Raid 1 & 5

[PCBrandon]

Hi all,

We have a server with two containers, one with two SCSI drives in a RAID 1 and the other with four SCSI drives in a RAID 5.

RAID 1 will store the OS and software. The files shouldn't get corrupted on either one since they are mirroring each other, correct? If a drive should fail, hardware wise, I should be able to pop in a brand new SCSI drive and the existing drive mirror its data onto the new drive? All the while everything still working fine on the one existing drive?

RAID 5 will store the data. Am I correct in thinking it strikes portions of the files across three of the four drives, and if one of the three drives dies the other drives copy the files onto the fourth drive, allowing us to replace the drive that died? If two or more die, we are pretty much screwed, right?


Thanks for the help!

[doctorgonzo]

Yes, depending on the RAID controller, you should be able to dynamically rebuild the RAID 1 array after the failure automatically and without any downtime.

With RAID 5, if one drive dies you will still be able to get the data from it. If two drives die simultaneously, then you won't. So after the first drive dies, replace it and rebuild the array as soon as you can. Some RAID controllers allow for "hot spares", drives that sit there blank until a drive fails, then the controller activates the spare and starts rebuilding the array right away. That reduces the chances of seeing two drives go bad simultaneously.

[PCBrandon]

Yes, I believe that is what ours does, for the RAID 5. The container only had 101 GB, and four 36 gig drives should add up to be around 144 gigs. So, I think it is not using one of the discs until another gets fried.

Thanks for the reply!

[doctorgonzo]

Yes, I believe that is what ours does, for the RAID 5. The container only had 101 GB, and four 36 gig drives should add up to be around 144 gigs. So, I think it is not using one of the discs until another gets fried.

No, there's a difference between having a hot spare and the data you lose to the array. With RAID 5, you lose one disk's worth of space. Since you have four 36 GB disks, that means you will have 108 GB of usable space spread across all four disks (25% of that data is the parity data). Thus, you do not have a hot spare. If you had a hot spare, you would only have 72 GB of space on the array: the array would be made up of three disks, you would disk's worth of space on the array, leaving you with 72 GB, and the fourth drive would not be doing anything.

[colecifer]

Hi all,
. . .RAID 1 will store the OS and software. The files shouldn't get corrupted on either one since they are mirroring each other, correct? If a drive should fail, hardware wise, I should be able to pop in a brand new SCSI drive and the existing drive mirror its data onto the new drive? All the while everything still working fine on the one existing drive?
. . .

Thats mainly correct except not exact on corruption. If one drive starts corrupting data as it writes the second drive will copy the corruption and it does you no good. But if the data is already on both drives and then one gets corrupted i believe it would safeguard against that.

[glc]

Raid 1 does not protect you from corruption, it only protects against drive failure.