26 Jun
2020
26 Jun
'20
7:27 a.m.
M=1 is never a good choice. Just use replication instead.
> On Jun 26, 2020, at 3:05 AM, Zhenshi Zhou <deaderzzs(a)gmail.com> wrote:
>
> Hi Janne,
>
> I use the default profile(2+1) and set failure-domain=host, is my best
> practice?
>
> Janne Johansson <icepic.dz(a)gmail.com> 于2020年6月26日周五 下午4:59写道：
>
>> Den fre 26 juni 2020 kl 10:32 skrev Zhenshi Zhou <deaderzzs(a)gmail.com>om>:
>>
>>> Hi all,
>>>
>>> I'm going to deploy a cluster with erasure code pool for cold storage.
>>> There are 3 servers for me to set up the cluster, 12 OSDs on each server.
>>> Does that mean the data is secure while 1/3 OSDs of the cluster is down,
>>> or only 2 of the OSDs is down , if I set the ec profile with k=4 and m=2.
>>>
>>
>> By default, crush will want to place each part (of 6 in your case for EC
>> 4+2)
>> on a host of its own, to maximize data safety. Since you can't do that
>> with 3
>> hosts, you must make sure no more than 2 pieces end up on a single host
>> ever,
>> so you can't just move from failure-domain=host to domain=osd, since that
>> would place all 6 pieces on the same host but different OSDs which would be
>> bad.
>>
>> You need to make the crush rule pick two different OSDs per host, but not
>> more.
>> One way could be to make a tree where hosts has half of its OSDs in one
>> branch
>> and the other half in another (lets call it subhost in this example), then
>> you get 3*2
>> subhosts, and you make crush pick placement from subhosts and it will
>> always put
>> two pieces per OSD host, never on the same OSD and it will allow one host
>> to be
>> down for a while.
>>
>> I would like to add that data is not very secure when you have no
>> redundancy at all
>> left. Machines will crash, they will require maintenance, patches, bios
>> updates and
>> things like that, and having NO redundancy while you have planned or
>> unplanned
>> downtime will be placing the data at huge risk, _any_ surprise in this
>> situation would
>> immediately lead to data loss.
>>
>> Also, if one box dies, the cluster can't run and can't recover until you
>> have a new
>> host back in, so you are already running at the edge of data safety in
>> your normal case.
>> Even if this will "work", ceph as being a cluster really should have
N+1
>> hosts or more
>> if your data split (replication factor or EC k+m) is equal to N.
>>
>> --
>> May the most significant bit of your life be positive.
>>
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