Ken Gramm wrote:
Well I can tell you from first hand experience, if all of the drive were purchased at the same time, have all been running in the same machine for the same amount of time, and have all been accessed around the same amount of times, your chances of a second drive failing during a rebuild is very high. Especially if you are getting close to the hard drives MTBF.
My thoughts exactly. Which is why I'm concerned. I've been watching this thread for a while and it definitely seems to me
Per Jessen wrote: that you are looking for an immediate backup to prevent data loss. RAID is not a backup method, it is a method of storing data. You are going to have a drive failure and recovering from it as quick as possible is the key. I would suggest that you write all your data onto RAID 5 array. The data is then mirrored on another disk which is backed up to tape or optical media. The problem is that with what I have suggested ie that all the disks would be bought at the same time and hence have same MTBF as they are akk used equally. The time taken to write to the mirror disk and then to the optical device could result in data loss so it does not provide the immediate backup solution you seek. Trying to find a solution by using RAID as your backup medium is not going to work as all the disks used will all have the same MTBF. A possible trick to thwart this would be to copy onto flash memory but unless you can have a mass of them connected as a single logical volume it cannot be done. Given theat disks are going to fail perhaps the only solution is to use a rolling RAID 5 +1 array by adding disks into and removing them from the RAID array. I have as an exaple used a 3 disk RAID 5 array but you could quite possibly use a RAID array of 200 disks. Start with 4 disks, 3 in RAID 5 and the 4th as the mirror drive of the data on the RAID array. A month down the line take out one of the RAID drives, as though it had failed and replace it with a new one, store the 'faulty' disk for use later, the same way that a backup tape is stored before its MTBF point. The following month buy another disk and again replace one of the RAID disks ie the mirrored one with the new one. Again store the 'failed' drive as before. On month three 'fail' one of the drives and replace it with the 'failed' drive you took out on month 1. And so on etc..etc.... The key is rotation and to backup from the mirrored drive as it will be the most up-to-date copy of the data sitting on the RAID array. This is the only way I can see that your data will be safe from dual RAID drive failures. If that were to happen you could rebuild the RAID array from the mirrored drive as it is a copy of the data on the RAID array. If that too fails as it will without rotation, you will have to resort to your backup media. HiH -- ======================================================================== Hylton Conacher - Linux user # 229959 at http://counter.li.org Currently using SuSE 9.2 Professional with KDE ========================================================================