[Linux-cluster] GFS2 as virtual machine disk store

[Steven Whitehouse]

Hi,


On 29/08/17 12:07, Gionatan Danti wrote:
> Il 29-08-2017 12:59 Steven Whitehouse ha scritto:
>>>
>> Yes, it definitely needs to be set to cache=none mode. Barrier passing
>> is only one issue, and as you say it is down to the cache coherency,
>> since the block layer is not aware of the caching requirements of the
>> upper layers in this case.
>
> Ok. Sorry to be pedantic, but I would be sure to grasp it: using 
> cache=writeback will *not* cause data corruption, rather low 
> performance if/when the cache coherency protocol is needed (ie: when 
> live migrate a VM from first to second host), correct?
Whatever kind of storage is being used with GFS2, it needs to act as if 
there was no cache or as if there is a common cache between all nodes - 
what we want to avoid is caches which are specific to each node. Using 
individual node caching will still cause issues in case, for example, 
one node has cached a block that another node has changed. In that case 
the node with the cached information will use that, rather than 
rereading from disk which is where the newly changed information is. So 
it is a question of ensuring that all nodes "see" the same data at all 
times,

Steve