In a 3 node cluster with EC k=2 m=1, you can turn off one of the nodes and the cluster will still operate normally. If you lose a disk during this state or another server goes offline, then you lose access to your data. But assuming that you bring up the third node and let it finish backfilling/recovering before restarting any other nodes, then you're fine.
On Thu, Oct 19, 2017 at 4:49 PM Jorge Pinilla López <jorpilo@xxxxxxxxx> wrote:
Imagine we have a 3 OSDs cluster and I make an erasure pool with k=2 m=1._______________________________________________
If I have an OSD fail, we can rebuild the data but (I think) the hole cluster won't be able to perform IOS.
Wouldn't be possible to make the cluster work in a degraded mode?
I think it would be a good idea to make the cluster work on degraded mode and promise to re balance/re build whenever a third OSD comes alive.
On reads, it could serve the data using the live data chunks and rebuilding (if necessary) the missing ones(using cpu to calculate the data before serving// with 0 RTA) or trying to rebuild the missing parts so it actually has the 2 data chunks on the 2 live OSDs (with some RTA and space usage) or even doing both things at the same time (with high network and cpu and storage cost).
On writes, it could write the 2 data parts into the live OSDs and whenever the third OSD comes up, the cluster could re balance rebuilding the parity chunk and re positioning the parts so all OSDs have the same amount of data/work.
would this be possible?
Jorge Pinilla López
jorpilo@xxxxxxxxx
Estudiante de ingenieria informática
Becario del area de sistemas (SICUZ)
Universidad de Zaragoza
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