[Linux-cluster] GFS2 as virtual machine disk store

[Steven Whitehouse]

Hi,


On 29/08/17 12:26, Gionatan Danti wrote:
> Il 29-08-2017 13:13 Steven Whitehouse ha scritto:
>> 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
>
> From my understanding (and I can be wrong...) GFS2 will itself take 
> care of cache coherency between hosts.
>
> For example, if:
> - node A read a file;
> - node B read and write the same file;
> - node A re-read the same file;
> GFS2 should be able to guarantee a consistent view on the file on both 
> nodes. Obviously this come with a price: a significant overhead when 
> reading/writing the same (cache) files.
>
> I am missing something?
> Thanks.
>

There is no siginificant overhead when reading the same file on multiple 
nodes. The overhead mostly applies when writes are involved in some 
form, whether mixed with other writes or reads. GFS2 does ensure cache 
coherency, but in order to do that it requires certain properties from 
the storage, and hence the requirement for a symmetric view of the storage,

Steve.