Disk Structure, and the Logical Block

Why the sums are a simplification

Real disks do not have a constant number of sectors per track, and a question that gives you one is giving you a model.

Two more reasons the mapping is not exact: bad sectors are remapped to spares elsewhere on the disk, so a block's neighbour may be far away, and the outermost tracks are sometimes reserved. The operating system cannot see any of it, which is the abstraction working as intended.

Disk attachment

MU's section names three ways a disk reaches a machine, and a question wants the distinction.

The distinction that carries the marks: NAS serves files and a SAN serves blocks. A machine using a SAN thinks it has its own disk; a machine using NAS knows it is talking to a file server.

Measured on the lab machine

The kernel publishes the block count of a device, which is all that is needed to work out its capacity.

$ cat /sys/block/vda/size
41943040
$ cat /sys/block/vda/queue/hw_sector_size
512
$ echo "capacity = $(( 41943040 * 512 )) bytes"
capacity = 21474836480 bytes
$ echo "that is $(( 41943040 * 512 / 1024 / 1024 / 1024 )) gigabytes"
that is 20 gigabytes

Forty one million sectors of 512 bytes: exactly twenty gigabytes. And the partition inside it is a range of block numbers and nothing more:

$ echo "starts at sector $(cat /sys/block/vda/vda1/start), and is $(cat /sys/block/vda/vda1/size) sectors long"
starts at sector 2099200, and is 39843807 sectors long

A partition is arithmetic on block numbers. Nothing about the geometry appears anywhere, and Chapter ninety two showed that the machine's claim to be a rotating disk is not even true.

Distinctions that carry marks

What it does not mean

A logical block is not a sector. It is one or more sectors, chosen by the file system.

Block order is not cylinder order. Blocks fill a whole cylinder, across all its surfaces, before moving to the next cylinder.

The geometry in a question is not a real disk's geometry. Zoned recording means sectors per track varies, and remapped bad sectors break the neighbour relation.

NAS is not a SAN. One serves files over the ordinary network; the other serves blocks over a network of its own.

A partition is not a physical division. It is a first block and a length.

Quick revision

• A disk is an array of logical blocks numbered from 0, and the block is the unit the

operating system transfers.

• The layout order:

sector by sector, then track by track inside the cylinder, then cylinder by cylinder outwards in. Track before cylinder, which is why sequential block numbers cost no seeking.

• Blocks per cylinder = surfaces × sectors per track. With 4 surfaces and 100 sectors, that

is 400.

• Block 1234 on that disk: 1234 = 3 × 400 + 34, so cylinder 3, surface 0, sector

34.

• Cylinder 5, surface 2, sector 17: 5 × 400 + 2 × 100 + 17 = 2217.
• Capacity = cylinders × surfaces × sectors per track × bytes per sector.

16,383 × 16 × 63 × 512 = 8,455,200,768 bytes.

• Real disks use zoned recording, so sectors per track varies; bad sectors are remapped;

the sums are a model.

• Host attached by SATA or SAS; NAS serves files over the network; a SAN serves

blocks over its own network.

• Measured: the lab disk is 41,943,040 sectors of 512 bytes, exactly 20 gigabytes,

and a partition is a start sector and a length.

Test yourself

1. What does the operating system use to address a disk? A logical block number, from 0 to

one less than the number of blocks; the geometry is hidden by the disk.

1. Give the order in which logical blocks are laid out. Sector by sector along a track, then

track by track within the same cylinder, then cylinder by cylinder from the outside inwards.

1. Why is track before cylinder the right order? Because the heads move together, so the

tracks of one cylinder need no arm movement: consecutive block numbers are cheap to read.

1. A disk has 4 surfaces and 100 sectors per track. Where is logical block 1234? 400 blocks

per cylinder, so 1234 = 3 × 400 + 34: cylinder 3, surface 0, sector 34.

1. Which block is cylinder 5, surface 2, sector 17 on that disk?

5 × 400 + 2 × 100 + 17 = 2217.

1. Find the capacity of a disk of 16,383 cylinders, 16 heads and 63 sectors of 512 bytes.

16,383 × 16 × 63 × 512 = 8,455,200,768 bytes.

1. Why is a constant sectors per track a simplification? Modern disks use zoned recording,

where outer tracks hold more sectors than inner ones, and bad sectors are remapped elsewhere.

1. Distinguish NAS from a SAN. Network attached storage serves whole files over the ordinary

network and runs the file system itself; a storage area network serves blocks over a dedicated network, and the client runs the file system.