Sunday, 5 April 2020

Disks performance in Azure

In our earliest post, we have discussed in detail about the factors that affect the disk performance and different types of disk offerings in the AWS platform. In this blog post, let us go through different types of disk available in the Azure Platform.

Azure offers three types of disk :
  • Standard SSD
  • Premium SSD 
  • Ultra Disk
Apart from this, Azure provides a standard HDD. These disks are low-cost disk support for VMs running low latency workload. Standard HDD offers write latencies under 10ms and read latencies under 20ms.

Before describing the characteristic of each disk type, let us go through some common facts:
  • I/O operation less than or equal to 256 KiB of size is considered as a single IO operation. AWS supports the IO size of 128 KiB for each IOPS. In azure, a disk with 1000 IOPS can attain maximum throughput of 1000X256=256000KB=250MiB/sec, but Azure put max throughput limit for each disk type and that will be less than or equal to the possible max throughput (IOPS X 256 KB)
  • Azure provides predefined disk size except for ultra disk. Users are allowed to create custom size disks but will be charged at the rate of next available size. For example, if the user creates a disk with 200 GB, he will be billed for the next available disk size offered by Azure. This is not applicable to Ultra Disk. With ultra disk, users can select the custom size and charge for that size.
  • Apart from the disk level maximum limit of  IOPS and throughput, Azure implements throttling at the VM level for IOPS and throughput. This cap works on the aggregate value of  IOPS and throughput of all disk attached to the VM.

Standard SSD: Standard SSD disks are low-cost SSD provides better latency than the standard HDD. Standard SSD disks are best suitable for OS disk, web servers, and low IOPS application servers. Azure offers fourteen different disk sizes ranging from 4GB to 32 TB with IOPS ranging from 500 IOPS to 6000 IOPS and throughput range between 25MiB/sec to 750MiB/sec. Actual IOPS and throughput may vary depending on the traffic pattern. As far as I understood from the documentation, these are the maximum IOPS and throughput but not guaranteed always. For exact details of size and  IOPS refer to azure page 

Premium SSD : Azure premium SSD disks are high performing and low-latency disks designed to support IO-intensive workloads. Premium SSDs can only be used with the VM series that are compatible with premium SSD disks. Azure offers fourteen different disk sizes ranging from 4GB to 32 TB with IOPS ranging from 120 IOPS to 20000 IOPS and throughput range between 25MiB/sec to 900MiB/sec. The major difference from the standard SSD, premium SSD provides more consistent performance in terms of IOPS and throughput. For exact details of size and  IOPS refer to azure page 

Apart from this, premium SSD supports bursting for disk size up to 512 GB. These disks can burst up to 3500 IOPS and throughput up to 170 MiB/sec. Bursting is based on a credit system. Credits are automatically accumulated when disk traffic is below the provisioned performance and credits are automatically consumed when traffic bursts beyond the provisioned performance. The max burst limit defines the ceiling of disk IOPS and throughput even if you have accumulated burst credits. This helps to support random unpredictable spikes of the IO workload for disk up to 512 GB in size.

For example, a disk with a size of 128 GB has provisioned 500 IOPS and 100MiB/sec throughput. This disk supports the burstable performance of 3500 IOPS and 170 MiB/sec for maximum of 30 minutes.  The burstable limit is the same for all the disk size. When the disk workload goes below the provisioned IOPS and throughput ( 500 IOPS and 100MiB/sec), it starts accumulating the burstable credits and the same will be used when the workload goes beyond the provisioned IOPS and throughput. For example, if the actual workload is 200 IOPS and 80MiB/sec in the past one-second interval, then the unused 300 IOPS and 20MB are accumulated to the burst bucket of the disk.

Disk bursting will never go beyond VM throttling on  IOPS and throughput. Irrespective of burstable credits available, the disk performance never goes beyond the burstable limit (1700 IOPS and 170MiB/sec). For example, even if the disk has 10,000 IOPS  in the burstable credit bucket, the disk can not support more than the max burst limit of 3500 IOPS. In the same way, if the disk is attached to a VM with an IOPS limit of 1000 IOPS and throughput limit of 150 MiB/sec, the disk can't burst beyond the VM limit.

When a VM starts with the burstable disk, it starts with a full burst credit bucket. This helps the booting speed of the VM if the boot disk is a burstable disk . A burstable disk  credit bucket can be in any of the three states :
  • Accruing: When the disk traffic is less than the provisioned IOPS and throughput. 
  • Declining: when the disk traffic is using more than the provisioned IOPS and throughput. The burst traffic will independently consume credits from IOPS and throughput.
  • Remaining Constant: When the disk performance is exactly at the provisioned IOPS and throughput level.

Ultra Disk : Azure ultra disks provide high throughput, high IOPS and consistent low latency disk storage for IO intensive mission-critical application. Ultra disk can only be used as a data disk(not as OS disk). Ultra disk supports the disk size ranging from 4 GB to 64 TB. When we configure the ultra disk, we can customize the size and performance variables (IOPS and throughput) 
  • Ultra disk support 300 IOPS per GB with a maximum limit of 160000 IOPS per disk. For example, an 8GB disk can be configured to support a maximum of 2400 IOPS. All disk above the size of 533 GB, support only  160K IOPS. (533*300=159900)
  • The minimum configurable IOPS for an ultra disk is 2 IOPS per GB with a baseline of 100 IOPS per disk. for example, a 32 GB disks can be configured minimum of 100 IOPS instead of 64 IOPS. A 1000 GB disk minimum configurable value for IOPS is 2000
  • To get the benefit of the provisioned IOPS, ensure that the VM IOPS limit is higher than the IOPS configured for the disk. 
  • The throughput limit is 256KiB/sec for each provisioned IOPS with a maximum of 2000 MiB/sec per disk. The minimum throughput per disk is 4KiB/sec for each provisioned IOPS with a baseline of 1MB/sec.
  • Ultra disk supports adjusting the performance variables (Throughput and IOPS) at runtime without detaching disk or restarting the VM. After changing the performance variable, it will take a maximum of one hour to get it effective.
There are lots of restrictions in using the ultra disk. Refer to the azure website for up to date information. 

Disk Stripping for better performance: Use multiple disks and stripes them together to get a combined higher IOPS and throughput. For example, in premium storage , P40 (2TB) and P50(4TB) provide the same  IOPS (7500) and throughput (250MiB/sec). Instead of going for P50 disk, we can allocate two P40 disks and stripes them in OS level to get combined IOPS of 15000 and 500MiB/sec throughput. 

Hope this explains well about the different types of disk available in the Azure portal and keep these points in mind while allocating disk for better performance. In the next blog, we will go through different parameters to measure the performance of the disk.  


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