Azure storage performance and management

Question

Could someone help me to understand following topics.

First of all I am on a trial account mode, maybe this is my issue, but I ma not sure yet.

I installed a Linux VM (OpenLogic CentOS6.3). I attached an empty data disk. Both disks OS and Data created on the same storage account. I tested IO performance using dd Linux tool (I understand that this is not a very relevant test especially for database VM, but still can give me an idea which disk is faster). The result was pretty similar from both disks. Here are my questions.

Azure storage performance:

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Is it true that Azure storage is SSD based storage?

Why the IO write performance on OS disk faster then on Data disk? I have red somewhere that it should be opposite. On the other hand reads are similar.

OS disk writes:

dd if=/dev/zero of=speedtest bs=64k count=3200 conv=fdatasync oflag=direct

3200+0 records in 3200+0 records out 209715200 bytes (210 MB) copied, 11.457 s, 18.3 MB/s

Data disk writes:

dd if=/dev/zero of=speedtest bs=64k count=3200 conv=fdatasync oflag=direct

3200+0 records in 3200+0 records out 209715200 bytes (210 MB) copied, 22.9221 s, 9.1 MB/s

OS disk reads:

dd if=speedtest of=/dev/null bs=64k count=24000

3200+0 records in 3200+0 records out 209715200 bytes (210 MB) copied, 0.463566 s, 452 MB/s

Data disk reads:

dd if=speedtest of=/dev/null bs=64k count=24000

3200+0 records in 3200+0 records out 209715200 bytes (210 MB) copied, 0.493757 s, 425 MB/s

Azure storage management:

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I have red in Azure documentation somewhere that it is always good idea to attach data disk from a separate storage account. When I tried to attach empty disk I didn't have an option to choose a different storage account. Everything is created on the same storage account where VM OS disk was created. Is it possible to archive that?

Answer 1

OS disks have read/write cache enabled by default, whereas attached data disks have no cache enabled by default (see this post for more details). This would give you different results when running IO perf tests.

Regarding storage accounts: The portal only lets you set up a data disk in the same storage account. Using PowerShell, you can choose any location. Here's the documentation for Add-AzureDataDisk - take a look at the -MediaLocation option. However: Depending on the workload you're driving, you might not really see a difference though. Each storage account is capable of 20K transactions / second, with up to 10Gbps inbound and 15Gbps outbound (see this post for details). I don't think a single VM (even an A7 VM with 2000Mbps on the NIC) could saturate a storage account's throughput target, though you might be able to hit the transactional target.

I would start off storing your disks in a single storage account. You can always copy the vhd to another account in the future, and then unmounts/remount the drive easily.