[pve-devel] [RFC qemu-server] increase timeout from guest-fsfreeze-freeze

[Thomas Lamprecht]

The qmp command 'guest-fsfreeze-freeze' issues in linux a FIFREEZE
ioctl call on all mounted guest FS.
This ioctl call locks the filesystem and gets it into an consistent
state. For this all caches must be synced after blocking new writes
to the FS, which may need a relative long time, especially under high
IO load on the backing storage.

In windows a VSS (Volume Shadow Copy Service) request_freeze will
issued. As of the closed Windows nature the exact mechanisms cannot
be checked but some microsoft blog posts and other forum post suggest
that it should return fast but certain workloads can still trigger a
long delay resulting an similar problems.

Thus try to minimize the error probability and increase the timeout
significantly.
We use 60 minutes as timeout as this seems a limit which should not
get trespassed in a somewhat healthy system.

See:
https://forum.proxmox.com/threads/22192/

see the 'freeze_super' and 'thaw_super' function in fs/super.c from
the linux kernel tree for more details on the freeze behavior in
Linux guests.

Signed-off-by: Thomas Lamprecht <t.lamprecht at proxmox.com>
---

The 60 mins are naturally questionable, imo it would be better to
remove the timeout entirely from this function as it seems that it
can only cause harm. Thats the reason I choose such a high value.

Over 60 minutes of syncing should normally (!) never be the case, on
the other hand. With 60 minutes of sync time a guest can sync 450
megabyte if he only has 125 kilobyte/s (1 mbit/s) bandwidth or 4.5 gigabyte
if he has 1.25 megabyte/s (10 mbit) bandwidth.


 PVE/QMPClient.pm | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/PVE/QMPClient.pm b/PVE/QMPClient.pm
index a527ffc..78111ac 100755
--- a/PVE/QMPClient.pm
+++ b/PVE/QMPClient.pm
@@ -107,8 +107,13 @@ sub cmd {
 	    $timeout = 60*60; # 1 hour
 	} elsif ($cmd->{execute} =~ m/^(eject|change)/) {
 	    $timeout = 60; # note: cdrom mount command is slow
-	} elsif ($cmd->{execute} eq 'guest-fsfreeze-freeze' ||
-		 $cmd->{execute} eq 'guest-fsfreeze-thaw') {
+	} elsif ($cmd->{execute} eq 'guest-fsfreeze-freeze') {
+	    # freeze syncs all guest FS, if we kill it it stays in an unfreezable
+	    # locked state with high probability, so use an generous timeout
+	    $timeout = 60*60; # 10 mins
+	} elsif ($cmd->{execute} eq 'guest-fsfreeze-thaw') {
+	    # thaw has no possible long blocking actions, either it returns
+	    # instantly or never (dead locked)
 	    $timeout = 10;
 	} elsif ($cmd->{execute} eq 'savevm-start' ||
 		 $cmd->{execute} eq 'savevm-end' ||
-- 
2.1.4