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Hi, I'm developing a simple "stacking" linux block device driver. In it's simplest form, this driver will only pass all requests to the "target" device by changing the bi_bdev on ...
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  1. #1
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    Ultra slow write speed with custom linux "stacking" block device drive


    Hi,

    I'm developing a simple "stacking" linux block device driver. In it's simplest form, this driver will only pass all requests to the "target" device by changing the bi_bdev on the bio.

    The driver uses the "make_request" to process incoming requests as opposed with having a standard request queue set up.

    The problem is that write performance using this setup makes it unusable; performance is about 10% of the target device. Read performance seem mostly unaffected.

    In concept, this is pretty much the same as the MD or DM drivers in linux, but with a "noop" personality; all requests are simply passed to one single target device, always. Of course MD/DM don't suffer from the write performance problem I'm describing, so I want to find out what I'm doing wrong.

    Here's the simplest driver I could write that exposes the problem:

    Code:
    https://github.com/wegel/passthrough
    It's been tested against 2.6.38. To try it, make sure you define TARGET_NAME to an existing device. Once loaded, /dev/passthrough will appear. To quickly test write speed, I do:

    WARNING: the following will effectively write zeroes to your target device, destroying it's MBR and partitions. Don't use a target device on which you have actual data!!
    Code:
    sudo dd if=/dev/zero of=/dev/passthrough bs=1024 count=10000
    Please let me know if you have any idea what I'm doing wrong, or if you have a suggestion on where to post my question to get a better chance to have it answered. Thanks!

  2. #2
    Linux Guru Rubberman's Avatar
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    You should probably move this to the kernel forum.
    Sometimes, real fast is almost as good as real time.
    Just remember, Semper Gumbi - always be flexible!

  3. #3
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    Oh right... Could a Mod please move this thread to Kernel? Thanks and sorry about that.

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