote:
> 
> 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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