[pve-devel] [RFC storage/qemu-server] Thin provisioning on LVM

[pve-devel] [RFC storage/qemu-server] Thin provisioning on LVM

[Joao Sousa via pve-devel]

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From: Joao Sousa <joao.sousa@eurotux.com>
To: Proxmox VE development discussion <pve-devel@lists.proxmox.com>
Subject: [RFC storage/qemu-server] Thin provisioning on LVM
Date: Fri, 25 Jul 2025 11:00:40 +0100
Message-ID: <1b1df67f-4c6a-4cf5-942f-b2ed752506a6@eurotux.com>

Hi,

As previously discussed with Alexandre, we talked about an architecture 
that enables the use of thin-provisioned LVs with LVM. The idea is to 
implement a daemon that processes LV extend requests from a queue.

We considered two possible implementations for the queue and the daemon:

1. One queue and daemon per node. This approach increases complexity, 
particularly for live migrations and node failures. If a node fails, 
other nodes would need to "steal" pending requests from the failed 
node’s queue. It also introduces challenges in preserving the execution 
order of extend operations, since multiple daemons would compete for a 
storage lock without a guaranteed order.

2. A centralized queue in /etc/pve and one daemon per node. Each daemon 
would check the first entry in the queue and process the extend request 
only if the target volume is local to that node. This approach is 
simpler and easier to manage. However, we’d need to ensure proper 
locking when writing to the queue. Is there a C-based alternative to 
cfs_lock_file that we can use to coordinate writes to the queue from 
qmeventd and pvestatd?

For both implementations, we need to configure a write threshold for 
each VM's block devices. When this threshold is reached, it should 
trigger an event that qmeventd catches. As a fallback, if a VM is locked 
due to an I/O error, pvestatd should also submit an extend request. This 
one should be prioritized by placing it at the front of the queue.

The write threshold must be applied to the top node of the block device 
chain (drive-$drive_id) during the QemuServer::Blockdev::attach function 
when the VM starts. It should also be updated each time the volume is 
extended, so the daemon must reset it accordingly.

Here’s a simplified flow of the architecture:

qemu -> qmeventd -> extend_queue <- storage_monitor_daemon
      -> pvestatd

I’m currently implementing the write threshold in the attach function 
but running into issues with debugging. Are there any recommended 
methods or tools for debugging qemu-server functions? I’m not seeing any 
relevant logs in syslog.


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Re: [pve-devel] [RFC storage/qemu-server] Thin provisioning on LVM

[Tiago Sousa via pve-devel]

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From: Tiago Sousa <joao.sousa@eurotux.com>
To: pve-devel@lists.proxmox.com
Subject: Re: [pve-devel] [RFC storage/qemu-server] Thin provisioning on LVM
Date: Fri, 25 Jul 2025 15:46:41 +0100
Message-ID: <6045867d-4fd6-4e29-b462-e37bfaba583c@eurotux.com>

Small correction, the write threshold should be applied to the file 
node, since it's the one that will have the correct filesystem 
wr_highest_offset.


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[pve-devel] [PATCH qemu-server 0/2] set write threshold logic

[Tiago Sousa via pve-devel]

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From: Tiago Sousa <joao.sousa@eurotux.com>
To: pve-devel@lists.proxmox.com
Subject: [PATCH qemu-server 0/2] set write threshold logic
Date: Sat,  2 Aug 2025 17:42:38 +0100
Message-ID: <20250802164240.21751-1-joao.sousa@eurotux.com>

Hello,

I'm looking for some assistance in understanding why the block_write_threshold
is not being set as expected.

When the VM starts and I run the query-named-block-nodes command, the
write_threshold value always appears as the default 0. However, if I set it
manually through the QMP socket after startup, the threshold is applied
correctly.

Is the attach function not the appropriate place to set this value? If not,
could you clarify the correct point in the VM startup sequence to set it? I
had assumed it would be applied during blockdev_add, but that doesn't seem
to be the case.

