RAID1 vs RAID5 with Oracle SAP

[Kurz]

We use Oracle 9 for an SAP database, currently on a VA7100.

We now plan to migrate to an EVA4000. We have set aside a separate disk group for the productive SAP system with 16 x 72 GB 15k disks.

We have been told that with protection level 1 there would be available:
806 GB with RAID5
504 GB with RAID1

806 GB seems to be (16-2)*72/1.25 ... ok
504 GB I cannot work out. Any ideas?

We are now starting with a ~ 250 GB database. The EVA is supposed to run at least 5 years. We might have to change the RAID level during that time because of disk space. Is this a problem?

And now the main question: Is it worth starting with RAID1? We have mainly read access vs. write access.

thx, Micha

[Rob Leadbeater]

Hi Micha,

(16-2) * 72 / 2 = 504GB

Note though that 72GB is actually more like 68GB in real terms due to the difference between marketing and real GB.

72 x 1,000,000 / 1,048,576 = 68.66GB

So in your example for VRAID-5 you'd have around 769GB available space, and for VRAID-1 you'd have around 480GB.

Hope this helps,

Regards,

Rob

[Kurz]

Hi Rob,

thanks a lot for your answer. The 1.25 is the correction factor for the marketing GB vs. real GB that I assumed. So the consultant missed this with RAID1, I should have seen.

Any ideas about RAID1 vs RAID5

[Uwe Zessin · Accepted solution]

> The 1.25 is the correction factor for the marketing GB vs. real GB

This is the VRAID-5 overhead, because the EVA always applies one parity chunk per 4 data chunks.

The difference between hardware and software GigaBytes is about 0.93. Command View EVA shows software GigaBytes, but the hardware GigaBytes value printed on the disk is often far from correct. A "72GB" disk has a bit more than 72 hardware GigaBytes, but one has to check the exact number of logical blocks.


If most of your I/O is read anyway, then I would start with VRAID-5 immediately. The EVA cannot do an in-place VRAID conversion, so you would have to spend some time with backup and restore.

[Kurz]

Thanks, Uwe, for clarifying.

lots of thanks, Michaela

[Kurz]

RAID5 seems the right choice.