RAID 0 with 2 different size hd's?

Is it possible to make a raid 0 with 2 different hard drives with different capacities? 150 gig and 70 gig both 10,000rpms.

Rule of thumb with raid is that if you connect 2 dissimilar drives to a raid array the entire array will run at the speed of the lowest performance drive.

So if you hooked up a 70 gig, 5200 rpm drive with 4 meg of cache up to an array with a 300 gig, 10,000 rpm 16 me of chache drive the entire thing would run at the latency of the 5200 rpm drive and would be no larger than 70 gig.

That's if, of course, you have a decent controller. Some controllers only support synchronous drives.

Rule of thumb with raid is that if you connect 2 dissimilar drives to a raid array the entire array will run at the speed of the lowest performance drive.

So if you hooked up a 70 gig, 5200 rpm drive with 4 meg of cache up to an array with a 300 gig, 10,000 rpm 16 me of chache drive the entire thing would run at the latency of the 5200 rpm drive and would be no larger than 70 gig.

That's if, of course, you have a decent controller. Some controllers only support synchronous drives.

so it goes by speed and not by size?

Basically, how a hard drive access works is an application calls an OS function to pull data off of a section off of a harddrive (and do whatever with it). The OS Function then translates this message to an LBA address by looking at it's file to lba address table (called a master file table for short), then it calls a driver routine with those addresses, which then calls and executes a bios routine. The BIOS Routine sends a coded message to the I/O address of the hard drive controller stating it wants a specific LBA address or LBA address range, and then the harddrive geometry controller translates that into a datastream and pulls the data physically off of the drive.

How RAID works is it inserts a physical controller between the harddrive controller and the drive itself. So whereas:

Processor->software->harddrive controller (motherboard)-> Harddrive Geomtery controller (board on the harddrive itself)

RAID works this way:

Processor->Software->RAID Controller->1 or more harddrive controllers-> harddrive geometry controller(s).

The controller is configured to have an array of drives in a specific configuration. For example, in RAID 0 with 2 drives, one drive is assigned even LBA addresses and the other Odd (or even clusters and odd clusters), so when a read or write command for a swath of the harddrive comes along, the controller splits/reassembles data as it goes back and forth to and from the drive.

In RAID 1 the controller duplexes the command (basically, it sends the commands to both drives); write commands are sent to both drives and when a read command comes in, it's data is checksummed and compaired before being sent along.

RAID goes by BOTH speed AND size.

If you have a 200GB disk and a 100GB disk in RAID1 (mirroring), the maximum size of the RAID disk will be 100GB. Why? Because you can only copy the data to both drives once. Moreover, once a drive is in an array, all the space on it is considered property of that array and the array is treated as one continuous hard drive. So there is no way to get the 100GB of wasted space to be usable without upgrading the array.

Similarly, the RAID controller configures itself to the speed of the hard drive controller can handle, and the harddrive controller to the speed the drive can handle. The RAID controller knows the maximum speed of the hardware under it as well as latency and other characteristics and will automatically adjust itself to the slowest speed.

So if your 200GB drive ran at 10,000RPM and was ATA133 compatable with 8.3MS seek time, and your 100GB drive ran at 5200RPM and ATA66 speeds with 15MS seektime, then you'd be stuck at the 100GB drives speed. Meaning, all drives on the array run at that speed. If you had a RAID 10 array with nice shiney 100GB SCSI3 Fibre channel running at 640MBPS and you installed an ATA100 drive onto it, every drive would run at ATA100 speeds.

You CAN create a RAID0 array out of those two drives, but you'd be limited to 140GB and the seek time/data transfer rate of the slower drive.

Your array will be 140 gigs unless your controller does JBOD.

dont listen to this confusing mess of copy/paste.