Expanding RAID 1 + 0 on a HP Proliant ML350 G4

Posted on 2006-05-23
Last Modified: 2008-01-09
What's the best way to expand storage space on a HP Proliant ML 350 G4 with a RAID 1 + 0.  It's currently only listing two physical drives which is a bit confusing, because I thought you need four for RAID 1 + 0.  I copied this from Array Configuration Utility  Thanks!

Controller  Smart Array 641 Controller
Bus Interface  64-Bit PCI  
Controller Location  Slot 3
RAID ADG status  Disabled  
Hardware Revision  B
Firmware Version  2.26
Rebuild Priority  Low  
Expand Priority  Low  
Current Surface Scan Delay  15 sec
Number of Arrays  1
Number of Logical Drives  1
Number of Physical Drives  2
Physical Drives Attached to Port 1 SCSI ID 0, 72.8 GB (Parallel SCSI)
SCSI ID 1, 72.8 GB (Parallel SCSI)
All Physical Drives Assigned  Yes  
Array Accelerator  
Present  Yes  
Cache Status  Enabled  
Accelerator Ratio  100% Read /0% Write  
Total Memory on Controller  64 MB  
Battery Pack Count  0
Question by:andrewjwd
    LVL 9

    Accepted Solution

    Technically you do need more than two drives for RAID 1+0 but it's just terminology, functionality is the same, they also call a single drive on it's own RAID 0 which isn't quite correct.

    Add your two new drives (can do that when powered on) and go into the ACU, click on configuration wizards and there should be an Expand Drive Array wizard, it will ask you which unassigned drives to add to the array and the rest is obvious. You can also click on the ? in the top right to get the help menu and under arrays: how to is expand drive array help with a flash demo that I've never been able to get to work.
    LVL 44

    Expert Comment

    A RAID 0 + 1 = RAID 10 needs 4 drives, unless you partition them, and that is not a good idea.  In fact, I am recommending RAID 0 + RAID 1 as a more secure alternative to RAID 5 now, since Ive had so much problems with RAID 5.  In a RAID 10, the first 2 drives are a RAID 0 integrated pair, and the second pair of drives, 3 and 4 are an integrated pair that is a MIRROR RAID 1 of the first two.  It is VERY stable, it is very resistant to failure, and you should be fine with it.  But in the end, you should see only ONE physical drive C out of all thes 4 drives, because the RAID 0 and 1 are the duplicates of C drive for the system -- i.e. 4 drives makes ONE "C:" drive and you are good to go with this setup.
    LVL 9

    Expert Comment

    You have that backwards Scratchy, the drives are mirrored before they are striped. Doesn't matter much with 4 disks but for example if there were 28 of them and you striped before mirroring you would end up with a 14 drive RAID0 array should any disk fail; then a subsequent failure on any of those 14 disks in the remaining stripe would bring the system down. Mirroring before striping is done so that if one drive fails you still have 13 mirrored pairs plus an unmirrored disk in the stripe so only if the disk that is unmirrored fails does the array die, any of the other 26 disks can fail without the array breaking. Much more fault tollerant and there are also performance benefits when one disk is down since either disk in the remaining mirrored pairs can perform reads.

    Admittedly it is confusing since Compaq called it RAID 0+1 and then HP took them over and called exactly the same algorithm RAID 1+0.
    LVL 44

    Expert Comment

    I dont think so, I have been using RAID 0 mirrored to RAID 1 flawlessly and the failover automatically kicks in.  Maybe you heve never tried it, therefore you dont know.  There is nothing confusing about my comment and success in 0 + 1, just because you dont know about it is no reason to criticize what you have not tried.
    LVL 9

    Expert Comment

    It's not something you can try with the Smart Array controller that is in a Proliant, you do not have the option to decide if you want the mirroring before the striping or vice-versa; you simply tell it which disks to use and it creates the array for you.

    I have verified the algorithm mirrors before striping by removing several disks from a RAID 10 array on a training course using test kit. Not something you would do with production kit since it increases the failure count on the disks and they start complaining to Insight Manager after being failed 3 times. The algorithm that decides which drive mirrors which other one is quite clever, it will mirror disks across SCSI channels if it is possible so if you have two enclosures with 14 disks in each it can tolerate a complete enclosure failure. I have not been able to independantly verify that it will maintain SCSI channel independance when hot spares kick in after a failure but HP say it will if it can.

    If you still don't believe me then look at page 44 of the user guide at - it says "When RAID 1+0 is used, drives are mirrored in pairs. Several drives can be in a failed condition simultaneously (and they can all be replaced
    simultaneously) without data loss, as long as no two failed drives belong
    to the same mirrored pair."

    It was not your comment I was referring to as being confusing but HP's terminology, both DEC and Compaq referred to RAID 10 mirroring before striping as 'RAID 0+1' but then after HP - Compaq merger HP went through all the old paperwork and changed the documentation to read 'RAID 1+0' where originally it said 0+1.
    LVL 44

    Expert Comment

    my comment was generally about RAID 0-1, not specifically about the HP Proliant in this question.  Knowing HP, they probably DO only allow one way, HP tends to think only one way, especially on RAID.  The most problems I have ever had with a RAID 5 was on an HP Netserver LP2000r where the RAID BIOS could not rebuild the 5 array if you just temp. removed one drive, booted, shut down, reinstered the drive, and rebooted.  It could not recover from this simple act !!!  SO in general, raid 0 works mirrored to raid 1, but in the case of this and other HP raid controllers, I dont doubt they have severe limitations.  ;)
    LVL 9

    Expert Comment

    It is never good to return a failed drive to the same array controller without wiping it if it can be avioded, I have had similar problems with RAID controllers not knowing which of the disks to believe have the most up to date data. Most controllers need you to go into diags and tell them it's OK to overwrite the possibly valuable data on the disk that's been put back although they will automatically rebuild if it's a new or clean disk that's inserted. The same problem will happen for andrewjwd if his extra two disks came from an old server with data on them.

    I had to attend site once where the customer claimed the server I configured for him had a faulty RAID controller only to find that he had filled it with old un-erased disks and the controller found bits of one array mixed with bits of others and of course it didn't know how to continue any more than I did until I asked the customer which disks he wanted to keep the data from.

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