RF=5 allows you to lose two hosts without losing quorum Many teams can calculate their hardware failure rate and replacement time. If you can do both of these things you can pick and RF that meets your durability and availability SLO. For sufficiently high SLOs you’ll need RF > 3
> On Jun 30, 2019, at 11:58 PM, Oleksandr Shulgin > <[email protected]> wrote: > >> On Sat, Jun 29, 2019 at 5:49 AM Jeff Jirsa <[email protected]> wrote: > >> If you’re at RF= 3 and read/write at quorum, you’ll have full visibility of >> all data if you switch to RF=4 and continue reading at quorum because quorum >> if 4 is 3, so you’re guaranteed to overlap with at least one of the two >> nodes that got all earlier writes >> >> Going from 3 to 4 to 5 requires a repair after 4. > > Understood, thanks for detailing it. > > At the same time, is it ever practical to use RF > 3? Is it practical to > switch to 5 if you already have 3? > > I imagine this question is popping up more often in a context of switching > from RF < 3 to =3. As well as switching from non-NTS to NTS, in which case > it is indeed quite troublesome, as you have pointed out. > > -- > Alex >
