JBOB with RAM Disk & HDD together

[Savage1701]

I would like to JBOD a 4GB i-RAM drive and a SATA I HDD drive together. Is there any way to control which of the 2 drives is written to first? In effect, I would like to use the 4GB i-RAM drive as a huge cache in tandem with the HDD so that info is written to & read from the i-RAM first. Can that be done under Win XP Pro? Thanks.

 

[DelJo63]

Not likely as the devices need to be on the same controller.

What you describe is known as a Write-Thru cache and takes special software to implement.

 

[Savage1701]

I assumed that when I said "JBOD" it meant it would be on its own controller. Technically, I am seeking to create a "write back cache" where the i-RAM would fill up first, not a "write through cache" as you describe. As long as the files I am working with do not exceed 4GB in size, they would, in effect, never be "flushed" to the mechanical HD, or at worst, the i-RAM would stand as a monster cache between the I/O and the mechanical hard drive.

So, my question stands: Is there any way of knowing which drive will be filled up first in a JBOD array on a seperate controller? E.G., on a 2-channel controller, does channel 1 always fill up first before proceeding to the drive attached to channel 2? Or is is purely random? I suspect there has to be some sort of ordering algorithim on these controllers for the case of JBOD so that all drives in the JBOD array are filled.

 

[DelJo63]

JBOD is not the same as Raid-0 spanning (or Logical Volume Support).

As I understand, each is accessed as if it were independently attached and mounted
via /etc/fstab.

 

[Savage1701]

from the Wikipedia:

Concatenation (JBOD or SPAN)
JBOD stands for Just a Bunch of Disks. The controller treats each drive as a stand-alone disk, therefore each drive is an independent logical drive. JBOD does not provide data redundancy.


Diagram of a JBOD setup with 3 unequally-sized disksConcatenation or Spanning of disks is not one of the numbered RAID levels, but it is a popular method for combining multiple physical disk drives into a single virtual disk. It provides no data redundancy. As the name implies, disks are merely concatenated together, end to beginning, so they appear to be a single large disk.

the concantenation diagram is located at: http://en.wikipedia.org/wiki/Standard_RAID_levels

Look at the diagram and you will see the crux of my question - can I control which drive the concantenation begins at? Look at the RAID 0 diagram - that's exactly what I don't want - splitting the data equally across the drives, and having my mechanical HDD reduced in size to that of the i-RAM.

So, back to my original question - can one glean/control how a RAID card decides what drive gets filled first, and what drive gets filled second? If I can control that, I get the equal of a 4GB cache between the system and the mechanical drive. I understand that at some point the 4GB space fills up, and I go to mechanical storage area, but that is ok. The point is to maximize the speed of what I am doing at moment.

 

[DelJo63]

great; thanks for the feedback. JBOD creates a Logical Volume