Re: [RFC PATCH 09/22] nbd: Add writethrough to block-export-add

[Eric Blake]

On 8/13/20 11:29 AM, Kevin Wolf wrote:
> qemu-nbd allows use of writethrough cache modes, which mean that write
> requests made through NBD will cause a flush before they complete.
> Expose the same functionality in block-export-add.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   qapi/block-export.json | 7 ++++++-
>   blockdev-nbd.c         | 2 +-
>   2 files changed, 7 insertions(+), 2 deletions(-)

> +++ b/blockdev-nbd.c
> @@ -218,7 +218,7 @@ BlockExport *nbd_export_create(BlockExportOptions 
> *exp_args, Error **errp)
>   
>       exp = nbd_export_new(bs, arg->name, arg->description, arg->bitmap,
>                            !arg->writable, !arg->writable,
> -                         NULL, false, errp);
> +                         NULL, exp_args->writethrough, errp);

It would someday be nice to get rid of exp_args->has_writethrough in 
QAPI generated code, but that's independent of this series.  Meanwhile, 
you are safe in relying on writethrough being false when 
has_writethrough is false.

This change makes sense to me interface-wise.  QAPI-wise, should we try 
harder to make writethrough settable only for writable exports (and an 
error for read-only exports)?  I'm not sure what QAPI contortions would 
have to look like to make that enforced by the QMP parser; but it's also 
not the end of the world if we just always permit the optional field and 
then apply a post-parse semantic check.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org