Any guidance would be appreciated.

Tiago Sousa (2):
  blockdev: add set write threshold
  qmeventd: add block write threshold event handling

 src/PVE/QemuServer/Blockdev.pm | 52 ++++++++++++++++++++++++++++++++++
 src/qmeventd/qmeventd.c        | 19 +++++++++++++
 2 files changed, 71 insertions(+)

--
2.39.5



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[pve-devel] [PATCH qemu-server 1/2] blockdev: add set write threshold

[Tiago Sousa via pve-devel]

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From: Tiago Sousa <joao.sousa@eurotux.com>
To: pve-devel@lists.proxmox.com
Subject: [PATCH qemu-server 1/2] blockdev: add set write threshold
Date: Sat,  2 Aug 2025 17:42:39 +0100
Message-ID: <20250802164240.21751-2-joao.sousa@eurotux.com>

Signed-off-by: Tiago Sousa <joao.sousa@eurotux.com>
---
 src/PVE/QemuServer/Blockdev.pm | 52 ++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)

diff --git a/src/PVE/QemuServer/Blockdev.pm b/src/PVE/QemuServer/Blockdev.pm
index 0cc4a0f6..a5d7a3b9 100644
--- a/src/PVE/QemuServer/Blockdev.pm
+++ b/src/PVE/QemuServer/Blockdev.pm
@@ -567,9 +567,61 @@ sub attach {
         die $err;
     }
 
+    set_write_threshold($storecfg, $vmid, $drive, $options);
+
     return $blockdev->{'node-name'};
 }
 
+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 ($scfg, $volid) = @_;
+    my $lv_size = PVE::Storage::volume_size_info($scfg, $volid, 5);
+
+    # FIX: change these vars to config inputs
+    my $chunksize = 1024 * 1024 * 1024; # 1 GB
+    my $alert_chunk_percentage = 0.5; # alert when percetage of chunk used
+
+    my $write_threshold = $lv_size - $chunksize * (1 - $alert_chunk_percentage);
+
+    return $write_threshold;
+}
+
+sub set_write_threshold {
+    my ($storecfg, $vmid, $drive, $options) = @_;
+
+    my $volid = $drive->{'file'};
+    my ($storeid) = PVE::Storage::parse_volume_id($volid);
+    my $support_qemu_snapshots = PVE::Storage::volume_qemu_snapshot_method($storecfg, $volid);
+    my $scfg = PVE::Storage::storage_config($storecfg, $storeid);
+
+    # set write threshold is only supported for lvm storage using
+    # qcow2+external snapshots
+    return if $scfg->{type} ne 'lvm' || $support_qemu_snapshots ne 'mixed';
+
+    my $snapshots = PVE::Storage::volume_snapshot_info($storecfg, $volid);
+    my $parentid = $snapshots->{'current'}->{parent};
+    # for now only set write_threshold for volumes that have snapshots
+    if ($parentid) {
+        my $drive_id = PVE::QemuServer::Drive::get_drive_id($drive);
+        my $nodename = get_node_name('file', $drive_id, $drive->{file}, $options);
+        my $write_threshold = compute_write_threshold($scfg, $volid);
+
+        block_set_write_threshold($vmid, $nodename, $write_threshold);
+    }
+}
+
 =pod
 
 =head3 detach
-- 
2.39.5



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[pve-devel] [PATCH qemu-server 2/2] qmeventd: add block write threshold event handling

[Tiago Sousa via pve-devel]

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From: Tiago Sousa <joao.sousa@eurotux.com>
To: pve-devel@lists.proxmox.com
Subject: [PATCH qemu-server 2/2] qmeventd: add block write threshold event handling
Date: Sat,  2 Aug 2025 17:42:40 +0100
Message-ID: <20250802164240.21751-3-joao.sousa@eurotux.com>

