incusd/storage/drivers/lvm: Cache VG extent size

Querying this value can get extremely expensive on busy systems.

Signed-off-by: Stéphane Graber <stgraber@stgraber.org>

internal/server/storage/drivers/driver_lvm.go

@@ -9,6 +9,7 @@ import (
 	"slices"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/lxc/incus/v6/internal/linux"
@@ -26,6 +27,9 @@ import (
 
 const lvmVgPoolMarker = "incus_pool" // Indicator tag used to mark volume groups as in use.
 
+var lvmExtentSize map[string]int64
+var lvmExtentSizeMu sync.Mutex
+
 var lvmLoaded bool
 var lvmVersion string
 

internal/server/storage/drivers/driver_lvm_utils.go

@@ -132,6 +132,16 @@ func (d *lvm) volumeGroupExists(vgName string) (bool, []string, error) {
 
 // volumeGroupExtentSize gets the volume group's physical extent size in bytes.
 func (d *lvm) volumeGroupExtentSize(vgName string) (int64, error) {
+	// Look for cached value.
+	lvmExtentSizeMu.Lock()
+	defer lvmExtentSizeMu.Unlock()
+
+	if lvmExtentSize == nil {
+		lvmExtentSize = map[string]int64{}
+	} else if lvmExtentSize[d.name] > 0 {
+		return lvmExtentSize[d.name], nil
+	}
+
 	output, err := subprocess.TryRunCommand("vgs", "--noheadings", "--nosuffix", "--units", "b", "-o", "vg_extent_size", vgName)
 	if err != nil {
 		if d.isLVMNotFoundExitError(err) {
@@ -142,7 +152,14 @@ func (d *lvm) volumeGroupExtentSize(vgName string) (int64, error) {
 	}
 
 	output = strings.TrimSpace(output)
-	return strconv.ParseInt(output, 10, 64)
+	val, err := strconv.ParseInt(output, 10, 64)
+	if err != nil {
+		return -1, err
+	}
+
+	lvmExtentSize[d.name] = val
+
+	return val, nil
 }
 
 // countLogicalVolumes gets the count of volumes (both normal and thin) in a volume group.