Hi Dan So will Kudu rebalance tablets on different disks dynamically when some gets bigger than other, in order not to fill a disk while the other disk space could remain mostly free ?
2017-03-02 18:26 GMT+01:00 Dan Burkert <[email protected]>: > Hi Alexandre, > > responses inline > > On Thu, Mar 2, 2017 at 9:18 AM, Alexandre Fouché <[email protected]> > wrote: >> >> >> When storing data on multiple JBOD disks, will Kudu assign data for >> tablets efficiently as far as tablet sizes or activity are concerned, or >> will it simply try to assign roughly the same number of tablets on each >> disk, regarless of tablets true size or activity (we have many empty >> tablets at this time). And will it rebalance tablets to one disk or another >> automatically ? Or is it still better to expose one RAID0 volume ? >> > > Kudu will evenly spread data from all tablets across all disks. This > allows Kudu to get good write throughput and balancing, but similarly to > RAID 0 it means that if one drive fails, all tablets on that tablet server > will become unavailable. Kudu will automatically recover by re-replicating > the tablets to a different tablet server as long as a majority of replicas > are still available. So, RAID 0 should provide no benefit for Kudu. It's > on the roadmap to make multi-disk configuration more flexible so that if a > single disk dies only a subset of the tablets will become unavailable, but > I don't have a timeline on that feature (no one is working on it to my > knowledge). > > >> Will using JBOD disks better than RAID stripes ? It seems from Bug >> reports that when WAL disk fails, or one of the JBOD data disks, Kudu is >> still unable to recover and keep or migrate good tablets. In that case, it >> shows no improvement over a failed disk on a RAID0 where in both cases the >> only recover option is to delete the whole Kudu data and WAL and let it >> resync from other nodes ? >> > > I think what I wrote previously answers this, if not I can clarify. > > I found comments that WAL can only be one one disk, is it still the case, >> or is this info obsolete ? >> > > This is currently the case. If you have many disks it's often advantageous > to put the WAL on it's own disk (ideally an SSD if it's available). The WAL > workload is more latency sensitive than the data workload. > > - Dan >
