Raid5 and Mirrors

[Greg Hall]

Hi,

has anyone ever used a mirrored raid-5 configuration? I mean raid-5 done at the controller level, then this array mirrored by the op system (using LVM say) to another raid-5 array.
I know its not the prescribed method and is probably overkill on the redundancy, but my client is reluctant to let go of the raid-5 functions.

thanks, Greg

[Uwe Zessin]

Greg,
even highly redundant storage systems can go down - I have seen such cases. In such a situation it does not matter if the data is protected by RAID-5, RAID-1 or something else. If the data is very important and/or continuous operations are required, then going with two storage boxes is certainly not overkill.

We have a number of customer who use two storage systems for desaster tolerance. Half of the storage and the servers is in one computer room and the other half is in another room or even in another building.

[Greg Hall]

Uwe,

thanks. The two storage arrays is just fine, this will be a high availabilty cluster. However there is a performance overhead with raid-5, and another one with op system mirroring, if I combine the two will it perform ok? I know its a very open ended question, I just wonder if anyone has seen this done.

thanks all the same, Greg

thanks, Greg

[Uwe Zessin]

Greg,
we have customers who even mirror RAID-1 and RAID-1+0 configurations between two data centers. Imagine one center is down and the company now runs its data unprotected: a single failed disk will leave the entire shop dead in the water!!

Any half-way competent RAID controller will try to optimize RAID-5 operations, e.g. by trying to collect an entire stripe in memory so that it can now calculate the parity and write everything in a single step.

You are right that this is additional overhead for the system, but that is a price one has to pay for high availability. We have several customers who mirror their SAP R/3 databases between two data centers.

[Greg Hall]

Cool. Thanks, Uwe, that's handy to know I'm not doing something totally silly.

cheers, Greg

[Mike Naime]

Greg:

Yes. On AIX and VMS systems. This was done for various reasons. But to answer your question, Yes.

Mike

[Greg Hall]

Hi Mike,

actually this is an AIX cluster I'm referring to. Did you have any issues, performance or otherwise, using RAID-5 and LVM mirroring? I'm not really expecting any but always good to talk to someone who's actually done it.

thanks, Greg

[Emilio Brusa]

Greg,
mirror disk will always be more slow that a controler, for this reason, mirror disk works to a softwate layer.
Depending on your application should choose the level of redundancy
for example raid 5 is good in readings but bad in writing.
If you have a controler and mirror disk and what you look for is performance,
you could configure in raid 0 the controler and in 1 the mirror disk for example