Signed-off-by: Tiago Sousa <joao.sousa@eurotux.com>
---
 src/qmeventd/qmeventd.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/src/qmeventd/qmeventd.c b/src/qmeventd/qmeventd.c
index 1d9eb74a..616b6521 100644
--- a/src/qmeventd/qmeventd.c
+++ b/src/qmeventd/qmeventd.c
@@ -209,6 +209,25 @@ void handle_qmp_event(struct Client *client, struct json_object *obj) {
 
         // check if a backup is running and kill QEMU process if not
         terminate_check(client);
+    } else if (!strcmp(json_object_get_string(event), "BLOCK_WRITE_THRESHOLD")) {
+        struct json_object *data;
+        struct json_object *nodename;
+        if (json_object_object_get_ex(obj, "data", &data) &&
+            json_object_object_get_ex(data, "node-name", &nodename)) {
+
+            // needs concurrency control
+            char extend_queue_path[] = "/etc/pve/extend_queue";
+            FILE *p_extend_queue = fopen(extend_queue_path, "a");
+            if (p_extend_queue == NULL) {
+                VERBOSE_PRINT(
+                    "%s: Couldn't open extend queue file %s", client->qemu.vmid, extend_queue_path
+                );
+            } else {
+                const char *nodename_string = json_object_get_string(nodename);
+                fprintf(p_extend_queue, "%s\n", nodename_string);
+            }
+            fclose(p_extend_queue);
+        }
     }
 }
 
-- 
2.39.5



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Re: [pve-devel] [PATCH qemu-server 0/2] set write threshold logic

[Fiona Ebner]

Am 02.08.25 um 6:42 PM schrieb Tiago Sousa via pve-devel:
> 
> Hello,
> 
> I'm looking for some assistance in understanding why the block_write_threshold
> is not being set as expected.
> 
> When the VM starts and I run the query-named-block-nodes command, the
> write_threshold value always appears as the default 0. However, if I set it
> manually through the QMP socket after startup, the threshold is applied
> correctly.
> 
> Is the attach function not the appropriate place to set this value? If not,
> could you clarify the correct point in the VM startup sequence to set it? I
> had assumed it would be applied during blockdev_add, but that doesn't seem
> to be the case.
> 
> Any guidance would be appreciated.

Hi,

blockdev_add()/Blockdev::attach() are only called for hot-plugged disks,
not disks already present at start-up time. You either need to set the
threshold as part of the blockdev options in the generate_*_blockdev()
functions (if that is possible, would be preferred) or issue the QMP
commands for the initially present disks right after VM start (in
vm_start_nolock(), we already do something similar for ballooning).

Best Regards,
Fiona


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Re: [pve-devel] [PATCH qemu-server 0/2] set write threshold logic

[Joao Sousa via pve-devel]

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From: Joao Sousa <joao.sousa@eurotux.com>
To: Fiona Ebner <f.ebner@proxmox.com>, Proxmox VE development discussion <pve-devel@lists.proxmox.com>
Subject: Re: [pve-devel] [PATCH qemu-server 0/2] set write threshold logic
Date: Mon, 11 Aug 2025 21:34:34 +0100
Message-ID: <65e2f286-320a-435c-9846-24aad68f9db7@eurotux.com>

On 8/4/25 10:14 AM, Fiona Ebner wrote:
> blockdev_add()/Blockdev::attach() are only called for hot-plugged disks,
> not disks already present at start-up time. You either need to set the
> threshold as part of the blockdev options in the generate_*_blockdev()
> functions (if that is possible, would be preferred) or issue the QMP
> commands for the initially present disks right after VM start (in
> vm_start_nolock(), we already do something similar for ballooning).

AFAIK the write threshold cannot be set when the blockdev is created, at 
least for now. As you suggested, I was able to loop over every volume in 
vm_start_lock() and the threshold is being set correctly.

Thanks!

Best regards,

Tiago


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