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.