View Full Version : MIRROR and/or RAID-5
07-02-2001, 01:00 PM
You can't do both with the same disk unit. Raid or Mirror depends on the necessity of keeping your system running in case of a disk outage. Mirror will keep you running, RAID won't but you should be able to recover your data. It's a matter of availability and cost.
07-02-2001, 01:38 PM
Bill, You can continue operating with a disk failure in a RAID environment. I believe that with the 720 you can run indefinitely with a failed drive, but you would need to bring the system down to replace the failed drive. Failure of more than one drive within a RAID set would result in a complete failure and would require a full reload. The term 'mirrored' in the ASP status column indicates that the ASP is protected and could be either mirrored or RAIDed. You may have a serious problem Bill. If all of your drives are in RAID sets but the status of the ASP in unprotected, then the logical assumption is that one of your drives has failed. Remember that your RAID set can tolerate the loss of only one drive. If you lose one, you are no longer protected, if you lose another within that set, you are toast and need to get your tapes. Check it out ASAP. Your 'Backup and Recovery Manual' is your best resource for interpreting the disk status displays. Andy
07-13-2001, 11:29 AM
If any of the drives do say unprotected there probably is another one out there that says failed. And in the 720's Some of them I am sure you can replace a drive concurrently, and it will rebuild and run in the background, you will see a performance hit because of it but I can think of worse things to happen when it comes to bad drives.....
07-17-2001, 10:35 PM
Bill, I know this reply is late, but in case anyone else looks in this thread, I have to set some things straight. RAID and Mirroring can co-exist. You can have RAID only, or Mirroring only, or both working together. It is just a matter of how much protection you need (and are willing to pay for). If you see DPY/Active for all your disk units, it means Raid is working and your disks are fine. If it says DPY/something else, then you have problems. If you see status 'Unprotected' on the ASP line (where the entries for disk units are blank), it means you do not have Mirroring. If the status says 'Mirrored'... Should you do both? Probably not. Here are my personal views on this subject: 1. RAID is cost-effective and good enough for most installations. 2. Mirroring offers superior protection and performance, but is much more expensive. 3. Mirroring and RAID are a great team. If you get Mirroring, you might as well pay the incremental cost to add-on RAID as well. With RAID, you are protected against single disk failures. With Mirroring, you are protected against multiple disk failures as long as they occur within the same "mirrored" set. With RAID and Mirroring, the only way your system will go down is when a parity failure (meaning: failure of at least 2 disks in the same parity set) occurs in both mirrored sets. So what are the chances of 4 disk units failing at the same time?
08-02-2001, 04:59 AM
I was always under the assumption that mirroring ment you loose half of your disk drive space. I recently setup an as400 and followed the backup and recovery book. You set parity on the drives and then mirror the ASP. I didn't loose half the disk space. It's not that I'm not happy to have the extra space but I don't understand why I didn't loose it. Thanks for any help.
08-02-2001, 05:23 AM
When you setup Mirroring, you are telling the AS/400 to store the exact same data on 2 seperate disk drives at the same time. That way if you loose one disk drive, the system continues processing with the data on the other drive. You effectively loose half your disk space because the OS is using 2 disk drives to hold each byte. In some cases, you may actually see a performance improvement since the system will decide to access the data on the disk unit it can get to fastest, based on all sorts of things, like, one drive is busy retreiving data or read head is closest to requested sector...
08-05-2001, 08:27 PM
I have 200 AS/400s (model 720s). When I do STRSST (Service Tools), option 3 (Work with Disk Units), then option 1 (display Disk configuration), then option 1 (display disk configuration status), I get a display that shows all my drives with a status of DPY/Active (Raid-5). I only have 1 ASP for all of my disk drives and the status for the ASP shows Mirrored on some AS/400s and Unprotected for others. My first question is: Does the "Unprotected" status mean only that I am not mirroring the drives in the ASP or indicating I have a problem? Again, all of the drives show DPY/Active so RAID-5 is in effect for all of the drives. Secondly: Does the "Mirrored" status mean I am doing both mirroring and RAID-5? I understand the difference between RAID-5 and Mirroring, but should I be doing both?
08-05-2001, 08:27 PM
Hi Tim, Sorry I took so long to respond. I saw your question last week, but was too dazed at the time to respond. Anyway, during that time I was running a DRP test and got to actually test mirroring. Mirroring does take half of your available disk space. But it all depends on how much mirroing was implemented. In my tests, I discovered that within an AS/400, you cannot implement Mirroring and RAID on the same disk unit. Beyond this restriction, Mirroring and RAID are still compatible. For example, in my test AS/400, there were some disks that do not support RAID. These same disks were eventually mirrored and I lost half the capacity of their disks. However, the RAID enabled disks were not affected. The above example may have happended to you as well. Or alternatively, you may have mirrored only your Load Source (I think this capability depends on your AS/400 model and OS Release). I would like to take this opportunity to apologize for not verifying before-hand my claim that RAID and Mirroring will work on the same disks. Apparently, this is true only if the ASP(s) or LPAR(s) are mirrored in a separate AS/400.
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