> On 5 Aug 2015, at 19:33, Anil Madhavapeddy <[email protected]> wrote: > > Reducing the dependency on qemu is very desirable given the recent rash > of security issues from the codebase as well. > > There's another alternative method which was used in the form of the > 'blktap' driver. This redirects block traffic to a userspace daemon > that can then write to (e.g.) a VHD or VMDK file. Blktap also doesnt > require using up a loop device. > > Blktap's been through a series of rewrites though, and never seems to > have been upstreamed even though it's the default storage mode used > in XenServer. CCing Dave Scott: do you know if using blktap is a viable > option these days?
I think George Dunlap (cc:d) has been adding support to the Xen build for the XenServer-derived blktap: http://lists.xen.org/archives/html/xen-devel/2015-04/msg01853.html I’ve not had a chance to try it myself, but it ought to be pretty good. What do you think, George? Note: the vhd support in blktap/tapdisk is well tested, but it doesn’t support vmdk IIRC (or at least if it does it won’t be anywhere near as well tested). It supports raw format too, which is equivalent to using a loop device + blkback. Cheers, Dave > > -anil > >> On 22 Jul 2015, at 18:31, Martin Lucina <[email protected]> wrote: >> >> Hi, >> >> I've managed to track down the Xen upstream change (will be in 4.6) which >> changes the default backend type for raw fles from 'qdisk' to 'phy'. In >> practice this means that a qemu process is no longer required to serve the >> qdisk protocol which saves both memory and domU startup time. >> >> I've tested this patch on my Debian jessie install by backporting it to the >> Debian Xen 4.4.1 packages and it works fine. However, note that what it >> actually does in practice (not obvious from the patch) is that it >> internally uses a loop device to back the file. It's not clear to me why >> the Linux blkback driver can't just use a file directly, but that's not >> something I want to investigate right now. >> >> The default maximum number of loop devices is (AFAIK) quite low, so you >> will want to increase that if you want to use this patch, by setting the >> max_loop parameter appropriately when the loop kernel module is loaded. >> >> Martin >> >> ---- >> >> commit a0a2dc45f1bdbabc661332e8530c480a77391d85 >> Author: Wei Liu <[email protected]> >> Date: Thu Jan 8 13:46:53 2015 +0000 >> >> libxl, hotplug/Linux: default to phy backend for raw format file, take 2 >> >> This patch resurrects 11a63a166. The previous patch had a bug that >> wrong "physical-device" was written to xenstore causing block script >> execution fail. This patch fixes that problem. >> >> Following configurations have been tested: >> >> 1. Raw file and PV >> 2. Raw file and HVM >> 3. Block device and PV >> 4. Block device and HVM >> >> Creation / destruction / local migration all worked. >> >> Original commit message from 11a63a166: >> >> Modify libxl and hotplug script to allow raw format file to use phy >> backend. >> >> The block script now tests the path and determine the actual type of >> file (block device or regular file) then use the actual type to >> determine which branch to run. >> >> With these changes, plus the current ordering of backend preference (phy >>> qdisk > tap), we will use phy backend for raw format file by default. >> >> Signed-off-by: Wei Liu <[email protected]> >> Cc: Ian Campbell <[email protected]> >> Cc: Stefano Stabellini <[email protected]> >> Cc: Ian Jackson <[email protected]> >> Cc: Roger Pau Monné <[email protected]> >> Acked-by: Roger Pau Monné <[email protected]> >> Acked-by: Ian Campbell <[email protected]> >> >> diff --git a/tools/hotplug/Linux/block b/tools/hotplug/Linux/block >> index da26e22..8d2ee9d 100644 >> --- a/tools/hotplug/Linux/block >> +++ b/tools/hotplug/Linux/block >> @@ -206,6 +206,13 @@ and so cannot be mounted ${m2}${when}." >> >> >> t=$(xenstore_read_default "$XENBUS_PATH/type" 'MISSING') >> +p=$(xenstore_read "$XENBUS_PATH/params") >> +mode=$(xenstore_read "$XENBUS_PATH/mode") >> +if [ -b "$p" ]; then >> + truetype="phy" >> +elif [ -f "$p" ]; then >> + truetype="file" >> +fi >> >> case "$command" in >> add) >> @@ -217,16 +224,11 @@ case "$command" in >> exit 0 >> fi >> >> - if [ -n "$t" ] >> - then >> - p=$(xenstore_read "$XENBUS_PATH/params") >> - mode=$(xenstore_read "$XENBUS_PATH/mode") >> - fi >> FRONTEND_ID=$(xenstore_read "$XENBUS_PATH/frontend-id") >> FRONTEND_UUID=$(xenstore_read_default \ >> "/local/domain/$FRONTEND_ID/vm" 'unknown') >> >> - case $t in >> + case $truetype in >> phy) >> dev=$(expand_dev $p) >> >> @@ -319,7 +321,7 @@ mount it read-write in a guest domain." >> ;; >> >> remove) >> - case $t in >> + case $truetype in >> phy) >> exit 0 >> ;; >> diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c >> index 372dd3b..11cf0e1 100644 >> --- a/tools/libxl/libxl.c >> +++ b/tools/libxl/libxl.c >> @@ -2416,9 +2416,9 @@ static void device_disk_add(libxl__egc *egc, uint32_t >> domid, >> if (!disk->script && >> disk->backend_domid == LIBXL_TOOLSTACK_DOMID) { >> int major, minor; >> - libxl__device_physdisk_major_minor(dev, &major, &minor); >> - flexarray_append_pair(back, "physical-device", >> - libxl__sprintf(gc, "%x:%x", major, minor)); >> + if (!libxl__device_physdisk_major_minor(dev, &major, >> &minor)) >> + flexarray_append_pair(back, "physical-device", >> + libxl__sprintf(gc, "%x:%x", >> major, minor)); >> } >> >> assert(device->backend_kind == LIBXL__DEVICE_KIND_VBD); >> diff --git a/tools/libxl/libxl_device.c b/tools/libxl/libxl_device.c >> index 4b51ded..0f50d04 100644 >> --- a/tools/libxl/libxl_device.c >> +++ b/tools/libxl/libxl_device.c >> @@ -332,6 +332,8 @@ int libxl__device_physdisk_major_minor(const char >> *physpath, int *major, int *mi >> struct stat buf; >> if (stat(physpath, &buf) < 0) >> return -1; >> + if (!S_ISBLK(buf.st_mode)) >> + return -1; >> *major = major(buf.st_rdev); >> *minor = minor(buf.st_rdev); >> return 0; >> diff --git a/tools/libxl/libxl_linux.c b/tools/libxl/libxl_linux.c >> index ea5d8c1..b51930c 100644 >> --- a/tools/libxl/libxl_linux.c >> +++ b/tools/libxl/libxl_linux.c >> @@ -19,11 +19,11 @@ >> >> int libxl__try_phy_backend(mode_t st_mode) >> { >> - if (!S_ISBLK(st_mode)) { >> - return 0; >> + if (S_ISBLK(st_mode) || S_ISREG(st_mode)) { >> + return 1; >> } >> >> - return 1; >> + return 0; >> } >> >> #define EXT_SHIFT 28 >> >
