On 18.03.2016 19:21, Kevin Wolf wrote: > Whether a write cache is used or not is a decision that concerns the > user (e.g. the guest device) rather than the backend. It was already > logically part of the BB level as bdrv_move_feature_fields() always kept > it on top of the BDS tree; with this patch, the core of it (the actual > flag and the additional flushes) is also implemented there. > > Direct callers of bdrv_open() must pass BDRV_O_CACHE_WB now if bs > doesn't have a BlockBackend attached. > > Signed-off-by: Kevin Wolf <kw...@redhat.com> > --- > block.c | 26 +++++++++++++++++--------- > block/block-backend.c | 42 +++++++++++++++++++++++++++--------------- > block/io.c | 2 +- > block/iscsi.c | 2 +- > include/block/block.h | 1 + > include/block/block_int.h | 3 --- > tests/qemu-iotests/142 | 4 ++-- > tests/qemu-iotests/142.out | 8 ++++---- > 8 files changed, 53 insertions(+), 35 deletions(-)
Reviewed-by: Max Reitz <mre...@redhat.com>
I'm not so sure about the state bdrv_{set_,}enable_write_cache() are in
after this patch (e.g. the NBD client will always think the write cache
is enabled; and bdrv_set_enable_write_cache() can be used to unset
BDRV_O_CACHE_WB on BDSs), but looking at the following patches' titles,
they'll clear that up.
It appears to me that multiwrite will ignore the writethrough status,
but then again, qemu-io seems to be the only multiwrite user.
> diff --git a/block.c b/block.c
> index 172f865..9271dbb 100644
> --- a/block.c
> +++ b/block.c
[...]
> @@ -3618,8 +3626,8 @@ void bdrv_img_create(const char *filename, const char
> *fmt,
> }
>
> /* backing files always opened read-only */
> - back_flags =
> - flags & ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
> + back_flags = flags | BDRV_O_CACHE_WB;
> + back_flags &= ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT |
> BDRV_O_NO_BACKING);
Actually, this is the only thing the @flags parameter of this function
is used for. Maybe it can be dropped since we already regulate the
back_flags pretty strictly.
>
> if (backing_fmt) {
> backing_options = qdict_new();
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