qemu-options: explain disk I/O throttling options

conversion of plain zero writes by the OS to driver specific optimized

zero write commands. You may even choose "unmap" if @var{discard} is set

to "unmap" to allow a zero write to be converted to an UNMAP operation.

@item bps=@var{b},bps_rd=@var{r},bps_wr=@var{w}

Specify bandwidth throttling limits in bytes per second, either for all request

types or for reads or writes only.  Small values can lead to timeouts or hangs

inside the guest.  A safe minimum for disks is 2 MB/s.

@item bps_max=@var{bm},bps_rd_max=@var{rm},bps_wr_max=@var{wm}

Specify bursts in bytes per second, either for all request types or for reads

or writes only.  Bursts allow the guest I/O to spike above the limit

temporarily.

@item iops=@var{i},iops_rd=@var{r},iops_wr=@var{w}

Specify request rate limits in requests per second, either for all request

types or for reads or writes only.

@item iops_max=@var{bm},iops_rd_max=@var{rm},iops_wr_max=@var{wm}

Specify bursts in requests per second, either for all request types or for reads

or writes only.  Bursts allow the guest I/O to spike above the limit

temporarily.

@item iops_size=@var{is}

Let every @var{is} bytes of a request count as a new request for iops

throttling purposes.  Use this option to prevent guests from circumventing iops

limits by sending fewer but larger requests.

@item group=@var{g}

Join a throttling quota group with given name @var{g}.  All drives that are

members of the same group are accounted for together.  Use this option to

prevent guests from circumventing throttling limits by using many small disks

instead of a single larger disk.

@end table

By default, the @option{cache=writeback} mode is used. It will report data