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From Dan Burkert <danburk...@apache.org>
Subject Re: stripes, JBOD: Assignment and rebalance ?
Date Thu, 02 Mar 2017 17:51:24 GMT
Individual tablets aren't assigned to a specific disk, they are spread
across all of the data disks.  So individual disks should fill up evenly.

- Dan

On Thu, Mar 2, 2017 at 9:38 AM, Alexandre Fouché <afouche@onfocus.io> wrote:

> 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 <danburkert@apache.org>:
>> Hi Alexandre,
>> responses inline
>> On Thu, Mar 2, 2017 at 9:18 AM, Alexandre Fouché <afouche@onfocus.io>
>> 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

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