SQL 2012 :: Read And Write Speeds Not Increased With Additional Disks

Why I see absolutely no performance improvement when I spread my primary file group over 8 separate files on 8 separate disks, as opposed to having the primary file group all in one file on one disk.

I have set up 2 identical databases, one spread over 8 disks and one on one disk. Each database has a table called DATA and a column called VALUE. Value is NVARCHAR(200). I have filled each table up in both databases with 20,000 rows.

I then perform a select on each table in each database using CHECKPOINT and DBCC DROPCLEANBUFFERS to ensure I am reading from disk before each query and the execution times are identical in both databases.

I then ran the same queries against each database using a load testing tool and the batch requests per second on each DB is identical under load.

Surely the database with data spread over 8 disks should be FAR faster than the single file database as you have the combined reading power of 8 disks as opposed to 2??

Also, the same is happening for write speeds. When I create the data on both databases, the time it takes is identical on both.

BOL says it should be faster with multiple disks.

Just FYI this is on an Azure virtual machine and each disk is a locally redundant data disk that I have attached to the virtual machine.

Whether write speeds should increase with multiple disks or just read speeds?