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We have been using an azure VM for hosting SQL Server. A4 size i.e. 4 cores & 7GB RAM We have noticed intermittent slow performance of database.

We are worried that since azure VM is multi-tenant instance, it could not be working at 4 cores performance always.

We are trying to understand that when we fire a 4 core VM, does it mean we always have that much compute power? or will it reduce depending on other users?

Abhijeet
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2 Answers2

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The first thing you should do is measure why your your database is performing so slow. Are you hitting the memory limit of your VM? The CPU limit? Or is the performance of the data disks an issue (IOPS)?

On MSDN there's a checklist with things you need to consider when hosting SQL Server in Virtual Machines:

  • Use minimum Standard Tier A2 for SQL Server VMs.
  • Keep the storage account and SQL Server VM in the same region.
  • Disable Azure geo-replication on the storage account.
  • Avoid using operating system or temporary disks for database storage or logging.
  • Avoid using Azure data disk caching options (caching policy = None).
  • Stripe multiple Azure data disks to get increased IO throughput.
  • Format with documented allocation sizes.
  • Separate data and log file I/O paths to obtain dedicated IOPs for data and log.
  • Enable database page compression.
  • Enable instant file initialization for data files.
  • Limit or disable autogrow on the database.
  • Disable autoshrink on the database.
  • Move all databases to data disks, including system databases.
  • Move SQL Server error log and trace file directories to data disks.
  • Apply SQL Server performance fixes.
  • Setup default locations.
  • Enable locked pages.
  • Backup directly to blob storage.
Sandrino Di Mattia
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    Very good advice. And to answer the actual question "does it mean we always have that much compute power?" --- Yes. You are always guaranteed the number of cores you allocate. A single server in Azure datacenter will not host more VMs than it has available cores. – kwill Jun 13 '14 at 16:49
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Azure will not share cores and memory as long as you are not choosing the smallest VM sizes.

However, keep in mind that other tenants can still interfere with you, mostly on the network traffic. I/O to and from persistent drives (any drive except D: ) also goes through network.

Evgeny Krivosheev
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