RE: RAID 6 rebuild time - can I slash it?

Jeff,

What % is your system at?
Do you know model of the disk controller, drive type and size?
Depending on your %, there might be other options for you.

Paul

-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx [mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of Jeff Crosby
Sent: Wednesday, March 19, 2014 4:39 PM
To: Midrange Systems Technical Discussion
Subject: Re: RAID 6 rebuild time - can I slash it?

It's i 7.1. I did not know that about getting the the drives for hot spares available ahead of time. Maybe the service provider is already planning this. We will be sitting down for a planning meeting soon.

Thanks.


On Wed, Mar 19, 2014 at 4:30 PM, DrFranken <midrange@xxxxxxxxxxxx> wrote:

I didn't see a release mentioned but with IBM i 7.1 you can completely
remove the two drives that will become hot spares in advance with
essentially zero impact to production and no outage. Determine which
drives you want to be the hot spares and then in Service Tools work
with disk units take them out of the ASP.

That will take some time but runs in the background. When done they
will appear at the bottom of WRKDSKSTS with no drive number. They are
still RAID drives at that point but that will change when you End
RAID5 in preparation for RAID 6. Since they are not in the ASP they
will be the chosen ones for hot spare. NOTE: In a Power5 the LS drive
must be drive 5, 6, or 7 counting from the LEFT, likely it's drive 5.
So pick either 6 or 7 to remove so it's a legit Hot Spare.

If you are on 6.1 then use STRASPBAL to *ENDALC followed by *MOVDTA to
drain them as low as possible, likely to 1 or 2 percent. This will
reduce the time needed in DST to remove them.

- Larry "DrFranken" Bolhuis

www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com

On 3/19/2014 3:58 PM, Jeff Crosby wrote:

I'm concerned about the amount of time we will be unprotected.

We're planning to do it all on 1 Saturday.

I don't think it's possible to move the data off the 2 drives that
will

be

the hot spares. One 8-drive RAID 5 set has the striping across 4
drives (CEC). The other 8-drive RAID 5 set has the striping across
all 8 drives (expansion unit).



On Wed, Mar 19, 2014 at 3:34 PM, Evan Harris <auctionitis@xxxxxxxxx>

wrote:

Hi Jeff

you didn't say (or I missed it) but are you moving the data off
that

drive

in advance ? Hopefully that's a "yes"

Also, is your concern around the timing based on the amount of time
you will be unprotected or the amount of time the system will
potentially be out of service ?




On Thu, Mar 20, 2014 at 8:26 AM, Jeff Crosby
<jlcrosby@xxxxxxxxxxxxxxxx

wrote:

Paul,

Not adding any drives. Moving data off 1 drive to make the hot spare.


On Wed, Mar 19, 2014 at 2:46 PM, Steinmetz, Paul <

PSteinmetz@xxxxxxxxxx

wrote:

Jeff,

Currently, you have 2 8 drive Raid5 sets, total of 16 drives Are
you adding 2 new drives for you're hot spare or remaining at 2 8

drive

Raid6 with hot spare.
If you're not adding drives, you will need to move data off 1
drive

for

each of your Raid5 sets, correct.

Paul

-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx [mailto:
midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of Jeff Crosby
Sent: Wednesday, March 19, 2014 2:34 PM
To: Midrange Systems Technical Discussion
Subject: Re: RAID 6 rebuild time - can I slash it?

Our QPFRADJ is set to 3.

You give times for drives with NO data and times for drives WITH data.
The drives WITH data are my question. Does it make a
difference,

for

example, if the drive is 20% full vs 40% full?




On Wed, Mar 19, 2014 at 2:07 PM, Steinmetz, Paul <

PSteinmetz@xxxxxxxxxx

wrote:

Jeff,

Disk controller, Disk type (SSD, 15K or 10k) Disk size are all
factors, IBM internals has a tool that will estimate this.

Below are times from a disk project years back on a 9406-550
Stop parity - 20 minutes for 8 drives if no data on drives - 4
hours / 40 minutes per drive if drives contain data

Start parity
Drives with no data - 5 minutes / drive
3 to 18 drives, all with no data, 5 minutes / drive; 15 to 90

minutes.

The time to start Raid on 3 drives vs 18 drives is not much
different = about 30 minutes with NO drives configured.
At least one drive with data, either customer data or LS - 40

minutes

/ drive The time to start Raid on 3 drives vs 18 drives is not
much different = about 5.5 to 5.75 hours with some drives configured.

Also check, QPFRADJ was set to 2. Machine Pool needed more,
adjuster was not giving it. I set QPFADJ to 0, forced machine
pool with more

memory.

This seemed to help.
What happens when at DST, is it possible machine pool needs more
memory, no way of changing, (to the best of my knowledge), at DST.


Below are additional Raid detail

Calculating RAID-5 and RAID-6 capacity All drives in each RAID-5
or
RAID-6 array must have the same capacity. To calculate the
usable capacity of each RAID-5 array, simply subtract 1 from the
actual number of drives in the array and multiply the result by
the individual drive capacity. For RAID-6, subtract 2 and
multiply by the capacity. Using the latest System i controllers,
the minimum number

of

drives in a single RAID-5 array is three and in a single RAID-6
array is four.
The maximum is the
lesser of 18 or the number of drives that can be placed in the
disk enclosure (12 for a
#5095/0595 or 15 for a #5094/5294).

