Introduction to Raid

Brief introduction

We've all heard of raid, and we often discuss raid as a SQL DBA, developer, or architect at work. But, in fact, many of us are not quite aware of the principles, levels of RAID, and how RAID affects SQL Server performance.

This article is to fill in this lesson.

Disk architecture

Today's disk, and the 70 's 45rpm (RPM) of the record machine is very similar (you can remember?), is only a rotation of the axis (track) of the media (face) and the data into a disk segment called sector.

Like the recording machine, the disk drive has a swing arm to control the needle, which can be called the "Head", to access the data. But for a disk, it's not as readable as a recording machine, but as readable and writable as it can be.

To read or write data from a particular sector, the disk must rotate and then swing the arm so that the head moves to the top of the specified sector to access the data.

This process is the basic input/output operation process (I/O).

IOPS

The term IOPS is also often used to put x, but again, not many people really understand the term.

Many people understand that IOPS is the abbreviation for input Output Operations per second, but translating this definition into the actual concept is a bit difficult for some people.

The basic understanding of IOPS is a measure of the average time required to meet a specific input-output request.

It is important to know that this metric is based on reading 0-byte files, only for statistical and standardized purposes because a disk sector size differs.

Limitations of physical disks

There are physical restrictions on the disk that limit the level of IOP that the disk can reach. This limitation is the track addressing time (seek times) and the rotation delay (rotational latency).

The track addressing time is used to allow the head to move to the sector to be read, and the average time it takes to move the swing arm.

The rotation delay is the time (usually millisecond) that the head reads the disk at a specific location on the disk.

The time formula for the unit IOP is as follows:

Unit IOP time = Track addressing time + rotational latency

So, with this formula, we can easily compute the maximum IOPS for a given disk.

The number of IOPS per second is also our most interesting, the formula is as follows:

1 sec/Track addressing time + rotation delay.

Let's look at the following examples:

HP 300GB 15k SAS Drive (200 knives)
Speed	15000
Average track addressing time	2.9ms
Average rotation delay	1.83ms

We can calculate IOPS with a formula:

IOPS = 1/(2.9ms + 1.83ms)
= 1/(4.73MS)
= 1/(0.00473)
= 211 IOPS

As we can see, the IOPS on this disk is 211 (not very miserable).

If we want to save more money, let's look at an example of a disk and how it differs from the disk above:

HP 300GB 7200 SATA Drive (100 knives)
Speed	7200
Average track addressing time	10ms
Average rotation delay	2.1ms