So: just don't ever set min_size to 1.
-Greg
On Tue, Mar 21, 2017 at 6:04 PM Anthony D'Atri <aad@xxxxxxxxxxxxxx> wrote:
>> a min_size of 1 is dangerous though because it means you are 1 hard disk failure away from losing the objects within that placement group entirely. a min_size of 2 is generally considered the minimum you want but many people ignore that advice, some wish they hadn't.
>
> I admit I am having difficulty following why this is the case
I think we have a case of fervently agreeing.
Setting min_size on a specific pool to 1 to allow PG’s to heal is absolutely a normal thing in certain circumstances, but it’s important to
1) Know _exactly_ what you’re doing, to which pool, and why
2) Do it very carefully, changing ‘size’ instead of ‘min_size’ on a busy pool with a bunch of PG’s and data can be quite the rude awakening.
3) Most importantly, _only_ set it for the minimum time needed, with eyes watching the healing, and set it back immediately after all affected PG’s have peered and healed.
The danger, which I think is what Wes was getting at, is in leaving it set to 1 all the time, or forgetting to revert it. THAT is, as we used to say, begging to lose.
— aad
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