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    SubjectRe: SCSI GENERIC command queueing for block storage is unstable.
    On 03/15/2010 12:25 PM, Mike Hayward wrote:
    > After discovering that O_NONBLOCK reads and writes were actually
    > blocking calls, I attempted to use the SCSI generic driver for
    > nonblocking io. The good news is that it is nonblocking; the bad news
    > is that it is not dependable in any of the systems I have tested with.
    > Does anyone know if these defects have been fixed in later kernels?
    > 1. When queueing, write can occassionally return errno 12 (ENOMEM, Cannot
    > allocate memory). This is documented in the SCSI GENERIC HOWTO,
    > however only for indirect io and it says extremely rare. I can cause
    > it easily within a few hours and it can return even for direct io when
    > no io's are queued and 80% of the ram is free or in buffer cache. The
    > fd polls as available for writing, but retrying never clears the error
    > and the fd is no longer usable. This is a complete show stopper.
    > Linux #1 SMP Wed Aug 1 14:30:16 EDT 2007 x86_64 x86_64 x86_64 GNU/Linux

    First off, have you tested any of these problems against a newer kernel?

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