DAS Stack: Let’s continue the journey – About the “Disk”

From a typical internal construction perspective, we can say the disks can be broadly classified as three types. The physical hard disk, a flash based Solid State Disk (SSD) and virtual disk from Storage Arrays (Network Disks).

The physical hard disks are usually made up of a hard metal disk (usually aluminum) coated with a magnetic material. The magnetic material records the data and the aluminum disk provides the rigid support.

The diagram below shows a quick view of insides of a typical hard disk drive.

More detail at http://news.bbc.co.uk/2/hi/technology/6677545.stm

Let’s quickly touch a few ares in a word or two. For further information¬†http://en.wikipedia.org/wiki/Hard_disk_drive¬†is an excellent source of detailed information.

The disk or platters here is held and rotated by the spindle. The Read/Write head moves about the surface of the disk and, of course, reads or writes the disk as it rotates. The head arm holds the Read/Write head(s) and is positioned by the voice coil actuator. The air-filter filters out the dust etc. from entering the hard drive compartment.

Just for notes, the head “flies” above the disk with a very thin gap between the head and the disk. If it ever touches the disk during spin, the head “crashes” and the disk is essentially useless. There are some data recovery technologies which can try and recover the data that is not on the crash path. However, as far I know, it is not practical to extract data under the tracks that is actually in the crash zone.

Disk Block Layout

This diagram shows a rough view of Disk Layout. The disk itself if split in to tracks and sectors. There are two possibilities with this kind of arrangement, as you might guess or know. The first is variable track length (or circumference, if you prefer). This is a typical concentric circle division. The center circles are smaller and hence will have smaller circumference and the outer circles are larger having greater circumference.

The other is fixed track length. This can normally be achieved by spirals instead of concentric circles. The track is of fixed length. Near the center of the disk, the circle can have lesser tracks than near the edge of the disk. This is more standard industry practice. The diagram below shows a typical tracks and sector division. Each sector is further divided in to blocks and we have that division on display here as well.

The block is the smallest individually addressable entity on the disk. It is usually 512 or 520 bytes, with 512 is most common. There is an advanced format proposal which makes the block size to 4096 (or 4K) bytes. The host system might need to have some support for this version, though. More on this later.

Shown at http://computershopper.com/feature/how-it-works-platter-based-hard-drive

The drive has the disk or platter(s) (1), spindle(2) to hold and spin them, the head arm (3) to hold the Read/Write heads, the voice coil (4) to move the head around, the heads (5), and the head landing zone(9) where the head can rest without crashing on the disk during power down. The tracks (6), sectors (8) and blocks (7) are the locations on the disk where data is stored and retrieved.

Typically, the disk is accessed as a set of serially numbered blocks – the Logical Block Address (LBA). The on disk electronics worry about translating disk LBA in to a location on the disk and store retrieve information. This also makes the life of host system easier as it only needs to worry about the block address as a logical block number – no need to remember on which platter, on which side, on what track in which sector is the block of our interest.

The logical block addressing also helps the on disk controller map out bad blocks without the host system worry about them. This is an unwanted behavior in some cases, though.

For the Solid State Disks, or flash drives – there are no spindles as you might guess. There are just a few chips under the hood storing and retrieving blocks of information with a bit of glue logic to attach these storage chips to the bus. The blocks can again be accessed with LBA and the controller translates that in to chip, sector/group and block address depending on flash chip organization. Let me try to touch SSDs in some more detail in a separate post. A typical small scale example of SSD is our USB pen drive or SD Card.

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