[pve-devel] [PATCH qemu-server 6/8] blockdev: add set write threshold

[Tiago Sousa via pve-devel <pve-devel@lists.proxmox.com>]

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From: Tiago Sousa <joao.sousa@eurotux.com>
To: pve-devel@lists.proxmox.com
Subject: [PATCH qemu-server 6/8] blockdev: add set write threshold
Date: Fri, 17 Oct 2025 12:25:31 +0100
Message-ID: <20251017112539.26471-7-joao.sousa@eurotux.com>

Signed-off-by: Tiago Sousa <joao.sousa@eurotux.com>
---
 src/PVE/QemuServer.pm          | 22 ++++++++++++++++
 src/PVE/QemuServer/Blockdev.pm | 47 ++++++++++++++++++++++++++++++++++
 src/PVE/QemuServer/Drive.pm    |  7 +++++
 3 files changed, 76 insertions(+)

diff --git a/src/PVE/QemuServer.pm b/src/PVE/QemuServer.pm
index 45daa06c..de1c39a5 100644
--- a/src/PVE/QemuServer.pm
+++ b/src/PVE/QemuServer.pm
@@ -5729,6 +5729,28 @@ sub vm_start_nolock {
         warn $@ if $@;
     }
 
+    # set write threshold for LVM thin provisioning disks
+    PVE::QemuConfig->foreach_volume(
+        $conf,
+        sub {
+            my ($ds, $drive) = @_;
+            return if PVE::QemuServer::drive_is_cdrom($drive, 1);
+            my $volid = $drive->{file};
+            if ( $volid ne 'none') {
+                my $drive_id = PVE::QemuServer::Drive::get_drive_id($drive);
+                my $snapshots = PVE::Storage::volume_snapshot_info($storecfg, $volid);
+                my $parentid = $snapshots->{'current'}->{parent};
+                # for now only set write_threshold for volumes that have snapshots
+                # FIX: Change to only thin drives
+                if ($parentid) {
+                    PVE::QemuServer::Blockdev::set_write_threshold(
+                        $storecfg, $vmid, $drive_id, $volid
+                    );
+                }
+            }
+        },
+    );
+
     #start nbd server for storage migration
     if (my $nbd = $migrate_opts->{nbd}) {
 
diff --git a/src/PVE/QemuServer/Blockdev.pm b/src/PVE/QemuServer/Blockdev.pm
index 8fa5eb51..38a1408a 100644
--- a/src/PVE/QemuServer/Blockdev.pm
+++ b/src/PVE/QemuServer/Blockdev.pm
@@ -830,6 +830,49 @@ sub set_io_throttle {
     }
 }
 
+sub block_set_write_threshold {
+    my ($vmid, $nodename, $threshold) = @_;
+
+    print "set threshold $nodename $threshold\n";
+
+    PVE::QemuServer::mon_cmd(
+        $vmid,
+        "block-set-write-threshold",
+        'node-name' => $nodename,
+        'write-threshold' => int($threshold),
+    );
+}
+
+sub compute_write_threshold {
+    my ($storecfg, $scfg, $volid) = @_;
+
+    my $lv_size = PVE::Storage::volume_size_info($storecfg, $volid, 5);
+
+    my $write_threshold = $lv_size - $scfg->{chunksize} * $scfg->{'chunk-percentage'};
+
+    return $write_threshold;
+}
+
+sub set_write_threshold {
+    my ($storecfg, $vmid, $drive_id, $volid, $options) = @_;
+
+    my ($storeid) = PVE::Storage::parse_volume_id($volid);
+    my $scfg = PVE::Storage::storage_config($storecfg, $storeid);
+    my $support_qemu_snapshots = PVE::Storage::volume_qemu_snapshot_method($storecfg, $volid);
+
+    # set write threshold is only supported for lvm storage using
+    # qcow2+external snapshots
+    return if $scfg->{type} ne 'lvm' || $support_qemu_snapshots ne 'mixed';
+    # return if drive is not set as thin
+    # return if !$drive->{thin};
+
+    my $nodename = get_node_name('file', $drive_id, $volid, $options);
+    my $write_threshold = compute_write_threshold($storecfg, $scfg, $volid);
+
+    print "setting threshold for $volid from $storeid\n";
+    block_set_write_threshold($vmid, $nodename, $write_threshold);
+}
+
 sub blockdev_external_snapshot {
     my ($storecfg, $vmid, $machine_version, $deviceid, $drive, $snap, $parent_snap) = @_;
 
@@ -878,6 +921,10 @@ sub blockdev_external_snapshot {
         node => $snap_fmt_blockdev->{'node-name'},
         overlay => $new_fmt_blockdev->{'node-name'},
     );
+
+    # FIX: only if thin
+    my $drive_id = PVE::QemuServer::Drive::get_drive_id($drive);
+    set_write_threshold($storecfg, $vmid, $drive_id, $volid);
 }
 
 sub blockdev_delete {
diff --git a/src/PVE/QemuServer/Drive.pm b/src/PVE/QemuServer/Drive.pm
index 79dd22e6..44f477a9 100644
--- a/src/PVE/QemuServer/Drive.pm
+++ b/src/PVE/QemuServer/Drive.pm
@@ -253,6 +253,13 @@ my %drivedesc_base = (
         optional => 1,
         default => 0,
     },
+    thin => {
+        type => 'boolean',
+        description =>
+            'Controls whether a drive should be thin provisioned',
+        optional => 1,
+        default => 0,
+    },
 );
 
 my %iothread_fmt = (
-- 
2.47.3



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