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fix(agent): avoid mismatched root disk I/O mapping in docker (#1737)

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- Stop using max-read fallback when mapping root filesystem to
diskstats.
- Keep root usage reporting even when root I/O cannot be mapped.
- Expand docker fallback mount detection to include /etc/resolv.conf and
/etc/hostname (in addition to /etc/hosts).
- Add clearer warnings when root block device detection is uncertain.
This commit is contained in:
henrygd
2026-02-10 18:12:04 -05:00
parent 48c35aa54d
commit dba3519b2c
3 changed files with 85 additions and 25 deletions
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50
agent/disk.go
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@@ -26,6 +26,15 @@ func parseFilesystemEntry(entry string) (device, customName string) {
return device, customName
}
func isDockerSpecialMountpoint(mountpoint string) bool {
switch mountpoint {
case "/etc/hosts", "/etc/resolv.conf", "/etc/hostname":
return true
default:
return false
}
}
// Sets up the filesystems to monitor for disk usage and I/O.
func (a *Agent) initializeDiskInfo() {
filesystem, _ := GetEnv("FILESYSTEM")
@@ -69,11 +78,15 @@ func (a *Agent) initializeDiskInfo() {
if _, exists := a.fsStats[key]; !exists {
if root {
slog.Info("Detected root device", "name", key)
// Check if root device is in /proc/diskstats, use fallback if not
// Check if root device is in /proc/diskstats. Do not guess a
// fallback device for root: that can misattribute root I/O to a
// different disk while usage remains tied to root mountpoint.
if _, ioMatch = diskIoCounters[key]; !ioMatch {
key, ioMatch = findIoDevice(filesystem, diskIoCounters, a.fsStats)
if !ioMatch {
slog.Info("Using I/O fallback", "device", device, "mountpoint", mountpoint, "fallback", key)
if matchedKey, match := findIoDevice(filesystem, diskIoCounters); match {
key = matchedKey
ioMatch = true
} else {
slog.Warn("Root I/O unmapped; set FILESYSTEM", "device", device, "mountpoint", mountpoint)
}
}
} else {
@@ -141,8 +154,8 @@ func (a *Agent) initializeDiskInfo() {
for _, p := range partitions {
// fmt.Println(p.Device, p.Mountpoint)
// Binary root fallback or docker root fallback
if !hasRoot && (p.Mountpoint == rootMountPoint || (p.Mountpoint == "/etc/hosts" && strings.HasPrefix(p.Device, "/dev"))) {
fs, match := findIoDevice(filepath.Base(p.Device), diskIoCounters, a.fsStats)
if !hasRoot && (p.Mountpoint == rootMountPoint || (isDockerSpecialMountpoint(p.Mountpoint) && strings.HasPrefix(p.Device, "/dev"))) {
fs, match := findIoDevice(filepath.Base(p.Device), diskIoCounters)
if match {
addFsStat(fs, p.Mountpoint, true)
hasRoot = true
@@ -176,33 +189,26 @@ func (a *Agent) initializeDiskInfo() {
// If no root filesystem set, use fallback
if !hasRoot {
rootDevice, _ := findIoDevice(filepath.Base(filesystem), diskIoCounters, a.fsStats)
slog.Info("Root disk", "mountpoint", rootMountPoint, "io", rootDevice)
a.fsStats[rootDevice] = &system.FsStats{Root: true, Mountpoint: rootMountPoint}
rootKey := filepath.Base(rootMountPoint)
if _, exists := a.fsStats[rootKey]; exists {
rootKey = "root"
}
slog.Warn("Root device not detected; root I/O disabled", "mountpoint", rootMountPoint)
a.fsStats[rootKey] = &system.FsStats{Root: true, Mountpoint: rootMountPoint}
}
a.initializeDiskIoStats(diskIoCounters)
}
// Returns matching device from /proc/diskstats,
// or the device with the most reads if no match is found.
// Returns matching device from /proc/diskstats.
// bool is true if a match was found.
func findIoDevice(filesystem string, diskIoCounters map[string]disk.IOCountersStat, fsStats map[string]*system.FsStats) (string, bool) {
var maxReadBytes uint64
maxReadDevice := "/"
func findIoDevice(filesystem string, diskIoCounters map[string]disk.IOCountersStat) (string, bool) {
for _, d := range diskIoCounters {
if d.Name == filesystem || (d.Label != "" && d.Label == filesystem) {
return d.Name, true
}
if d.ReadBytes > maxReadBytes {
// don't use if device already exists in fsStats
if _, exists := fsStats[d.Name]; !exists {
maxReadBytes = d.ReadBytes
maxReadDevice = d.Name
}
}
}
return maxReadDevice, false
return "", false
}
// Sets start values for disk I/O stats.