There is significant performance overhead involved in RAID-6
environments with heavy writes to disk application workload. A
RAID-5 environment requires four physical disk accesses for each
system issued write operation. A RAID-6 environment requires six
physical disk accesses for the single system write operation.
This is because two parity sets are maintained rather than just
one. We discuss this more fully in the next section ("The RAID-5
and RAID-6 Write Penalty"). While it can protect against disk
outages (which should be relatively rare with the SCSI drives
used by the System i, but less

so

for the Serial ATA (SATA) drives where RAID-6 was initially
implemented on other platforms), it does nothing to protect
against disk controller or other component outages. If you need
something greater than
RAID-5 protection, mirroring (with double capacity disks) is
usually preferable to RAID-6 (from both an availability and a
performance perspective).


"RAID levels 3, 4, 5, and 6 all use a similar parity-based
approach

to

data protection. Simple arithmetic is used to maintain an extra
drive
(2 for RAID-6) that contains parity information. In the

implementation

for RAID-5 and RAID-6 from IBM, it is the capacity equivalent of
one or two extra drive(s), and the parity data is physically
striped across multiple drives in the RAID array. If a drive
fails, simple arithmetic is used to reconstruct the missing
data. It is beyond the scope of this paper to discuss this in
more detail. It works. So well in fact, that the RAID-5 is widely used throughout the industry."

The RAID-5 and RAID-6 Write Penalty Parity information is used
to provide disk protection in RAID-5 and
RAID-6 (also in RAID-3 and RAID-4, but because they are not used
on System i and function similarly to RAID-5, we do not discuss
them further). To maintain the parity, whenever data changes on
any of the disks, the parity must be updated. For RAID-5, this requires:
_ An extra read of the data on the drive being updated (to
determine the current data on the drive that will be updated
(changed)). This

is

needed to allow the calculation of any needed parity changes.
_ A read of the "parity information" (to retrieve the current
parity data that will need to be updated), and _ After some
quick math to determine the new parity values, a write of the
updated parity information about the drive containing the parity.
This nets out to 2 extra reads and 1 extra write that all occur

behind

the scenes and are not reported on the standard performance
reports; plus the actual writing of the data-a total of 4 I/O operations.
With RAID-6, these activities also occur, but for 2 sets of
parity

drives.

Therefore, in addition
to the initial data read, there are 2 parity reads and 2 parity
writes, and the actual write of the data. This means three extra
reads, and two extra writes, plus the actual data update for a
total of six I/O operations.
These extra activities have been termed the RAID Write Penalty.

System

i
RAID-5
subsystems reduce/eliminate the write penalty via a true write
cache (not just a write buffer) See Appendix B, "Understanding
disk write cache" on page 69 for more information.
This cache disconnects the physical disk write from the
reporting of the write complete back to the processor. The
physical write

(usually)

occurs after System i has been told that the write has been

performed.

This hides the write penalty (actually write cache allows the
write penalty to overlap with other disk and system activities
so that it usually does not have a performance impact). The
write cache also supports greater levels of disk activity in
mirrored environments as well.

Paul

-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx [mailto:
midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of DrFranken
Sent: Wednesday, March 19, 2014 12:34 PM
To: Midrange Systems Technical Discussion
Subject: Re: RAID 6 rebuild time - can I slash it?

Mostly the size of the drives is the major factor.

Yes data does need to be moved about to allow for the various
RAID stripes so if the system will END UP more than 70% full
then the prepare step will take longer as well. If it's cool to
delete that
130+ GB virtual tape then I would do so as it can't hurt for sure!

- Larry "DrFranken" Bolhuis

www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com

On 3/19/2014 12:29 PM, Jeff Crosby wrote:

All,

On a Saturday of my choosing, our HW service provider is going
to take our System i 520 from 2 8-drive RAID 5 sets to 2
8-drive RAID

6

sets w/hot spare. Yes I'm taking a SAVE21 before.

Building the RAID 6 will be time-consuming. My question is this:
Is the amount of time it will take based on the size of the
*drives*? Or on the amount of *data* on the drives?

I'm asking because we have a virtual tape drive (TAPVRT01) that
is taking up 123GB. If I understand virtual tape correctly, it
cannot be

"shrunk."

I could remove that device (or whatever "piece" of the image
catalog
structure) beforehand and recreate it afterward. I could also
do other cleanup, like old job logs, etc.

But only if it actually saves time. If the RAID 6 build time
is tied to drive size to where a cleanup won't make a
difference, then I won't

bother.

Thanks.

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Jeff Crosby
VP Information Systems
UniPro FoodService/Dilgard
P.O. Box 13369
Ft. Wayne, IN 46868-3369
260-422-7531
www.dilgardfoods.com

The opinions expressed are my own and not necessarily the opinion
of

my

company. Unless I say so.
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Jeff Crosby
VP Information Systems
UniPro FoodService/Dilgard
P.O. Box 13369
Ft. Wayne, IN 46868-3369
260-422-7531
www.dilgardfoods.com

The opinions expressed are my own and not necessarily the opinion
of my company. Unless I say so.
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Evan Harris
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Jeff Crosby
VP Information Systems
UniPro FoodService/Dilgard
P.O. Box 13369
Ft. Wayne, IN 46868-3369
260-422-7531
www.dilgardfoods.com

The opinions expressed are my own and not necessarily the opinion of my company. Unless I say so.
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