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Thread: Swap file tweaking with multiple hard drives

  1. #21
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    Quote Originally Posted by Darth Brooks View Post
    Why not RAID0? Not trying to say it's wrong, I just want to know why. I could definitely understand redundancy arrays like a RAID1 or even 5. But I would have thought a RAID 0 array would give a little more throughput to a bandwidth starved process.
    If you have several swap areas on different drives they are all used at the same time. The OS knows which ones are on several disks.

    Let's say it want to page out 1000 page (4 MB):
    - then it pushes page 1,3,5, 999 to disk 1
    - page 2.4...1000 go to disk 2

    (except it isn't this sharp, random block can go to random locations, but the point is, both disks are use the same way, unless they have different priorities).

    Obviously, this is very fast as both disks work in parallel. While one is seeking and writing the other one can also seek and write without waiting.

    If you put swapspace in one area on a rwo-disk raid0 instead of two areas on two disks then the OS has no idea which block in the paging area is located on which drive and it essentially becomes random. And you will inevitably end up loading one disk more than the other and hence cause a slowdown compared to two plain areas.

    The above is ten times worse on Windows than on typical Linux/FreeBSD installations since Windows usually has swapspace in files, not partitions. On swap partitions you might accidentally end up with a pattern that load both disks in raid-0 about even, but if it is in files in a filesystem it is entirely thrown off.

  2. #22
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    Ok. I would buy that. Thanks for the explain.

    But, lets say you have 2 or 3 different RAID 0 arrays. One would be the main array with your OS partition. Then on each subsequent array, you make a 2 or 4 GB partition and set a paging file to each. Then you leave a small one on the C: drive for the kernel debugging thing.

    Would that still fall under what you pointed out?

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    Quote Originally Posted by Darth Brooks View Post
    Ok. I would buy that. Thanks for the explain.

    But, lets say you have 2 or 3 different RAID 0 arrays. One would be the main array with your OS partition. Then on each subsequent array, you make a 2 or 4 GB partition and set a paging file to each. Then you leave a small one on the C: drive for the kernel debugging thing.

    Would that still fall under what you pointed out?
    2 o 3 different raid-0 sets over 4-6 or more disks would be much better than one raid-0 over 4-6 disks, as far as paging space is concerned.

    What you really want is partitioning below the raid levels so that you can keep the paging space unraided and the filesystem space raided.

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    Very interesting. Although not done easily with low-end onboard RAID controllers. Thanks again. Good info.

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    Quote Originally Posted by davidhammock200 View Post
    Page/swap file size should be 2X to 3X the size of usable system RAM, not to exceed 4GB, per physical drive.
    Hi David,

    This 2x to 3x advice is a leftover from the days of systems with 128 MB max RAM. Actually the more RAM you have the less swap space you need. Of course this amount change for different usage patterns. For most people with 1 GB or more RAM a pagefile of 1 GB is more than enough.

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