On 02/04/2019 18.27, Christian Balzer wrote:
I did a quick peek at my test cluster (20 OSDs, 5 hosts) and a replica 2
pool with 1024 PGs.
(20 choose 2) is 190, so you're never going to have more than that many
unique sets of OSDs.
I just looked at the OSD distribution for a replica 3 pool across 48
OSDs with 4096 PGs that I have and the result is reasonable. There are
3782 unique OSD tuples, out of (48 choose 3) = 17296 options. Since this
is a random process, due to the birthday paradox, some duplicates are
expected after only the order of 17296^0.5 = ~131 PGs; at 4096 PGs
having 3782 unique choices seems to pass the gut feeling test. Too lazy
to do the math closed form, but here's a quick simulation:
>>> len(set(random.randrange(17296) for i in range(4096)))
3671
So I'm actually slightly ahead.
At the numbers in my previous example (1500 OSDs, 50k pool PGs),
statistically you should get something like ~3 collisions on average, so
negligible.
Another thing to look at here is of course critical period and disk
failure probabilities, these guys explain the logic behind their
calculator, would be delighted if you could have a peek and comment.
https://www.memset.com/support/resources/raid-calculator/
I'll take a look tonight :)
--
Hector Martin (hec...@marcansoft.com)
Public Key: https://mrcn.st/pub
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