HPE Storage Users Group

Hi All,

We have got a very interesting request from the customer and it is given below.

Is there any way to maintain a continuos synching between the base volume and its physical copy.

Awaiting answers ..

Regards,
KK

Why ?

Wouldn't you just get rid of the physical copy and take a snapshot on demand and remove the capacity requirement and copy overhead of a physical copy. You could still use physical copies if you really wanted but keep an intermediate snap between primary and secondary for fast resync. If it's some kind of availability play, then remember 3PAR is wide striped so the primary and secondary likely live on the same physical disks anyway unless you want to start filtering CPG's and make the array look like a traditional box.

Hi JohnHM,

Thanks for the reply.

Sorry, I have to correct my query.

Request is like, Need to mirror a drive for existing drive where the data between these 2 drives synched and maintained continuosly.

Thanks in advance.

Regards,
KK

Think about what your customer asked...
The physical copy would be RW from the array and from the host...

Then you maybe better use a snapshot and update it quite often.

Just because something is possible doesn't necessarily mean it's a good idea.

If you have a synchronous mirror internally, then the only reliable way to expose the secondary copy to an O/S is to freeze it at a point in time, hence the need for a snapshot, You need to do this because the primary will continue to be updated and the secondary host will have no knowledge of these updates at a file system level, which will at some point lead to the secondary host reporting the volume as corrupt. Note this is not an array issue but a filesystem issue as there is no lock management between the hosts.

If it's for availability in case the primary fails then consider that the primary is on the same array and will also replicate corruptions to the secondary copy in real time and more than likely they will both also reside on the same underlying physical disks so what have you achieved other than lots of overhead ?

If it's a performance thing then consider the overhead of mirroring (write overhead and doubling capacity) and also remember as above the disks are shared so you aren't really segregating resources. Also any snapshot delta's as well as the original volume are wide striped across the same number of disks, so again what have you really gained.

Today 3PAR supports via createvvcopy, on demand clones (online - immediately exportable) and resyncable clones (offline - which uses a snapshot to capture changes and resync only the changes since the last split). You should probably look at the latter but even then you'll require a snapshot to be taken to export the data.

So based on the requirement and the above, Raid 10 with an extra mirror on the primary and a subsequent snapshot could give you the same, but probably not what your Customer had in mind TBH you may as well just use a snapshot of the primary volume on demand without all the complexity and overheads of clones, take a look at the updatevv command.

As RitonLaBevue asked above what is the Customer trying to achieve with this request ? I don't mean what does he want to do at the array level but what is his end goal.