About Hard Disk Drive Interfaces

Hard Disk Drive Connectivity:

A hard disk drive. You know what that is! It is the most important component of your computer/server/mainframe/storage device or whatever you use to save large amounts of data. In this page, I'm just going to put a brief about what I learnt today about the hard drive connectivity, be it a a PC or a Server or anything.

What connects a hard drive to the system and how do you access it?

You probably know where the hard drive is. A hard drive connects to a motherboard that sits inside your cabinet. There are several ways you can connect a hard drive to your motherboard depending on your motherboard's compatibility with the interfaces. Here's a set of the most commonly used hard drive connectivity interfaces and the protocols that are used on your hard drive's controller. Most of these are used for servers and storage devices for quicker access. 

3. SAS
5. FC


IDE/ATA stands for Integrated Data Electronics/Advanced Technology Attachment. To be honest, when I read about it, I was more curious to know about its functionality rather than where it got its name from. So here's what I got. It looks like a bus with a 40-pin fat cable coming from your motherboard and connects to your hard drive. Yes, that's the white colored, fat, old and dusty wire thing. IDE/ATA is an interface that is being used since the very beginning of desktop computers. They operate at a good transfer speed of 133 MB/s and are connected in a Master-Slave style. That means that you can connect this device to two different hard drives. One major disadvantage here is that the slave gets the extension of the bus that is connecting to the master which means, you use the SAME bus to connect both the hard drives. And that means lower performance and speed when you are randomly accessing data on both your hard drives. This was why it was stopped at desktop PCs and not implemented on servers. 

2. SATA 

SATA just stands for Serial ATA. This interface was developed after ATA and is highly efficient if you are looking for performance. Serial ATA is just a serial version of the old age ATA connectivity. This connects the drive by just using a 7-pin connector instead of a 40-pin connector that we saw in ATA. SATA offers a transfer speed of 150 MB/s and can be tuned to a whopping 600 MB/s. This makes SATA superfast when you compare it with ATA. There is no master-slave configuration here meaning that this interface offers point to point connectivity to the motherboard for each hard drive. So the traffic passing through the cable is not shared. In addition to all these advantages, SATA also makes your hard drives hot-pluggable, meaning your hard drive can just be pulled out while your server is on. You better save the data on that drive before you do it! SATA is most popular on servers and storage appliances. These interfaces may be present on your desktop PCs but I would not advice you to pull the HDD out while your PC is turned on. 

3. Parallel SCSI

Before SCSI came in, every component had its own interface to communicate with the system. SCSI introduced an interface-independent mechanism where it was compatible with several devices. Printers and scanners connect using the SCSI interface.

4. Serial Attached SCSI (SAS)

Serial Attached SCSI was introduced for a faster means of HDD communication. The latest SAS cables range up to transfer speeds of 640 MB/s. These are mostly built into servers and storage devices and are well known for their speeds. SAS also uses the SCSI's protocols of transferring the data from the HDD to the system.

5. Fibre Channel

Fibre Channel (FC) was an adaption of the SCSI-3 architecture with a little more of a modification. This protocol is mostly used in the high-end storage devices and fewer servers. These devices offer high speeds up to 8 Gb/s.

More About SCSI

SCSI is the majorly used protocol for transferring data between two mediums be it servers or storage devices in a datacenter. It follows a client-server architecture where there is a SCSI initiator device and a SCSI target. SCSI initiator transmits the data to the target and the target receives the data. Any of the two devices can act as a initiator or a target depending on the direction of data flow. SCSI assigns a SCSI ID to identify the devices it is connected to. The SCSI ids can range from 0-7 where the SCSI ID 7 is meant to have the highest priority among all the devices. When you look at it from a storage perspective, SCSI initiator is mostly the application server connected to the storage array port that acts as the SCSI target. This array port in turn connects to the LUNS assigned to it. If you have ever logged on to a SUN Solaris operating system, you would notice that the OS detects all the drives in the CTD addressing format such as c0t0d1. This is nothing but the exact address of the SCSI target where C is the controller, T is the target and D is the device ID. These are divided into LUNS that contain the filesystems in the UNIX operating system. SCSI follows a very similar communication procedure. This page was just supposed to be a brief intro on these interfaces. When you try to explain it, it keeps going on and on and on..
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