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.
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