There are a lot of “best practice” and “performance guide” links available on the Internet to provide guidance on migrating SQL Server instances and databases from on-prem to Azure on VMs (aka Azure IaaS). In this post, I wanted to share a few findings from the field that I’ve encountered along the way recently working with customers on projects that migrate from on-prem SQL Server to Azure VMs. This is not a guide to migrating to Azure SQL Database and I do not cover all of the issues, migration steps or performance measures in an exhaustive manner. I will share a number of good links to help in that respect. But Azure as a platform is changing and evolving very rapidly and so it seemed like a good time to catch my breath and share a few tips here.
Tip #1: Take the time to analyze and evaluate current on-prem workloads prior to database lift and shift
Don’t just jump right into SSMS and use “Migrate to Azure”. Benchmark your current database and resource utilization. Measure IOPS, TX/s, CPU, RAM, network … all classic SQL Server constraints. This will help to determine the proper class of VM to choose for your SQL Server instances. Azure VMs come in classes of resources and you need to make sure that you have enough capacity. I generally recommend DS_v2 and G-series VMs. You can always scale-up to higher-numbered VM in the same series in Azure. But note that there are resource capacity limits grouped together (RAM, IOPS, disks, network) in each series and they all scale-up together when you upgrade your VMs.
Tip #2: Look at Azure Managed Disks for production workloads
Microsoft recently released Managed Disks (https://azure.microsoft.com/en-us/services/managed-disks/) as a way to eliminate the need to manually manage the details of the Azure IO subsystem in terms of storage accounts, relaxes limitations on bandwidth and IOPs that storage accounts imposed and provides a higher level of VM reliability. The absolute best detailed description of Managed Disks is here by Igor Pagliai.
You can choose MDs if you choose to build a SQL Server VM from a base Windows image in Azure or a pre-built SQL Server image. If you choose MDs or UMDs, make sure the use premium disks for production environments. Azure provides a 99.9% SLA with single-instance VMs but they must be using premium disks.
Tip #3: Backup to URL
You are running your SQL Server locally on a VM in the Cloud. Make sure that you use the Backup-to-URL feature in SQL Server to store your backups in Azure. You can use standard storage for your backups and consider GRS to keep geo-redundant copies of your backups, relying on Azure’s behind-the-scenes async replication.
Tip #4: Understand the Azure storage subsystem
If you are not going to use Managed Disks (see above), aka UMDs, then you will need to manage the storage accounts yourself. Here is a good SQL Server on Azure IaaS VMs perf guide with an Azure storage explainer that I like: http://www.faceofit.com/performance-tuning-best-practices-for-sql-on-azure-vm/.
Here are some tips below on storage, disk and LUN usage within SQL Server VMs in Azure. I indicate relevance to managed disks (MDs) and unmanaged disks (UMDs):
- (UMD) Build 2 additional Storage Accounts (SAs) after provisioning the VM. Keep the original VHDs where they are and add new premium disks for data and log in separate SAs. You do this from the VM’s Disks blade. Make sure to do this entire process one at a time. This way you can keep better track of the LUN #s to be sure that the SA being presented to the Windows OS is the LOG vs. the DATA LUN. SQL Server also allows you to also map database files directly to blob storage using URLs, as opposed to presenting LUNs to the OS, see here. There are a number of limitations listed in that document and since mapping raw disks to volumes and drives is very common and storage/database admins are comfortable with that approach, I typically do not recommend it at this time.
- (UMD/MD) Keep TempDB mapped to local disks, which are SSDs on the VM. It is also a good practice to use the local SSDs for BPE: https://blogs.technet.microsoft.com/dataplatforminsider/2014/09/25/using-ssds-in-azure-vms-to-store-sql-server-tempdb-and-buffer-pool-extensions/.
- (UMD/MD) We need to determine if the database needs optimization for read or write. For read workloads, set disk caching policy to “ReadOnly”. For write-intensive (and for SQL log files disk & tempdb disk, if you store tempdb on separate VHDs) set disk caching policy to “None”. This is on the Attach New Disk blade.
- (UMD/MD) Those are raw volumes, so we need to map them disk letters in Server Manager from Windows Server on the VM. Create a folder for LOG and a folder for DATA in each of the new volumes from the respective drive letter home dir.
- (UMD/MD) For TempDB, create a data (rows) file for each CPU core exposed to the VM. I.e. if the VM has 8 cores, generate 8 TempDB data files.http://sqlmag.com/storage/sql-server-storage-best-practices
- (UMD) You are now striped across multiple disks, SAs and multi-threaded maximizing available resources and ensuring that you are not getting IOPs and TX/sec capped.
- (UMD/MD) Azure disks are limited to 1 TB, so if you need files and drives larger than 1 TB, create a virtual volume using Windows Storage Spaces to pool multiple VHDs together.
Tip #5: SQL Server images from the Azure Gallery are easy and quick, but be careful
By far, the quickest and easiest way to start-up a new SQL Server in Azure is to pick the SQL Server image that you want to spin-up from the Azure gallery. Just be aware that you must (a) pick bring-your-own-license to use your existing MSFT SQL Server licenses or (b) pay-as-you-go pricing that is baked into the monthly $$ cost that you see on your bill for the amount of time that the VM is running. Also, you cannot upgrade SQL Server on VM created from the gallery. You will need to migrate your databases to the next version of SQL Server on a new image.
Tip #6: High Availability & Disaster Recovery
Best practice in production is to use Availability Groups. There is a pre-built image option available in the Azure gallery for SQL Server that builds out the networking and secondary nodes for an AG for you. It also created the ILB/ELB that Azure requires to handle routing to primary nodes. If you run single-instance VMs for your SQL Server, make sure that you choose Premium Storage to gain that 99.9% SLA from MSFT Azure.
That’s all that I’ll cover for now. These are some of the basics, updated as of April 2017. Since joining the Azure team @ MSFT 2 years ago, I’ve learned to be very careful about regularly updating these sorts of lists and indicating their dates of validity / spoil-by-date!