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Re: [libvirt] [PATCHv2 11/16] save: let iohelper work on O_DIRECT fds



On Tue, Jul 19, 2011 at 10:20:34PM -0600, Eric Blake wrote:
> Required for a coming patch where iohelper will operate on O_DIRECT
> fds.  There, the user-space memory must be aligned to file system
> boundaries (at least 512, but using page-aligned works better, and
> some file systems prefer 64k).  Made tougher by the fact that
> VIR_ALLOC won't work on void *, but posix_memalign won't work on
> char * and isn't available everywhere.
> 
> This patch makes some simplifying assumptions - namely, output
> to an O_DIRECT fd will only be attempted on an empty seekable
> file (hence, no need to worry about preserving existing data
> on a partial block, and ftruncate will work to undo the effects
> of having to round up the size of the last block written), and
> input from an O_DIRECT fd will only be attempted on a complete
> seekable file with the only possible short read at EOF.
> 
> * configure.ac (AC_CHECK_FUNCS_ONCE): Check for posix_memalign.
> * src/util/iohelper.c (runIO): Use aligned memory, and handle
> quirks of O_DIRECT on last write.
> ---
> 
> v2: merge patch 6 and 15 of v1.
> Sorry, I didn't add a testsuite of this yet, but agree that
> it would be a nice project
> 
>  configure.ac        |    6 ++--
>  src/util/iohelper.c |   52 ++++++++++++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 52 insertions(+), 6 deletions(-)

ACK

Daniel
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