Whenever you are planning to buy SQL Server Database Service under Azure, you have a question What is DTU and eDTU?
Under Azure SQL Server plans, we can measure database in two units, DTU and eDTU
What is DTU in Azure SQL Server?
DTU is Database Transaction Unit. So, what exactly it means? We have heard about CPU, RAM, IOPs and all, but what is that new thing DTU?
What Microsoft Describes it on azure website
A database transaction unit (DTU) represents a blended measure of CPU, memory, reads, and writes.
In simple words, DTU is combined unit of CPU, RAM and IO operations where planner can decide the overall usage requirement of his database.
DTU measure is used while selecting Single Database Plan
Single Database Plan
This plan is for the user/customer who need single or couple of databases which performs individually. None of them should affect the performance of other database. Single database can have minimum 5 DTU plan where user get maximum 2GB of storage limit. As name says, its really a Basic tier. 5 DTU is useful for very small database and which has no importance of performance to user.
There are multiple tiers which starts from 10 DTU to 4000 DTUs with included storage limit starting from 250GB to 4TB. User can also extend storage limit with additional costs
What is SQL Server Elastic Pool in Azure ?
As we discussed above, DTU is blended measure of CPU, memory and IO operations, in the same manner eDTU is also a measure of collective DTUs for multiple resources/ databases.
Lets say we have a SAAS software and we have 20 databases under one Elastic Pool. We have created each database per client. When we say 50 eDTU for a pool means that. 50 DTU is shared between all the active databases who are currently operating. When we have 20 clients under single elastic pool sharing 50 eDTU. It is not necessary that all the clients are active and doing overloaded operations on database. When 5 databases are doing some operations at the same time, they will operate on 10 DTU or near number as per requirement. This is automated system, whenever any database requires higher performance, it will get more DTU capacity upto 50 DTUs or given maximum limit.
What to choose now?
Elastic pools start with 50 eDTU and included storage 50 GB per pool having 500GB capacity (extendable) and 100 Maximum number of database per pool. As per my personal experience this package is good for small SAAS softwares or limited requirement up to 25 to 30 database with a good performance on heavy database.
You can still choose Single Database model with any DTU tier if you have only one database requirement. In case you have limited budget with strong security and medium level performance requirement you can give it try with 5 or 10 DTU tier. You can scale up in future too.
Because Experience is the best teacher ever.