Disk Caching in Azure

•Cache settings can make considerable improvement when read-write operations with large amount of data are done. However, if lot of random I/O operations are done, turning the cache off is preferable as operations on cache incur charges on the basis of number of transactions.

• Random operations will not make any signification improvement in performance.

•Can be configured using portal and powershell

ReadOnly

By configuring ReadOnly caching on Premium Storage data disks, you can achieve low Read latency and get very high Read IOPS and Throughput for your application. This is due two reasons,

Reads performed from cache, which is on the VM memory and local SSD, are much faster than reads from the data disk, which is on the Azure blob storage.

Premium Storage does not count the Reads served from cache, towards the disk IOPS and Throughput. Therefore, your application is able to achieve higher total IOPS and Throughput.

ReadWrite
By default, the OS disks have ReadWrite caching enabled. We have recently added support for ReadWrite caching on data disks as well. If you are using ReadWrite caching, you must have a proper way to write the data from cache to persistent disks. For example, SQL Server handles writing cached data to the persistent storage disks on its own. Using ReadWrite cache with an application that does not handle persisting the required data can lead to data loss, if the VM crashes.

As an example, you can apply these guidelines to SQL Server running on Premium Storage by doing the following,

Configure “ReadOnly” cache on premium storage disks hosting data files.
a. The fast reads from cache lower the SQL Server query time since data pages are retrieved much faster from the cache compared to directly from the data disks.
b. Serving reads from cache, means there is additional Throughput available from premium data disks. SQL Server can use this additional Throughput towards retrieving more data pages and other operations like backup/restore, batch loads, and index rebuilds.

Configure “None” cache on premium storage disks hosting the log files.
a. Log files have primarily write-heavy operations. Therefore, they do not benefit from the ReadOnly cache.

source : https://docs.microsoft.com/en-us/azure/storage/storage-premium-storage-performance#disk-caching/ 

Disk Type	Default Cache Setting
OS disk	ReadWrite
Data disk	None

Following are the recommended disk cache settings for data disks,

Disk Caching Setting	Recommendation on when to use this setting
None	Configure host-cache as None for write-only and write-heavy disks.
ReadOnly	Configure host-cache as ReadOnly for read-only and read-write disks.
ReadWrite	Configure host-cache as ReadWrite only if your application properly handles  writing cached data to persistent disks when needed.