Migrate Persistent Disk to Hyperdisk for SAP HANA

This document describes how you can migrate the Persistent Disk volumes used by your SAP HANA databases to Google Cloud Hyperdisk volumes.

Hyperdisk, especially, Hyperdisk Extreme, provides better performance for SAP HANA than the SSD-based Persistent Disk types.

To migrate the Persistent Disk volumes used by your SAP HANA database to Hyperdisk volumes, you use Google Cloud's disk snapshots and the SAP HANA Fast Restart Option.

Review migration considerations

Migration data: The SAP HANA Fast Restart option is used as a helper. It reduces downtime while grating the disks by eliminating the need to wait for your database tables to load. However, make sure to take into account the time needed to reload row store and Binary Large Object (BLOB) data types.
Downtime: Although the migration process requires minimal downtime, the actual duration of the downtime depends on the time taken to complete the following tasks:
- Creating disk snapshots.
  
  To reduce downtime while taking disk snapshots, you can take snapshots of the disks before the planned migration activity, and then take some additional snapshots closer to the activity, which results in a smaller difference between the snapshots.
- Creating Hyperdisk volumes by using the snapshots of your Persistent Disk volumes.
- Reloading the SAP HANA tables into SAP HANA memory.
Reverting to existing disks: In the event of any problem during the migration, you can revert to the existing disks because they are unaffected by this procedure, and are available till you delete them yourself. For more information, see Fall back to existing disks.

Before you begin

Before you migrate the Persistent Disk volumes used by your SAP HANA database to Hyperdisk volumes, make sure the following conditions are met:

SAP HANA is running on SAP-certified Compute Engine instances that support Hyperdisk.
The /hana/data and /hana/log volumes are hosted on separate disks.
Linux logical volume management is used for SAP HANA storage persistence. While direct storage can be used, it would require explicit device remapping through the /etc/fstab table.
The SAP HANA Fast Restart Option is enabled for your SAP HANA system.

For information about how to enable this, see Enabling SAP HANA Fast Restart.
A valid backup of the SAP HANA database is available. This backup can be used to restore the database, if required.
If the target Compute Engine instance is part of a high-availability (HA) cluster, then make sure that the cluster is in maintenance mode.
If your SAP HANA database uses a scale-out deployment, then repeat the procedure in this guide for each SAP HANA instance.
The SAP HANA database is up and running.

The tmpfs file system is fully loaded with the content of the MAIN data fragments. To view the file system utilization, run the following command:

df -Th

The output is similar to the following example:

#  df -Th
Filesystem                        Type      Size  Used Avail Use% Mounted on
...
/dev/mapper/vg_hana_shared-shared xfs       1.0T   56G  968G   6% /hana/shared
/dev/mapper/vg_hana_data-data     xfs        14T  5.7T  8.2T  41% /hana/data
/dev/mapper/vg_hana_log-log       xfs       512G  7.2G  505G   2% /hana/log
/dev/mapper/vg_hana_usrsap-usrsap xfs        32G  276M   32G   1% /usr/sap
tmpfsDB10                         tmpfs     5.7T  800G  4.9T  14% /hana/tmpfs0/DB1
tmpfsDB11                         tmpfs     5.7T  796G  4.9T  14% /hana/tmpfs1/DB1
tmpfsDB12                         tmpfs     5.7T  783G  4.9T  14% /hana/tmpfs2/DB1
tmpfsDB13                         tmpfs     5.7T  780G  4.9T  14% /hana/tmpfs3/DB1
tmpfsDB14                         tmpfs     5.7T  816G  4.9T  15% /hana/tmpfs4/DB1
tmpfsDB15                         tmpfs     5.7T  780G  4.9T  14% /hana/tmpfs5/DB1
tmpfsDB16                         tmpfs     5.7T  816G  4.9T  15% /hana/tmpfs6/DB1
tmpfsDB17                         tmpfs     5.7T  780G  4.9T  14% /hana/tmpfs7/DB1

Migrate Persistent Disk volumes to Hyperdisk volumes

This section describes how you migrate the /hana/data and /hana/logvolumes from Persistent Disk volumes to Hyperdisk volumes.

