Since the PGs will be splitted when you add new OSDs you don't need to
merge them beforehand, your PG per OSD ratio will improve (if you have
too many PGs/OSD at the moment). After they have been remapped and the
rebalancing is done you can check how the PGs/OSD ratio is and then
decide whether it makes sense to make further changes.
Zitat von Christoph Adomeit <Christoph.Adomeit@xxxxxxxxxxx>:
I have 15 Nvme 6.4 TB and 4200 Pgs. This has historical reasons
since the same cluster once had 100 spinning drives and at that
ancient times it was not possible to shrink the number of pgs in an
existing cluster.
Do you think it is a good idea for snaptrim performance to reduce
the number of PGs ? Also I want to add more Nvmes. Ist it better to
first add the new NVMEs and then reduce PGs or is it better to first
reduche the number of PGs and then add the NVMEs ?
On Wed, Nov 10, 2021 at 04:04:52PM -0800, Anthony D'Atri wrote:
> How many osd you have on 1 nvme drives?
> We increased 2/nvme to 4/nvme and it improved the snap-trimming
quite a lot.
Interesting. Most analyses I’ve seen report diminishing returns
with more than two OSDs per.
There are definitely serialization bottlenecks in the PG and OSD
code, so I’m curious re the number and size of the NVMe devices
you’re using, and especially their PG ratio. Not lowballing the
PGs per OSD can have a similar effect with less impact to CPU and
RAM. ymmv.
> I guess the utilisation of the nvmes when you snaptrim is not 100%.
Take the iostat %util field with a grain of salt, like the load
average. Both are traditional metrics whose meanings have diffused
as systems have evolved over the years.
— aad
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