HW raid & software raid

[SM_3]

Hello

There's something I can't follow with regards to hardware raid and software raid.
Lets say we have a hardware raid enabled disks, EMC or HDS.
Now the mirroing, say RAID1+0 is done at the hardware level.

In addition to hardware raid, why would we want to raid at software level using LVM or VXVM?

So why would we need to raid at the hardware and from the operating system?

Thanks.

[Steven Protter]

Shalom,

You would not want to.

It is a waste of CPU resources and provides no extra protection.

SEP

[Ninad_1]

Hi,

To add to your point, I would like to say that if you mirror at hardware level, then its better to stripe at OS/software level to get better distributed data and hence good response time for the IO. Thus while creating your lvols use striping over these hardware raid disks.

Regards,
Ninad

[santhanakrishnan_1]

HI SM,

Its not recommended to do Software Raid while its in hardware Raid.Bcoz ur wasting the CPU resource.The Hardware raid is more intelligent than the software raid..

[AwadheshPandey]

http://www.3ware.com/products/pdf/HWvsSW_111804.pdf
http://www.redhat.com/docs/manuals/linux/RHL-9-Manual/custom-guide/s1-raid-approaches.html
See these docs,

Awadhesh

[Alzhy]

SM,

Greetings. Peace & Good Earth.

I care to disagree BIG TIME with the poster earlier who claimed "You would not want to" and "a waste of CPU Resources and provides no Extra Protecttion". Sorry SEP, the post was very specific -- HDS/EMC arrays..

In Fact with HDS based Arrays (i.e. the Tagmastore, The Lightning Series, HP's HP Series) and EMC - best practice is to do additional RAIDing at the host level to scale performance. Normally the RAIDing you will need to do is striping (RAID0) - and you need to have members of your stripe sucn that each comes from a different RAID group inside your array.

I believe the same is true for the EMC Symmetrix/DMX series.

In some very critical environments, Software based striping and mirroring is even employed so you mirror and stripe accross arrays so no single array failures can render a server environment down.

With EVA arrays and the like - I agree you need not do any host-based/software striping (RAID0) BUT if you do not trust a single EVA, you may want to look at implementing software based mirroring to another Array or another EVA.

Hope this helps.

[SM_3]

ok
So you can mirror at OS level even if you have hardware raid.
Would this only make sense if you had hardware raid 1 then you stripe raid 1 at os level?

Also if a HP-UX server is connected to an EVA would this server require LVM software with a mirroring license?

[Alzhy]

"ok
So you can mirror at OS level even if you have hardware raid.
Would this only make sense if you had hardware raid 1 then you stripe raid 1 at os level?"

Yes you can. And in certain applications (i.e. you are a Bank and you do not trust single array stability) = you NEED to.


"Also if a HP-UX server is connected to an EVA would this server require LVM software with a mirroring license?"

If it is an EVA array, you do not need to have Mirroring software IF you are not mirroring accross 2 EVAs. no need to mirror if you are dealing with only ONE EVA.

[Ninad_1]

To reiterate and clarify.
If you have mirrored at hardware level then there is absolutely no need and you should not MIRROR at OS level.
If you do not have RAID/mirror capability at hardware level then only you should use mirroring at LVM level (For the required volumes - vg00 must )
Now if you have mirrored at hardware level, you have achieved the fault tolerance in case of a disk failure - now you should look to get some performance out of the storage as well - this you do by configuring your lvols as striped across these disks (which you have already mirrored at hardware level and hence at OS level you will see the mirrored pari as a single disk device and not as 2 different disk devices )
Hope this clarifies.

regards,
Ninad

[SM_3]

ok
So if data is Ultra Critical like in a bank, you could mirror one volume on one EVA to another connected to the same server.

Thanks everyone.

[SM_3]

Although I'm happy with the answers to this thread I have not assigned the rabbit so as to leave open this thread in case anyone wants to add a comment.

[Alzhy]

SM,

If you will be using EVA architecture, then HP will most likely insist you use Continous Access instead of host-based mirroring. Not a bad idea BUT what if you decide to someday move off to a brand-spankin-new array from XYZ Co.? Remember most Hardware based mirroring solutions work on the arrays vendors only.

Again, If you use EMC or HDS arrays (so called "cache-centric" arrays) - regardless of whether you have more than one array or not - for performance and scalability reasons and to get the most out of your array - You will need to do some additional RAIDing (RAID0 or striping) on your OS.

IF you do not trust single arrays and would want to invest on another array (dissimilar or not) - then it's up to you really to pick a mirroring solution - use RAID1 features of your VOlume Manager (software based) or the array vendor's solutions (hardware or appliance based).

Hope this helps.

[Chan 007]

Hi SM,

SAN does every thing that you do via LVM/VXVM. Many of the organisations are now going towards the JOBS (LUN)/SAN leaving the software once.

This is just a overhead and waste of your cost.

Assuming a Company plan for ROI (Return on Investment) they will cut off unnessary overheads, so remove s/w mirroring which is overhead for your systems io & performance.

Next, Lesser overhead for a SA to maintain your LVM/VXVM.

Next, Nowadays SAN takes care of your DR & BC and set a best standards for Business critical IT domains like Finance etc, you can utilise SAN with (HP's Continuous Access, EMC's SRDF, HDS's Truecopy) etc to have your LUNs moved kilometers aways. While your software lacks many of this features.

Finally to conclude, the IS/IT is totally depended on the Organisation Goals, I mean you can use any thing a SA / IT team wishes, but what and how that will benefit is the key question....!!! Which truely depends on Skills, IS/IT & finally the Organisation's goal.

Hope I have comfused a Lottttttttt...:-(

Chan

[Alzhy]

Sure SANs can do it ALL but mostly ONLY IF YOUR ARRAYS AND OTHER COMPONENTS are from one vendor. In effect you will be under what is called "Vendor Lock-in". If it's okay under your Company's policies - then by all means - be locked in and at the mercy of your vendor.

I have yet to see an in-array/hardware based solution that can handle mirroring to another array (different vendor). Can CA/TrueCopy/SRDF work accross different vendors' arrays accross long distances?

There might be "appliance type" hardware out there that does SAN based virtualization and mirroring etc. but the problem is they're not really 100% certified by vendor's arrays and you will just need to trust them. And would you risk all your SAN being managed (mirrors, LUNS) on one appliance that could break? I have seen so called early adopters of these new generation of SAN appliances and believe me - their experiences are horrific. Sure they are excellent technologies - as long as they're up and running and working, but when they do go down, so do all your SAN connected machines.