Is overlapping two RAID5 arrays on same drives a bad idea ??

[Saed]

is placing two RAID5 arrays on disk as shown below Is advisable? Will this create performance problems?

           sda-(500GB)        sdb-(1TB)       sdc-(1TB)         sdd-(1TB) 
              (250MB)----------(250MB) ---------unused------------unused------->(/dev/md0) RAID1
              (470GB)----------(470GB) ---------(500GB)-----------(500GB)------>(/dev/md1) RAID5
               unused----------(500GB) ---------(500GB)-----------(500GB)------>(/dev/md2) RAID5

[tboland]

I would consider it pretty much evil.

While the overlapping RAID5's are an issue, the overlapping RAID1 and both RAID5's on to one disk is the real issue. That's pretty much a no-no. One disk failure (sdb) can now affect all three RAIDs. Overlapping any RAID's onto shared drives is NOT a good idea.

I hope I understand what you seem to be doing - getting the most out of the drives that are not the same size and get some RAID redundancy at the same time.

The figures you gave for sda and sdb don't add up. Please check on that and get back to us.

RAID's (other than RAID0) are not created to maximize space. They are created to increase reliability as cheaply as possible. If you want to get every byte you can out of these things, combine them into one big volume and back them up. A lot. And plan for a total failure within 2 years.

If you want to get some true RAID consistency and security, I would suggest combining them all into one RAID5 and then creating volumes and partitions inside Linux.

If your RAID setup does not allow using mixed drive sizes, I would consider making the smallest drive a standalone for boot and system only, and RAID5 the three 1TB drives together for data, then partition as needed. In this setup, you would end up with 500GB for system use and somewhere around 1.8TB of usable RAID storage.

[Saed]

hi tboland ,

I would consider making the smallest drive a standalone for boot and system only, and RAID5 the three 1TB drives together for data, then partition as needed. In this setup, you would end up with 500GB for system use and somewhere around 1.8TB of usable RAID storage.

that indeed what i did at the first place , but as i wanted my system to survive a worst HW failure Scenario , i needed to lay the /boot on a RAID1 Array , which lead me to a question "how to make use of the remaining space on the sda (500GB) ?
thanks alot for helpin tboland