Re: SCSI GENERIC command queueing for block storage is unstable.

[Robert Hancock <hancockrwd@gmail.com>]

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 2.6.22.1-32.fc6 #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?