storage: avoid excessive walking of the auxiliary directory

Pebble incrementally accounts its disk usage and this accounting is used to
cheaply service Capacity calls. The auxiliary directory is a subdirectory of
the data directory, and various Cockroach subsystems write to it directly
without any incremental accounting of their disk usage. As a result, Capacity
calculations walk the auxiliary directory to calculate the directory's disk
usage anew each time.

Previously, this calculation occurred during every call to Capacity. This
commit adapts Pebble.Capacity to cache the computed capacity and recalculate it
at most every minute. Ideally, we would incrementally account disk usage within
the auxiliary directory (#96344). This change is a stopgap, avoiding the bulk
of the CPU usage incurred by Capacity recalculations during IMPORTs.

Epic: none
Release note: none

Author: jbowens

pkg/storage/pebble.go

@@ -803,10 +803,15 @@ type engineConfig struct {
 
 // Pebble is a wrapper around a Pebble database instance.
 type Pebble struct {
-	cfg         engineConfig
-	db          *pebble.DB
-	closed      atomic.Bool
-	auxDir      string
+	cfg           engineConfig
+	db            *pebble.DB
+	closed        atomic.Bool
+	auxDir        string
+	auxiliarySize struct {
+		mu         sync.Mutex
+		computedAt time.Time
+		size       int64
+	}
 	ballastPath string
 	properties  roachpb.StoreProperties
 
@@ -1919,37 +1924,11 @@ func (p *Pebble) Capacity() (roachpb.StoreCapacity, error) {
 	m := p.db.Metrics()
 	totalUsedBytes := int64(m.DiskSpaceUsage())
 
-	// We don't have incremental accounting of the disk space usage of files
-	// in the auxiliary directory. Walk the auxiliary directory and all its
-	// subdirectories, adding to the total used bytes.
-	if errOuter := filepath.Walk(p.auxDir, func(path string, info os.FileInfo, err error) error {
-		if err != nil {
-			// This can happen if CockroachDB removes files out from under us -
-			// just keep going to get the best estimate we can.
-			if oserror.IsNotExist(err) {
-				return nil
-			}
-			// Special-case: if the store-dir is configured using the root of some fs,
-			// e.g. "/mnt/db", we might have special fs-created files like lost+found
-			// that we can't read, so just ignore them rather than crashing.
-			if oserror.IsPermission(err) && filepath.Base(path) == "lost+found" {
-				return nil
-			}
-			return err
-		}
-		if path == p.ballastPath {
-			// Skip the ballast. Counting it as used is likely to confuse
-			// users, and it's more akin to space that is just unavailable
-			// like disk space often restricted to a root user.
-			return nil
-		}
-		if info.Mode().IsRegular() {
-			totalUsedBytes += info.Size()
-		}
-		return nil
-	}); errOuter != nil {
-		return roachpb.StoreCapacity{}, errOuter
+	auxiliarySize, err := p.auxiliaryDirSize()
+	if err != nil {
+		return roachpb.StoreCapacity{}, err
 	}
+	totalUsedBytes += auxiliarySize
 
 	// If no size limitation have been placed on the store size or if the
 	// limitation is greater than what's available, just return the actual
@@ -1982,6 +1961,54 @@ func (p *Pebble) Capacity() (roachpb.StoreCapacity, error) {
 	}, nil
 }
 
+// auxiliaryDirSize computes the size of the auxiliary directory. There are
+// multiple Cockroach subsystems that write into the auxiliary directory, and
+// they don't incrementally account for their disk space usage. This function
+// walks the auxiliary directory and all its subdirectories, summing the file
+// sizes. This walk can be expensive, so we cache the result and only recompute
+// if it's over 1 minute stale.
+//
+// TODO(jackson): Eventually we should update the various subsystems writing
+// into the auxiliary directory to incrementally account for their disk space
+// usage.  See #96344.
+func (p *Pebble) auxiliaryDirSize() (int64, error) {
+	p.auxiliarySize.mu.Lock()
+	defer p.auxiliarySize.mu.Unlock()
+	if time.Since(p.auxiliarySize.computedAt) < time.Minute {
+		return p.auxiliarySize.size, nil
+	}
+
+	p.auxiliarySize.size = 0
+	err := filepath.Walk(p.auxDir, func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			// This can happen if CockroachDB removes files out from under us -
+			// just keep going to get the best estimate we can.
+			if oserror.IsNotExist(err) {
+				return nil
+			}
+			// Special-case: if the store-dir is configured using the root of some fs,
+			// e.g. "/mnt/db", we might have special fs-created files like lost+found
+			// that we can't read, so just ignore them rather than crashing.
+			if oserror.IsPermission(err) && filepath.Base(path) == "lost+found" {
+				return nil
+			}
+			return err
+		}
+		if path == p.ballastPath {
+			// Skip the ballast. Counting it as used is likely to confuse
+			// users, and it's more akin to space that is just unavailable
+			// like disk space often restricted to a root user.
+			return nil
+		}
+		if info.Mode().IsRegular() {
+			p.auxiliarySize.size += info.Size()
+		}
+		return nil
+	})
+	p.auxiliarySize.computedAt = time.Now()
+	return p.auxiliarySize.size, err
+}
+
 // Flush implements the Engine interface.
 func (p *Pebble) Flush() error {
 	return p.db.Flush()