| Message ID | 1485362572-7246-1-git-send-email-den@openvz.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
[CC-ing John] On 25.01.2017 17:42, Denis V. Lunev wrote: > Technically there is a problem when the guest DVD is created by libvirt > with AIO mode 'native' on Linux. Current QEMU is unable to start the > domain configured as follows: > <disk type='file' device='cdrom'> > <driver name='qemu' type='raw' cache='none' io='native'/> > <target dev='sdb' bus='scsi'/> > <readonly/> > </disk> > The problem comes from the combination of 'cache' and 'io' options. > > 'io' option is common option and it is removed from block driver > specific options. 'cache' originally is not. The patch makes 'cache' > option common. This works fine as long as cdrom media insertion > later on. > > Signed-off-by: Denis V. Lunev <den@openvz.org> > CC: Kevin Wolf <kwolf@redhat.com> > CC: Max Reitz <mreitz@redhat.com> > --- > blockdev.c | 18 +++++++++++++++--- > 1 file changed, 15 insertions(+), 3 deletions(-) There was a Red Hat BZ for this: https://bugzilla.redhat.com/show_bug.cgi?id=1342999 There is now a corresponding BZ for libvirt: https://bugzilla.redhat.com/show_bug.cgi?id=1377321 The gist is that it was determined to be a problem with libvirt. RHEL has a downstream commit to work around this issue by ignoring the cache mode set for empty CD-ROMs -- but that is only because that was simpler than fixing libvirt. (Note that it's ignoring the cache mode instead of actually evaluating it.) > May be this has already discussed, but still. AIO=native for CDROM without > media seems important case. First, I personally don't find aio=native very important for CD-ROM drives. Yes, we shouldn't make IDE CD-ROM slower than necessary, but I don't think aio=native will make the difference between "Slow like CD-ROMs are in reality" and "As fast as virtio". Second, all this patch does is revert some changes done by commit 91a097e7478940483e76d52217f05bc05b98d5a5, which was very deliberate. Third, you may then be asking for the recommended way to put an aio=native medium into a CD-ROM drive. Good thing you ask, because we have a way that we want to recommend but can't because it's still considered experimental: The BDS is added using blockdev-add, with all of the appropriate caching and aio options. Then it's inserted into the drive using the x-blockdev-insert-medium command, and the drive is closed using blockdev-close-tray. There are a couple of issues with this: First, blockdev-add and x-blockdev-insert-medium are still experimental. The good news are that I think the reason why the latter is experimental has actually disappeared and we can just drop the x- by now. The not-so-good-but-not-so-bad-either news are that we plan to get blockdev-add declared non-experimental for 2.9. Let's see how it goes. The other issue is that of course it's more cumbersome to use than a simple 'change' via HMP. I argue that if someone communicates with a VM by hand, they either have to deal with this added complexity or they cannot use aio=native for CD-ROM drives -- which, as I said, I don't think is too bad. However, there is a reason why you cannot set cache/aio for an empty drive and it's simply that it doesn't make sense. Max
diff --git a/blockdev.c b/blockdev.c index 245e1e1..004dcde 100644 --- a/blockdev.c +++ b/blockdev.c @@ -374,6 +374,13 @@ static void extract_common_blockdev_options(QemuOpts *opts, int *bdrv_flags, return; } } + + if (qemu_opt_get_bool(opts, BDRV_OPT_CACHE_NO_FLUSH, false)) { + *bdrv_flags |= BDRV_O_NO_FLUSH; + } + if (qemu_opt_get_bool(opts, BDRV_OPT_CACHE_DIRECT, false)) { + *bdrv_flags |= BDRV_O_NOCACHE; + } } /* disk I/O throttling */ @@ -569,11 +576,8 @@ static BlockBackend *blockdev_init(const char *file, QDict *bs_opts, /* bdrv_open() defaults to the values in bdrv_flags (for compatibility * with other callers) rather than what we want as the real defaults. * Apply the defaults here instead. */ - qdict_set_default_str(bs_opts, BDRV_OPT_CACHE_DIRECT, "off"); - qdict_set_default_str(bs_opts, BDRV_OPT_CACHE_NO_FLUSH, "off"); qdict_set_default_str(bs_opts, BDRV_OPT_READ_ONLY, read_only ? "on" : "off"); - assert((bdrv_flags & BDRV_O_CACHE_MASK) == 0); if (runstate_check(RUN_STATE_INMIGRATE)) { bdrv_flags |= BDRV_O_INACTIVE; @@ -3996,6 +4000,14 @@ QemuOptsList qemu_common_drive_opts = { .type = QEMU_OPT_STRING, .help = "write error action", },{ + .name = BDRV_OPT_CACHE_DIRECT, + .type = QEMU_OPT_BOOL, + .help = "Bypass software writeback cache on the host", + }, { + .name = BDRV_OPT_CACHE_NO_FLUSH, + .type = QEMU_OPT_BOOL, + .help = "Ignore flush requests", + }, { .name = BDRV_OPT_READ_ONLY, .type = QEMU_OPT_BOOL, .help = "open drive file as read-only",
Technically there is a problem when the guest DVD is created by libvirt with AIO mode 'native' on Linux. Current QEMU is unable to start the domain configured as follows: <disk type='file' device='cdrom'> <driver name='qemu' type='raw' cache='none' io='native'/> <target dev='sdb' bus='scsi'/> <readonly/> </disk> The problem comes from the combination of 'cache' and 'io' options. 'io' option is common option and it is removed from block driver specific options. 'cache' originally is not. The patch makes 'cache' option common. This works fine as long as cdrom media insertion later on. Signed-off-by: Denis V. Lunev <den@openvz.org> CC: Kevin Wolf <kwolf@redhat.com> CC: Max Reitz <mreitz@redhat.com> --- blockdev.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) May be this has already discussed, but still. AIO=native for CDROM without media seems important case.