add ability to set cache mode for disk

Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>

builder/qemu/builder.go

@@ -56,6 +56,14 @@ var diskInterface = map[string]bool{
 	"virtio": true,
 }
 
+var diskCache = map[string]bool{
+	"writethrough": true,
+	"writeback":    true,
+	"none":         true,
+	"unsafe":       true,
+	"directsync":   true,
+}
+
 type Builder struct {
 	config config
 	runner multistep.Runner
@@ -68,6 +76,7 @@ type config struct {
 	BootCommand     []string   `mapstructure:"boot_command"`
 	DiskInterface   string     `mapstructure:"disk_interface"`
 	DiskSize        uint       `mapstructure:"disk_size"`
+	DiskCache       string     `mapstructure:"disk_cache`
 	FloppyFiles     []string   `mapstructure:"floppy_files"`
 	Format          string     `mapstructure:"format"`
 	Headless        bool       `mapstructure:"headless"`
@@ -126,6 +135,10 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		b.config.DiskSize = 40000
 	}
 
+	if b.config.DiskCache == "" {
+		b.config.DiskCache = "writeback"
+	}
+
 	if b.config.Accelerator == "" {
 		b.config.Accelerator = "kvm"
 	}
@@ -280,6 +293,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 			errs, errors.New("unrecognized disk interface type"))
 	}
 
+	if _, ok := diskCache[b.config.DiskCache]; !ok {
+		errs = packer.MultiErrorAppend(
+			errs, errors.New("unrecognized disk cache type"))
+	}
+
 	if b.config.HTTPPortMin > b.config.HTTPPortMax {
 		errs = packer.MultiErrorAppend(
 			errs, errors.New("http_port_min must be less than http_port_max"))

builder/qemu/step_run.go

@@ -2,11 +2,12 @@ package qemu
 
 import (
 	"fmt"
-	"github.com/mitchellh/multistep"
-	"github.com/mitchellh/packer/packer"
 	"log"
 	"path/filepath"
 	"strings"
+
+	"github.com/mitchellh/multistep"
+	"github.com/mitchellh/packer/packer"
 )
 
 // stepRun runs the virtual machine
@@ -80,7 +81,7 @@ func getCommandArgs(bootDrive string, state multistep.StateBag) ([]string, error
 	defaultArgs["-machine"] = fmt.Sprintf("type=%s", config.MachineType)
 	defaultArgs["-netdev"] = "user,id=user.0"
 	defaultArgs["-device"] = fmt.Sprintf("%s,netdev=user.0", config.NetDevice)
-	defaultArgs["-drive"] = fmt.Sprintf("file=%s,if=%s", imgPath, config.DiskInterface)
+	defaultArgs["-drive"] = fmt.Sprintf("file=%s,if=%s,cache=%s", imgPath, config.DiskInterface, config.DiskCache)
 	defaultArgs["-cdrom"] = isoPath
 	defaultArgs["-boot"] = bootDrive
 	defaultArgs["-m"] = "512M"

website/source/docs/builders/qemu.html.markdown

@@ -117,6 +117,10 @@ each category, the available options are alphabetized and described.
   commands or kickstart type scripts must have proper adjustments for
   resulting device names. The Qemu builder uses "virtio" by default.
 
+* `disk_cache` (string) - The cache mode to use for disk. Allowed values
+  values include any of "writethrough", "writeback", "none", "unsafe" or
+  "directsync".
+
 * `floppy_files` (array of strings) - A list of files to place onto a floppy
   disk that is attached when the VM is booted. This is most useful
   for unattended Windows installs, which look for an `Autounattend.xml` file