Re: Adding hot spare using a disk from an existing RAID 5 string

It's not really a Power7 thing it's as SAS vs SCSI thing. But since Power7 is 100% SAS it will be true on Power7. Power5 was all SCSI and Power6 was the stepping stone where you could mix SAS and SCSI disks with i 7.1.

So the issue is that on SCSI disks when we RAIDed a set of disk the RAID card looked at the number of drives you had selected for that set and wrote the RAID stripes onto only some disks. For 3 drives it used 2. For 4-7 it used 4 and for 8-11 it was 8 and I believe it used 12 for 12-18 drives which was the largest RAID sets of the day. The other disks were protected but didn't contain RAID stripe data so theses disks could be removed from the set without breaking it. Internal to the RAID card it will simply be declared to contain all zeros. With that the RAID calculations worked. You can also ADD to the RAID sets and those new drives are protected also but with no Stripe. The raid card assumes they contain all zeroes, initializes the drive to contain all zeros, and protection begins from there.

With SAS however the game changed. No matter how many drives you have when you start RAID they all get a percentage of the RAID data. 8 9 10 disks, whatever, it's evenly split. This is better for performance all around. However that means in a Power7 CEC with it's 8 drives you're going to have all 8 with strip. (Curiously with SCSI you would have also since it's 8 :-) ) Back in the Power5 days we saw a lot of companies putting in lesser numbers of drives to start but with the price of drives dropping so fast, most Power7 systems shipped with the full compliment of 8. I cannot recall a single customer who did not install 8 drives to start.

I mentioned that you MIGHT find that a drive could be removed and that would occur if you had started with say 6 drives and then added two more to the set at a later time. This is still allowed with SAS as it was with SCSI but it is NOT recommended. If this had been done then 6 drives in the set will appear smaller than the other two. Those two would be candidates for step 6 in this case. So possible but unlikely.

Hope that helps!


- Larry "DrFranken" Bolhuis

www.Frankeni.com
www.iDevCloud.com - Personal Development IBM i timeshare service.
www.iInTheCloud.com - Commercial IBM i Cloud Hosting.

On 6/2/2017 7:08 AM, Marco Facchinetti wrote:

Thanks Doc, do you mind explaine why on a power7 step 6 will drive to step
7? I'm not a specialist so this is not obvious to me.

TIA

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Marco Facchinetti

Mr S.r.l.

Tel. 035 962885
Cel. 393 9620498

Skype: facchinettimarco

2017-06-01 20:49 GMT+02:00 DrFranken <midrange@xxxxxxxxxxxx>:

Error! Note the added ** NOT ** in the first line below.

On 6/1/2017 2:32 PM, DrFranken wrote:

You will with near certainty be ** NOT ** able to use option 6 on a Power7

machine. Yes there is a scenario I can imagine where it could happen but
not likely.

That means you will be using option 7 (Stop RAID) and THAT means you
WILL have a full backup of your system before you attempt this
procedure. It is NOT an option.

Rob is right that you could use ASP balancing to drain the drive but why
not go into SST and simply remove the drive. This will completely drain
it AND remove it from the ASP. This can run while the system is still up.

After that you can do your backup, boot to SSD, End RAID, then start
that drive as a hot spare and then start RAID again.

- Larry "DrFranken" Bolhuis

www.Frankeni.com
www.iDevCloud.com - Personal Development IBM i timeshare service.
www.iInTheCloud.com - Commercial IBM i Cloud Hosting.

On 6/1/2017 2:23 PM, Rob Berendt wrote:

Verify from others.
That being said I wonder if you could minimize your downtime by removing
data from the disk, or at least as much as you can, beforehand.
See:
STRASPBAL TYPE(*ENDALC)
STRASPBAL TYPE(*MOVDTA)


Rob Berendt

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