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JBOB with RAM Disk & HDD together

By Savage1701
Aug 13, 2008
  1. 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.
     
  2. jobeard

    jobeard TS Ambassador Posts: 9,317   +618

    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.
     
  3. Savage1701

    Savage1701 TS Enthusiast Topic Starter Posts: 153

    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.
     
  4. jobeard

    jobeard TS Ambassador Posts: 9,317   +618

    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.
     
  5. Savage1701

    Savage1701 TS Enthusiast Topic Starter Posts: 153

    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.
     
  6. jobeard

    jobeard TS Ambassador Posts: 9,317   +618

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