Persistent Disk: ACE Exam Study Guide

Image source: Google Cloud Documentation

1. Overview

Persistent Disk is a durable storage solution for Google Cloud VMs. Data is replicated automatically for durability and resides independently from VM lifecycle.

Key Characteristics:

• Block storage (like a physical hard drive)
• Automatically encrypted by default (AES-256)
• Can be attached to only one VM at a time (except hyperdisk multi-writer mode)
• Survives VM termination/deletion
• Regional PD replicates data across zones automatically

2. Disk Types

Standard Hard Disks

SSD Hard Disks

Extreme (Extreme Persistent Disk)

3. Disk Performance

Factors affecting performance:

• Disk size (larger disks = better baseline performance)
• Instance machine type (instance must support high IOPS)
• Number of vCPUs on the instance

4. Disk Size Limits

Important: You can only increase disk size, not decrease it.

5. Local Solid-State Drive (Local SSD)

Local SSDs are physically attached to the host server, not the network.

Characteristics:

• Temporary: Data is lost when VM stops or is preempted
• Highest performance: Slower latency, higher IOPS than Persistent Disk
• Use case: Scratch space, caches, temporary data
• Cost: Charged while VM is running (not when stopped)
• Encryption: Always encrypted; keys managed by Google
• Limit: Maximum 8 Local SSD partitions (375 GB each = 3 TB total)
  • up to 24 Local SSD, depending on machine type

Not for: Databases, anything requiring durability

6. Regional Persistent Disk (High Availability)

Regional PD replicates data across two zones in the same region automatically.

Use when:

• High availability is required
• Cannot tolerate zone failure
• Running production workloads

Trade-offs:

• ~2x cost of zonal PD
• Higher write latency (data written to two zones)
• Cannot be used for disk sharing between instances

7. Snapshots

Snapshots are incremental backups of Persistent Disks stored in Cloud Storage.

Characteristics:

• Incremental: Only changes since last snapshot are stored (reduces cost)
• Cross-region: Can be used to create disks in different regions
• Encryption: Encrypted by default (Google-managed keys, or CMEK if configured)
• Consistency: For consistent snapshots of multiple disks, use snapshot-schedule with application-consistent quiescing

Creating a snapshot:

gcloud compute disks snapshot [DISK_NAME] --region=[REGION]

Restoring from snapshot:

gcloud compute disks create [NEW_DISK] --source-snapshot=[SNAPSHOT]

8. Disk Operations

Attaching/Detaching

Rules:

• Disk must be in same zone as VM
• Can attach while VM is running (hot-add)
• Must unmount filesystem before detaching

Resizing

gcloud compute disks resize [DISK] --size=[NEW_SIZE_GB]

• Always possible: Increase disk size online (no restart needed for most OS)
• Never possible: Decrease disk size (must recreate disk at smaller size)
• After resizing: Must extend filesystem within the VM (resize2fs, diskpart, etc.)

Moving Disks Between Zones

gcloud compute disks move [DISK] --destination-zone=[ZONE] --zone=[CURRENT_ZONE]

9. Sharing Disks

Read-only Sharing

• Attach a single Persistent Disk to multiple VMs in read-only mode
• Use case: Sharing OS images, read-only data

Multi-writer Mode (Hyperdisk)

• Allows attaching a disk to multiple VMs in read-write mode
• Requires hyperdisk type (Extreme, Throughput, or Balanced)
• Use case: Clustered databases (GCS, GlusterFS, etc.)

10. Encryption Options

11. GKE Integration

Google Kubernetes Engine uses Persistent Disk primarily through Kubernetes PersistentVolumes (PV) and PersistentVolumeClaims (PVC).

Storage Classes

GKE uses predefined Storage Classes to provision Persistent Disks:

Volume Modes

• Filesystem (default): Mount as directory; supports ReadWriteOnce and ReadOnlyMany
• Block: Raw block device; supports ReadWriteOnce and ReadWriteMany (hyperdisk only)

Access Modes

StatefulSets

Use Persistent Disk with StatefulSets for workloads requiring stable identity and persistent storage:

• Each pod gets a unique PersistentVolumeClaim
• Pods are ordered for deployment/deletion
• Volume persists across pod rescheduling

Key Points for Exam

• Zonal: GKE nodes and PD must be in the same zone
• Regional clusters: Use Regional PD for HA across zones
• Node affinity: PD auto-attaches to the node where the pod is scheduled
• Disk size: Cannot decrease PVC size (same as standalone PD)
• Regional PD: Requires GKE 1.26+ or GKE Standard mode for multi-zone volume placement

Kubernetes config files

PersistentVolume (PV):

gcloud compute disks create my-gke-disk \
    --size=20Gi --zone=europe-central2-a

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 20Gi
  accessModes:
    - ReadWriteOnce
  persistentVolumeReclaimPolicy: Retain
  storageClassName: "my-pv"
  gcePersistentDisk:
    pdName: my-gke-disk
    fsType: ext4

PVC using default GKE StorageClass:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: my-pvc
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 10Gi
  storageClassName: "my-pv"
  volumeName: my-pv

Pod mounting the Persistent Disk:

apiVersion: v1
kind: Pod
metadata:
  name: my-pod
spec:
  containers:
    - name: app
      image: nginx
      volumeMounts:
        - mountPath: "/data"
          name: my-storage
  volumes:
    - name: my-storage
      persistentVolumeClaim:
        claimName: my-pvc

12. Common Exam Gotchas

1. Data loss on stop: Local SSD data is lost when VM stops (not Persistent Disk)
2. Cannot decrease size: You can only increase disk size
3. Zone requirement: Disk and VM must be in same zone
4. Single attachment: Standard PD can only attach to one VM at a time
5. Snapshot deletion: Deleting a snapshot does not delete the disk (incremental)
6. Disk performance scales with size: Larger disks = better IOPS/throughput
7. Instance vCPU limits: Instance must have enough vCPUs to utilize disk performance
8. Boot disk deletion: By default, boot disk is deleted when VM is deleted (check “Delete boot disk” checkbox to keep)

13. Practice Questions

Q1: You need storage that survives VM deletion. What should you use?

Answer: Persistent Disk (local SSD is ephemeral)

Q2: A VM needs maximum IOPS for a database. What disk type?

Answer: pd-extreme (or pd-ssd if not extreme needed)

Q3: A disk needs to be attached to multiple VMs simultaneously. What mode/type?

Answer: Hyperdisk with multi-writer mode

14. Quick Reference Summary

15. External Links

• Persistent Disk - The Cloud Girl
• Which Storage Should I Use - The Cloud Girl
• What are different storage types - The Cloud Girl