On Mon, 05/15 12:02, Peter Lieven wrote:
> Hi Block developers,
>
> I would like to add a feature to Qemu to drain all traffic from a block so
> that
> I can take external snaphosts without the risk to that in the middle of a
> write
> operation. Its meant for cases where where QGA freeze/thaw is not available.
>
> For me its enough to have this through qemu-io, but Kevin asked me to check
> if its not worth to have a stable API for it and present it via QMP/HMP.
>
> What are your thoughts?
For debugging purpose or a "hacky" usage where you know what you are doing, it
may be fine to have this. The only issue is it should be a separate flag, like
BlockJob.user_paused.
What happens from guest perspective? In the case of virtio, the request queue is
not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still handled,
the command is not effective (or rather the implementation is not complete).
Fam
>
> Thanks,
> Peter
>
> ---
> qemu-io-cmds.c | 78
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 78 insertions(+)
>
> diff --git a/qemu-io-cmds.c b/qemu-io-cmds.c
> index 312fc6d..49d82fe 100644
> --- a/qemu-io-cmds.c
> +++ b/qemu-io-cmds.c
> @@ -1565,6 +1565,82 @@ static const cmdinfo_t flush_cmd = {
> .oneline = "flush all in-core file state to disk",
> };
>
> +static const cmdinfo_t drain_cmd;
> +
> +static int drain_f(BlockBackend *blk, int argc, char **argv)
> +{
> + BlockDriverState *bs = blk_bs(blk);
> + bool flush = false;
> + int c;
> +
> + while ((c = getopt(argc, argv, "f")) != -1) {
> + switch (c) {
> + case 'f':
> + flush = true;
> + break;
> + default:
> + return qemuio_command_usage(&drain_cmd);
> + }
> + }
> +
> + if (optind != argc) {
> + return qemuio_command_usage(&drain_cmd);
> + }
> +
> +
> + if (bs->quiesce_counter) {
> + printf("drain failed: device is already drained!\n");
> + return 1;
> + }
> +
> + bdrv_drained_begin(bs); /* complete I/O */
> + if (flush) {
> + bdrv_flush(bs);
> + bdrv_drain(bs); /* in case flush left pending I/O */
> + printf("flushed all pending I/O\n");
> + }
> + printf("drain successful\n");
> + return 0;
> +}
> +
> +static void drain_help(void)
> +{
> + printf(
> +"\n"
> +" Drains all external I/O from the device\n"
> +"\n"
> +" -f, -- flush all in-core file state to disk\n"
> +"\n");
> +}
> +
> +static const cmdinfo_t drain_cmd = {
> + .name = "drain",
> + .cfunc = drain_f,
> + .args = "[-f]",
> + .argmin = 0,
> + .argmax = -1,
> + .oneline = "cease to send I/O to the device",
> + .help = drain_help
> +};
> +
> +static int undrain_f(BlockBackend *blk, int argc, char **argv)
> +{
> + BlockDriverState *bs = blk_bs(blk);
> + if (!bs->quiesce_counter) {
> + printf("undrain failed: device is not drained!\n");
> + return 1;
> + }
> + bdrv_drained_end(bs);
> + printf("undrain successful\n");
> + return 0;
> +}
> +
> +static const cmdinfo_t undrain_cmd = {
> + .name = "undrain",
> + .cfunc = undrain_f,
> + .oneline = "continue I/O to a drained device",
> +};
> +
> static int truncate_f(BlockBackend *blk, int argc, char **argv)
> {
> int64_t offset;
> @@ -2296,6 +2372,8 @@ static void __attribute((constructor))
> init_qemuio_commands(void)
> qemuio_add_command(&aio_write_cmd);
> qemuio_add_command(&aio_flush_cmd);
> qemuio_add_command(&flush_cmd);
> + qemuio_add_command(&drain_cmd);
> + qemuio_add_command(&undrain_cmd);
> qemuio_add_command(&truncate_cmd);
> qemuio_add_command(&length_cmd);
> qemuio_add_command(&info_cmd);
> --
> 1.9.1
>
>
