Again about "Mixed drive size in Auto Raid 12H"

[Massimo Carozzo]

Looking at posted message I noticed the problem posted by John Love and the interesting answers coming from Thierry Poels, Andreas Voss, Mark Mitchell and Bill McNamara.
I have a similar problem with an 12H with four 4.3 GB disk with hot spare enabled and, despite we have a small amount of data in our Oracle database (roughtly 6 GB), only 1 GB of disk space remains free and it is not sufficent.
Our consultants proposed solution is to add four (or six) 18.2 GB disk and, because I have a limited downtime possibility, I should like do not have suprises during this upgrade.
I have some doubts about:
1) I guess that installing the first 18.2 GB disk this will be immediately and automatically elected as the new hot spare disk and AutoRaid will balance the existing data between the four old 4.3 GB disk. Is it correct?
2) In which way it is necessary proceed adding and configuring the other three (or five) new disk to mantain the possibility than AutoRaid switches between RAID 5 and RAID 1 (if it still remains possible after upgrade) in consequence of the increased RAID capacity?

Thanks and regards.
Massimo.

[Bill McNAMARA_1]

See here for capacity planning.

http://www.hp.com/essd/capacity.html

The autoraid does not use a fixed disk approach to RAID. You'll notice that when you read from one of the autoraid luns that all leds will blink on the array.. thus the autoraid spreads the lun over every disk in the array. When you add that 18G disk, it's true that Host Spare requirement will increase to 18G seeing as that is the largest disk that can now fail, but it won't happen until you add 2 18G drives.

What the host spare is on autoraid is active. which means that if a disk fails the autoraid will rebuild the data into a redundant state by pushing data down into R5, by doing this the autoraid calculates that the ammount of data storage saved in R5 is the same as the disk that has failed.

ie:
imagine
4 disks in R10
(even though autoraid isn't fixed disk)
Each disk is 10G
that means 20G data and 20G mirror

If a disk fails
3 remain
push the data into R5
is 20G data and 10G parity
the data is back to N+1 state but in R5.

By moving to R5 we have saved 10G of data.
Think of this in reverse.
Fixed disk raid system,
20G data and 10G parity in R5 fixed.
10G hot spare doing nothing until a disk fails.
disk fails 20G data + 10G parity after rebuild.

The autoraid is making sense. Use the disk, it's not going to fail every week.

You might want to turn auto include off so that you can include the disks you insert later because on insertion the autoraid will redistribute data across the new disks which will take up significant io and slow down normal usage, although previously you probably don't have all that much data there with only 4G drives.

Important thing to note on the autoraid is that you must never remove 2 disks at once.
Or even remove 1 when the array state from arraydsp -a is in anything other than READY.

Mixed drive sizes are okay, just make sure too that your 18G drives if they're 10krpm, they need 10krpm fan modules (bubble plastic on the fan module not like the 7200rpm fan sticker)

Hope that helps a bit...
Later,
Bill

[Bill McNAMARA_1]

I think this question would be better in the storage forum.. Can somebody push it over!
Those guys in particular, Bob Inglis...

Later,
Bill

[Massimo Carozzo]

Thank Bill.
I guess that if I were less ignorant about the way mirroring works I could understand better your example.
Herebelow what you wrote:

------------------Begin
imagine
4 disks in R10
(even though autoraid isn't fixed disk)
Each disk is 10G
that means 20G data and 20G mirror
------------------End

I thought that "mirror" means "the same data written exactly twice" so if I have, let say, 18 GB of data and one mirror disk fail you say that passing to R5 the only 10GB mirror remaining disk is sufficent to allocate 18 GB of recovery info. Is it correct?

As regard the opportunity to submit my question to another forum may I simply repost there or repost addressing (how?) to the gentlemen you mentioned?
Thanks againg.
Massimo.

[Bill McNAMARA_1 · Accepted solution]

Yes R5 will store 4 disks worth of R10 on 3 disks. The autoraid will do this when a disk fails. R5 is N+1, when a mirror disk in a R10 group fails your data is not N+1.
I've attached a little picture!
Hope you like it!

The reason though you'd prefer not to be in RAID 5 is because for every bit of data you read, you must read all equivalent bits recalculate parity and write it down.


Parity is based on XOR

A B XOR
0 0 0
0 1 1
1 0 1
1 1 0


So consider 2 disks
with the following data

1011 0101
parity would be 1110 which would be written on the parity drive.
Imagine one of the disks fails, we need to do the same parity calculation to find out what went missing. With Mirroring the procedure is much more straight forward, you know what went missing because your still have the other half of the mirror... but disadvantage is that you use up a lot of disk space


To get to Bob!, Click on Forums >
Then select the storage -> disk array section although Bob is sometimes around here too.
Sometimes someone, don't know who can push questions into the more relevant section.

Bill

[Bill McNAMARA_1]

I forgot my silly picture! :)

Bill