To illustrate the migration process, the following example configuration is used:

Machine type: m2-ultramem-416 (12 TB memory, 416 vCPUs)
SAP HANA scale-up system deployed by using the Google Cloud document Terraform: SAP HANA scale-up deployment guide.
- OS: SLES for SAP 15 SP1
- SAP HANA: HANA 2 SPS06, Patch 63
- Default disk type: SSD Persistent Disk (pd-ssd)
- /hana/data and /hana/log volumes are mounted on separate disks and are built using LVM and XFS
- The SAP HANA Fast Restart Option is enabled and 6 TB of data is loaded into the database. For information about how to do this, see Enabling SAP HANA Fast Restart.

To migrate Persistent Disk volumes to Hyperdisk volumes, perform the following steps:

Stop the SAP HANA database by running the HDB stop command:
```
HDB stop
```
Alternatively, you can run the sapcontrol command:
```
sapcontrol -nr INSTANCE_NUMBER -function StopSystem HDB
```
Replace INSTANCE_NUMBER with the instance number for your SAP HANA system.

For more information, see the SAP document Starting and Stopping SAP HANA Systems.
Unmount the /hana/data and /hana/log volumes from the file system:
```
umount /hana/data
umount /hana/log
```

Determine the names disks hosting the /hana/data and /hana/log volumes by using any of the following methods:

Run the ls command:

ls -l /dev/disk/by-id/

The output shows the mapping of disk names to devices, similar to the following example:

...
lrwxrwxrwx 1 root root  9 May 18 20:14 google-hana-vm-data00001 -> ../../sdb
lrwxrwxrwx 1 root root  9 May 18 20:14 google-hana-vm-log00001 -> ../../sdc
...

Run the gcloud compute instances describe command:

gcloud compute instances describe INSTANCE_NAME
    --zone=ZONE

Replace the following:

INSTANCE_NAME: the name of the compute instance.
ZONE: the zone of the compute instance.

The output shows details about the compute instance, including the disks attached to it. For example:

gcloud compute instances describe hana-vm --zone europe-west4-a
...
disks:
- autoDelete: false
deviceName: hana-vm-shared00001
diskSizeGb: '1024'
- autoDelete: false
deviceName: hana-vm-usrsap00001
diskSizeGb: '32'
- autoDelete: false
deviceName: hana-vm-data00001
diskSizeGb: '14093'
- autoDelete: false
deviceName: hana-vm-log00001
diskSizeGb: '512'

In the Google Cloud console, go to the Compute Engine VM instances page, and click the compute instance name. The Storage section displays the associated disk information.

Create snapshots of the disks hosting the /hana/data and /hana/log volumes:
```
gcloud compute snapshots create DATA_DISK-snapshot \
  --project=PROJECT_ID \
  --source-disk-zone=SOURCE_DISK_ZONE  \
  --source-disk=DATA_DISK \
gcloud compute snapshots create LOG_DISK-snapshot \
  --project=PROJECT_ID \
  --source-disk-zone=SOURCE_DISK_ZONE \
  --source-disk=LOG_DISK
```
Replace the following:
- DATA_DISK: the name of the disk that hosts the /hana/data volume, which is set as a prefix for the data disk's snapshot
- LOG_DISK: the name of the disk that hosts the /hana/log volume, which is set as a prefix for the log disk's snapshot
- PROJECT_ID: the ID of the Google Cloud project where the disk is deployed
- SOURCE_DISK_ZONE: the Compute Engine zone where the disk is deployed
For more information about creating snapshots, see Create and manage disk snapshots.

