Re: [Qemu-block] [PATCH 1/1] block: add cache mode with direct IO and without flushes

[Kevin Wolf]

Am 16.09.2016 um 10:15 hat Denis V. Lunev geschrieben:
> On 09/16/2016 11:08 AM, Kevin Wolf wrote:
> > Am 15.09.2016 um 18:39 hat Denis V. Lunev geschrieben:
> >> On 09/15/2016 07:09 PM, Kevin Wolf wrote:
> >>> Am 15.09.2016 um 15:19 hat Denis V. Lunev geschrieben:
> >>>> This mode could be very useful for flush bottlenecks evaluation and for
> >>>> running non-persistent VMs, when host crash is considered not fatal.
> >>>>
> >>>> Signed-off-by: Denis V. Lunev <address@hidden>
> >>>> CC: Kevin Wolf <address@hidden>
> >>>> CC: Max Reitz <address@hidden>
> >>> Why not just specify the individual options?
> >>>
> >>>     -drive file=...,cache.direct=on,cache.no-flush=on
> >>>
> >>> Kevin
> >> then we need to have three options:
> >>   cache.direct=on/off
> >>   cache.flush=on/off (default on)
> >>   cache.writethrough=on/off
> >> What will have preference, old style option, new style option, rightmost
> >> option?
> > The individual options take precedence, see the code that handles the
> > "cache=..." option in drive_init():
> >
> >     value = qemu_opt_get(all_opts, "cache");
> >     if (value) {
> >         int flags = 0;
> >         bool writethrough;
> >
> >         if (bdrv_parse_cache_mode(value, &flags, &writethrough) != 0) {
> >             error_report("invalid cache option");
> >             return NULL;
> >         }
> >
> >         /* Specific options take precedence */
> >         if (!qemu_opt_get(all_opts, BDRV_OPT_CACHE_WB)) {
> >             qemu_opt_set_bool(all_opts, BDRV_OPT_CACHE_WB,
> >                               !writethrough, &error_abort);
> >         }
> >         if (!qemu_opt_get(all_opts, BDRV_OPT_CACHE_DIRECT)) {
> >             qemu_opt_set_bool(all_opts, BDRV_OPT_CACHE_DIRECT,
> >                               !!(flags & BDRV_O_NOCACHE), &error_abort);
> >         }
> >         if (!qemu_opt_get(all_opts, BDRV_OPT_CACHE_NO_FLUSH)) {
> >             qemu_opt_set_bool(all_opts, BDRV_OPT_CACHE_NO_FLUSH,
> >                               !!(flags & BDRV_O_NO_FLUSH), &error_abort);
> >         }
> >         qemu_opt_unset(all_opts, "cache");
> >     }
> >
> > Kevin
> thank for an idea!

Just to be clear, everything that I mentioned is working today, so it's
not just an idea that still needs to implemented.

You just need to check the actual names of the options. Instead of your
proposed writethrough/flush options, we have the reversed options
writeback/no-flush.

Kevin