----- Original Message -----
> From: "Kevin Wolf" <kwolf@xxxxxxxxxx>
> To: "Avi Kivity" <avi@xxxxxxxxxx>
> Cc: "Federico Simoncelli" <fsimonce@xxxxxxxxxx>, kvm@xxxxxxxxxxxxxxx
> Sent: Wednesday, June 15, 2011 12:50:21 PM
> Subject: Re: [PATCH] qemu-img: Add nocache command line option
> Am 14.06.2011 15:13, schrieb Avi Kivity:
> > On 06/14/2011 03:09 PM, Federico Simoncelli wrote:
> >> qemu-img currently writes images using writeback and filling up
> >> the cache buffers which are then flushed by the kernel preventing
> >> other processes from accessing the storage.
> >> This is particularly bad in cluster environments where time-based
> >> algorithms might be in place and accessing the storage within
> >> certain timeouts is critical.
> >> This patch adds the option to use nocache when other solutions
> >> (eg: cgroups) are not available.
>
> Your patch confuses a write-through cache (cache=writethrough ==
> O_SYNC)
> with bypassing the cache (cache=none == O_DIRECT), which are entirely
> different.
I know the difference, how am I confusing them in the patch?
I am using BDRV_O_NOCACHE which is translated to O_DIRECT, which is
what I wanted to use (as per commit message: nocache).
Maybe I overlooked something, can you point me to the code parts you
suppose are wrong?
> > Please copy qemu-devel@xxxxxxxxxx as this is not a kvm-specific
> > patch.
> >
> >> diff --git a/qemu-img.c b/qemu-img.c
> >> index 4f162d1..c10e4a6 100644
> >> --- a/qemu-img.c
> >> +++ b/qemu-img.c
> >> @@ -78,6 +78,7 @@ static void help(void)
> >> " rebasing in this case (useful for renaming the
> >> backing file)\n"
> >> " '-h' with or without a command shows this help and
> >> lists the supported formats\n"
> >> " '-p' show progress of command (only certain
> >> commands)\n"
> >> + " '-t' use write-through (no cache), valid with: convert, commit
> >> and rebase\n"
> >> "\n"
> >> "Parameters to snapshot subcommand:\n"
> >> " 'snapshot' is the name of the snapshot to create,
> >> apply or delete\n"
> >
> > IMO better to use the existing qemu option format, say -o
> > cache=writeback|writethrough|none|volatile.
>
> No, -o is used with qemu-img create for options that change what the
> image will look like (e.g. setting flags in the image header). This
> options is about the cache mode that is used for various operations.
Actually I am just working over this. I am going to add a qemu-img
specific option, roughly something like this:
static QEMUOptionParameter qemuimg_options[] = {
{
.name = QEMUIMG_OPT_CACHE,
.type = OPT_STRING,
.help = "Write cache method"
},
{ NULL }
};
[...]
create_options = append_option_parameters(create_options,
qemuimg_options);
[...]
/* Setting output cache flag */
out_flags = BDRV_O_RDWR | BDRV_O_NO_FLUSH;
ret = qemuimg_option_cache(param, &out_flags);
if (ret < 0) {
goto out;
}
--
Federico
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
