While preparing my session about In-Memory OLTP for internal audience, I learned something new about placement of checkpoint files. In case you don’t know about checkpoint files, I would like you to read about checkpoint (part 1 and part 2).

Data files and Delta files are created in round robin fashion. Let’s assume that we have two drives W and X and we want to distribute the IO load of In-Memory table. In general, for disk based table, what we used to do? Create the file on each drive? Yes, that would work for disk bases table but there is a little behavior which we should know before doing such thing for in-memory tables.

For demo purpose, I am going to use two folder on same drive to mimic two drives.

USE master
GO
IF EXISTS (
        SELECT *
        FROM sys.databases
        WHERE NAME = 'ObjectsDemo'
        )
    ALTER DATABASE ObjectsDemo SET  SINGLE_USER WITH ROLLBACK IMMEDIATE;
    DROP DATABASE ObjectsDemo
GO

CREATE DATABASE ObjectsDemo
GO

ALTER DATABASE ObjectsDemo 
ADD filegroup ObjectsDemo_mod CONTAINS memory_optimized_data
GO

-- Add Files 
ALTER DATABASE ObjectsDemo 
ADD FILE (
    NAME = 'ObjectsDemo_mod_W'
    ,filename = 'c:\dataHK\ObjectsDemo_W'
    ) TO filegroup ObjectsDemo_mod
GO

-- Add Files 
ALTER DATABASE ObjectsDemo 
ADD FILE (
    NAME = 'ObjectsDemo_mod_X'
    ,filename = 'c:\dataHK\ObjectsDemo_X'
    ) TO filegroup ObjectsDemo_mod
GO

So, I have two folders “ObjectsDemo_W” and “ObjectsDemo_X” to represent W and X drives. Once we create a in-memory table, we should see checkpoint file pairs (CFP) created. 

USE ObjectsDemo
GO

CREATE TABLE dbo.t1 (
    c1 INT NOT NULL PRIMARY KEY NONCLUSTERED
    ,c2 INT
    )
    WITH (memory_optimized = ON)
GO

Let’s have a look at folders now.

One folder (Drive W) contains only data files (pre-created at 16 MB each) and another folder (Drive X) contains only delta file (pre-created at 1 MB each). That would not put uniform load on the both folders. The files are placed in this fashion because data and delta files are created in container into round-robin fashion

Here is the tip: Since we have even number of drives, we should create two folders on each drive and place files in 4 containers instead of 2 containers. We need to remember that first two containers should be from same drive.

use master
go
IF EXISTS (
        SELECT *
        FROM sys.databases
        WHERE NAME = 'ObjectsDemo'
        )
    ALTER DATABASE ObjectsDemo SET  SINGLE_USER WITH ROLLBACK IMMEDIATE;
    DROP DATABASE ObjectsDemo
GO

CREATE DATABASE ObjectsDemo
GO

ALTER DATABASE ObjectsDemo 
ADD filegroup ObjectsDemo_mod CONTAINS memory_optimized_data
GO

-- Add Files 
ALTER DATABASE ObjectsDemo 
ADD FILE (
    NAME = 'ObjectsDemo_mod_W1'
    ,filename = 'c:\dataHK\ObjectsDemo_W1'
    ) TO filegroup ObjectsDemo_mod
GO

-- Add Files 
ALTER DATABASE ObjectsDemo 
ADD FILE (
    NAME = 'ObjectsDemo_mod_W2'
    ,filename = 'c:\dataHK\ObjectsDemo_W2'
    ) TO filegroup ObjectsDemo_mod
GO

-- Add Files 
ALTER DATABASE ObjectsDemo 
ADD FILE (
    NAME = 'ObjectsDemo_mod_X1'
    ,filename = 'c:\dataHK\ObjectsDemo_X1'
    ) TO filegroup ObjectsDemo_mod
GO

-- Add Files 
ALTER DATABASE ObjectsDemo 
ADD FILE (
    NAME = 'ObjectsDemo_mod_X2'
    ,filename = 'c:\dataHK\ObjectsDemo_X2'
    ) TO filegroup ObjectsDemo_mod
GO

W1 and W2 are two container for mimicking two folder on W and same for X as well. Now, if we create a table, due to round robin, we should see below.

Now we have DATA file distributed and should have better usage of both driver. What you should do if you have ODD number of drives? Well, nothing because of round robin, data and delta files would be distributed automatically. 

Bottom line: If you have plans to create file stream containers on odd number of drives, create them using trick mentioned above. If you have even number of containers then no special consideration.

Hope you have learned something new today.