[Linux-cluster] GFS2 support EMC storage SRDF??
Steven Whitehouse
swhiteho at redhat.com
Thu Dec 15 10:06:11 UTC 2011
Hi,
On Thu, 2011-12-15 at 13:06 +1100, yu song wrote:
> Gents,
>
> beauty!! it is great to see your ideas.
>
> I found a doc from redhat kb, which has the following statement
>
> "
>
> Multi-Site Disaster Recovery Clusters
> A multi-site cluster established for disaster recovery comprises two
> completely different clusters. These clusters typically have the same
> configuration, with one active and the other passive (and sometimes
> powered off). If the primary site fails, the secondary site is
> manually activated and takes over all services.
>
>
>
> Multi-site clusters are supported since implementation involves two
> separate clusters with the same configuration/architecture at two
> physical locations. Shared storage must be replicated from the primary
> to the back-up site using array-based replication. During a site
> failover, the cluster administrator must first toggle the
> directionality of the storage replication so that the back-up site
> becomes the primary and then start up the back-up cluster. These steps
> cannot be automated since using heuristics like site-to-site link
> failure might result in primary/back-up toggling when there are
> intermittent network failures.
>
> "
>
> It does give me an answer what I am after.
>
> have a great Christmas!!
>
Also, just to clarify, these multi-site clusters are not supported when
combined with GFS2,
Steve.
>
> On Thu, Dec 15, 2011 at 8:56 AM, <michael.trachtman at emc.com> wrote:
> Actually, another product that implements "geographically
> distributed storage" is VPlex (from EMC). VPlex is a product
> for geographically distributed storage. It works quite
> nicely. And, yes you can put a HA file system on top of
> that. I haven't tried it with GSF2 yet; but I have tried it
> with OCFS2, and there is no reason why GFS2 would be any
> different. I.e. there is every reason why it should work.
>
> ..m
>
>
> >>>To implement a HA cluster that uses a cluster filesystem
> such as GFS2 across geographical area you need a
> >>> different type of storage - a geographically distributed
> storage to have a chance of the cluster surviving the
> >>> inter-site link failure or site failure. Standard
> unidirectional replication won't do for this. I know of only
> >>>one such storage - Left Hand Networks iSCSI arrays (now
> owned by HP - the P4300, P4500 and P4800 storage
> >>>arrays). Again, implementation of such cluster is very
> complex. IMHO it is easier to have local HA clusters on
> >>>both sites and a good DR process based on replication.
>
> -----Original Message-----
> From: linux-cluster-bounces at redhat.com
> [mailto:linux-cluster-bounces at redhat.com] On Behalf Of
> Jankowski, Chris
> Sent: Wednesday, December 14, 2011 3:03 AM
> To: linux clustering
> Subject: Re: [Linux-cluster] GFS2 support EMC storage SRDF??
>
> *Unidirectional* replication is probably a better phrase to
> describe what EMC SRDF and all other typical block mode
> storage arrays do for replication.
>
> Typically this is used for manual or semi-automated DR systems
> and works very well for this purpose. This approach splits the
> HA and DR domains.
>
> It can also be used with a HA stretched cluster configuration
> for failing over services from one site to the other. You
> need to integrate into the service scripts unmounting of the
> filesystems for the service on one site, changing the
> direction of the replication and mounting the filesystem on
> the other site. This is quite complex and fiddly to say the
> least. I have yet to see an implementation where the users
> will be really happy with the robustness of the integrated
> solution.
>
> To implement a HA cluster that uses a cluster filesystem such
> as GFS2 across geographical area you need a different type of
> storage - a geographically distributed storage to have a
> chance of the cluster surviving the inter-site link failure or
> site failure. Standard unidirectional replication won't do for
> this. I know of only one such storage - Left Hand Networks
> iSCSI arrays (now owned by HP - the P4300, P4500 and P4800
> storage arrays). Again, implementation of such cluster is very
> complex. IMHO it is easier to have local HA clusters on both
> sites and a good DR process based on replication.
>
> You could also try to implement the stretched cluster purely
> in software using separate LUNs on storage arrays in two sites
> and mirroring them. Personally, I believe that this will not
> yield a robust solution with the current versions of software.
>
> Regards,
>
> Chris Jankowski
>
> -----Original Message-----
> From: linux-cluster-bounces at redhat.com
> [mailto:linux-cluster-bounces at redhat.com] On Behalf Of Bryn M.
> Reeves
> Sent: Wednesday, 14 December 2011 01:43
> To: linux clustering
> Subject: Re: [Linux-cluster] GFS2 support EMC storage SRDF??
>
> On 12/12/2011 03:29 AM, yu song wrote:
> > My question is that GFS2 supports SRDF ?? looking at KB in
> redhat site, it
> > only says that GFS2 does not support using asynchronous or
> active/passive
> > array based replication. but it seems like does not apply
> for SRDF.
>
>
> SRDF offers both synchronous and asynchronous replication but
> is
> active/passive. I.e. the administrator can configure whether
> the primary
> (R1) site waits for write acknowledgement from the remote (R2)
> site or
> not but at any one time it is only possible to write to either
> the R1 or
> the R2 device.
>
> Synchronous replication guarantees write order fidelity for
> the R2 copy
> and ensures the remote copy is crash consistent at all times.
>
> Asynchronous replication allows SRDF to support longer
> distances (or
> lower bandwidth / higher latency inter site links) by
> packaging multiple
> writes into delta sets to be sent to the remote site.
>
> More complex modes and combinations exist that allow
> consistency to be
> maintained among a group of devices, for example a database's
> data store
> and redo logs, or that relax some of the synchronous
> replication
> guarantees to improve efficiency (semi-synchronous operation).
>
> Active/passive in the context of storage replication usually
> refers to
> the states of the devices on the two sites. In active/active
> replication
> both sides are fully active at all times and writes may be
> issued on
> either side of the replication (a bit like multi-master
> application
> layer replication). An active/passive design only allows one
> side to be
> active for writes at a time.
>
> Most array based implementations are active/passive and offer
> asynchronous, synchronous or semi-synchronous operation.
>
> Regards,
> Bryn.
>
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