In your example of EC 5 + 3, your min_size is 5. As long as you have 5 hosts up, you should still be serving content. My home cluster uses 2+1 and has 3 nodes. I can reboot any node (leaving 2 online) as long as the PGs in the cluster are healthy. If I were to actually lose a node, I would have to replace it and backfill into it before the cluster would be healthy again. During that time it would still be capable of serving content.
On Mon, Aug 28, 2017, 9:20 PM Nick Tan <nick.tan@xxxxxxxxx> wrote:
On Wed, Aug 23, 2017 at 2:28 PM, Christian Balzer <chibi@xxxxxxx> wrote:On Wed, 23 Aug 2017 13:38:25 +0800 Nick Tan wrote:
> Thanks for the advice Christian. I think I'm leaning more towards the
> 'traditional' storage server with 12 disks - as you say they give a lot
> more flexibility with the performance tuning/network options etc.
>
> The cache pool is an interesting idea but as you say it can get quite
> expensive for the capacities we're looking at. I'm interested in how
> bluestore performs without a flash/SSD WAL/DB. In my small scale testing
> it seems much better than filestore so I was planning on building something
> without any flash/SSD. There's always the option of adding it later if
> required.
>
Given the lack (for large writes) of double writes with Bluestore that's
to be expected.
Since you're looking mostly at largish, sequential writes and reads, a
pure HDDs cluster may be feasible.
_______________________________________________I have a final question. Assume I'll have 96 OSD's of 10TB each and I use erasure coding with k=5 and m=3 with the failure domain of host.It's my understanding that with erasure coding and host failure domains, the minimum number of hosts required is equal to k+m. Is this correct?So, if I use 8 hosts with 12 OSDs each this means I can lose any 3 OSD's but can I lose a host, assuming there's enough free space in the pool to accomodate the missing 12 OSD's? Or will the cluster block at this point because there's only 7 hosts which is less than k+m?If I use 48 hosts with 2 OSDs each then I can lose up to any 3 OSD's and the cluster can recover. And I could lose 1 host at a time assuming the recovery completes before the next host is lost (until there's 7 hosts left or there's not enough free space in the pool). Is this a correct assessment?Thanks,Nick
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