RAID 1 Queries

[_SBradley_]

Hi guys!

I'm about to set up my first RAID array. It's going to be pretty primitive: RAID 1 using two 1.5 TB Samsung F2s. I've done a lot of Googling this evening, but I still have a few specific queries that I suspect some knowledgeable people here might be able to answer.

What happens if, when you set up your array, one of the drives already has some data on it (but the other does not)? (Note: this is hypothetical, as my drives will be brand new; I'm just eternally curious!) Will the OS show the data from the single drive, or will it be hidden, as it's not on both? Or will this somehow just not work?

Do the disks in a RAID 1 array remain identical, byte-for-byte? Or are the writes to each disk entirely independent (i.e., the disk controller could put files in different places on each drive)? Related: What happens when you defragment a RAID 1 array? (Is this recommended? Or do weird things happen?)

Finally, are the disks in a RAID 1 array essentially normal disks? Could you take one of them out of the array and put it into a different PC, as though it were never part of the array?

Thanks!

 

[CBJ]

When you set up the RAID array you'll effectively be reformatting both drives, so existing data will be lost. The disks in a RAID 1 array are identical, so all actions including defragmentation will be replicated on both. Taking disks out of a RAID array is not always as easy as it sounds. Don't rely on even being able to read the disks if you transfer both of them to a new system with a different brand of RAID controller, let alone being able to read one disk when used outside the RAID array.

A word of warning about setting up a PC with drives in a RAID 1 array. If you suffer power loss or a hard crash such as a bluescreen error then your RAID controller may decide that it's not sure whether everything was correctly written to both disks. If this happens then it will start a resynchronisation process. This can take a long time and will affect the performance of your system, with the main issue obviously being access to the drives. If Windows itself is installed on the RAID drives then this can make your PC grind almost to a complete standstill during the early stages of the resynchronisation process. On my current PC, which has two 320GB drives in the RAID array, for example the resynchronisation process takes between 2 and 4 hours, with the worst of the slowdown occurring during the start-up that follows the crash and in the 10-20 minutes after that.

The resynchronisation issue has, frankly, been a pain in the neck, and has led to me deciding not to use RAID 1 in any future Windows PC I set up. The RAID 1 array in my NAS box on the other hand has been great, mainly because the NAS box never crashes, and I'd have no qualms about using RAID 1 in a NAS box in future.

 

[miniyazz]

Finally, are the disks in a RAID 1 array essentially normal disks? Could you take one of them out of the array and put it into a different PC, as though it were never part of the array?

AFAIK this is normally doable, but if you write to it odd things will likely happen.. or more likely it'll just assume new drive when you put it back, wipe and resynch.