Important: Because snapshots incrementally back up data from your Persistent Disk volumes, the initial snapshot typically takes longer to create than subsequent snapshots, especially for large disks. To reduce the downtime while taking snapshots, you can take snapshots of the disks before the migration process starts and the SAP HANA database is stopped. You can also take multiple snapshots before maintenance.
By using the disk snapshots that you created, create Hyperdisk volumes for the /hana/data and /hana/log volumes:
```
gcloud compute disks create DATA_DISK-hd \
     --project=PROJECT_ID \
     --zone=ZONE \
     --type=HYPERDISK_TYPE \
     --provisioned-iops=IOPS_DATA_DISK \
     --source-snapshot=DATA_DISK-snapshot \
 gcloud compute disks create LOG_DISK-hd \
     --project=PROJECT_ID \
     --zone=ZONE \
     --type=HYPERDISK_TYPE \
     --provisioned-iops=IOPS_LOG_DISK \
     --source-snapshot=LOG_DISK-snapshot
```
Replace the following:
- DATA_DISK: the name of the Persistent Disk volume that hosts the /hana/data volume, which is set as a prefix to the name of the Hyperdisk volume that you create
- LOG_DISK: the name of the Persistent Disk volume that hosts the /hana/log volume, which is is set as a prefix to the name of the Hyperdisk volume that you create
- PROJECT_ID: the ID of the Google Cloud project
- HYPERDISK_TYPE: the type of Hyperdisk that you want to create, such as hyperdisk-extreme
- ZONE: the Compute Engine zone where you need to deploy the Hyperdisk volumes
- IOPS_DATA_DISK: the IOPS that you want to set for the Hyperdisk you're creating to host /hana/data. You set the IOPS according to your performance requirements.
- IOPS_LOG_DISK: the IOPS that you want to set for the Hyperdisk you're creating to host /hana/log. You set the IOPS according to your performance requirements.
If you're creating Hyperdisk Balanced (hyperdisk-balanced) volumes, then you can also set its throughput.

For information about the minimum size, IOPS, and throughput that you need to set for using Hyperdisk volumes with SAP HANA, see the row corresponding to your Compute Engine machine type in Minimum sizes for SSD-based Persistent Disk and Hyperdisk volumes.

For more information about restoring a disk from a snapshot, see Restore from a snapshot.
From the Compute Engine instance hosting your SAP HANA database, detach the Persistent Disk volumes that are hosting your /hana/data and /hana/log volumes:
```
gcloud compute instances detach-disk INSTANCE_NAME \
  --disk=DATA_DISK \
  --zone=ZONE
gcloud compute instances detach-disk INSTANCE_NAME \
  --disk=LOG_DISK \
  --zone=ZONE
```
Replace the following:
- INSTANCE_NAME: the name of your Compute Engine instance
- DATA_DISK: the name of the Persistent Disk volume hosting your /hana/data volume, which you want to detach
- LOG_DISK: the name of the Persistent Disk volume hosting your /hana/log volume, which you want to detach
- ZONE: the Compute Engine zone where the disk is deployed
Important: Don't delete the Persistent Disk volumes used by your SAP HANA database till you've completed the migration procedure successfully.
Attach the Hyperdisk volumes that you created to the Compute Engine instance hosting your SAP HANA database:
```
gcloud compute instances attach-disk INSTANCE_NAME \
    --disk=DATA_DISK-hd \
    --zone=ZONE
gcloud compute instances attach-disk INSTANCE_NAME \
   --disk=LOG_DISK-hd \
    --zone=ZONE
```
Replace the following:
- INSTANCE_NAME: the name of the Compute Engine instance
- DATA_DISK: the name of the Hyperdisk volume that you want to use to host the /hana/data volume
- LOG_DISK: the name of the Hyperdisk volume that you want to use to host the /hana/log volume
- ZONE: the Compute Engine zone where the Hyperdisk is deployed

To mount the Hyperdisk volumes, perform the following steps as root or a user having sudo access:

Remove all device mapping definitions to avoid LVM device mapping conflicts:
```
dmsetup remove_all
```

Scan all disks for volume groups, rebuild caches, and create missing volumes, including LVM:

vgscan -v --mknodes