Block devices and block sizes

Blocks, block devices and block sizes must be considered together.

What is a block device
A block device is a device you can read blocks from. For example hard disks, cdrom drives and floppies are block devices, but not the keyboard. You can receive data from the keyboard and regard them as blocks, but you cannot seek on the keyboard. You can tell a hard disk "give me block 5433", then block 7707, then block 1807 and you cannot do this with a keyboard, so, a keyboard is no block device.

Block sizes
It is important to understand the ideas behind the block sizes.

Disk block sizes
Typically, a hard disk cannot read less than 512 bytes, if you want to read less, read 512 bytes and discard the rest. This is why dd reads one block à 512 bytes in the following example: tweedleburg:~ # dd if=/dev/sda1 of=/dev/null count=1 1+0 records in 1+0 records out 512 bytes (512 B) copied, 1.8977e-05 s, 27.0 MB/s

File system block sizes
On the other hand, every file system needs to split up a partition into blocks to store files and file parts. This is why there is a different block size for a file system as you can see here: tweedleburg:/mnt/sdb2 # stat -f. File: "." ID: 236d62321492c2ce Namelen: 255    Type: ext2/ext3 Block size: 4096      Fundamental block size: 4096 Blocks: Total: 76899893  Free: 8380419    Available: 4474114 Inodes: Total: 39075840  Free: 38013010 So, if you store a file in this file system, it will be stored in a 4096-byte-block, that means, even if your file only contains 5 bytes, it will take away 4096 bytes from your disk's capacity: tweedleburg:/mnt/sdb2 # df. Filesystem          1K-blocks      Used Available Use% Mounted on /dev/sdb2            307599572 274077896  17896456  94% /mnt/sdb2 tweedleburg:/mnt/sdb2 # echo hallo>welt tweedleburg:/mnt/sdb2 # df. Filesystem          1K-blocks      Used Available Use% Mounted on /dev/sdb2            307599572 274077900  17896452  94% /mnt/sdb2 tweedleburg:/mnt/sdb2 # du -csh welt 4.0K   welt 4.0K   total

Kernel block size
Also the kernel has its own block size. This is relevant e.g. for vmstat. In the vmstat man page you find the statement All linux blocks are currently 1024 bytes. So, again another block size when you work with vmstat. This is the block size the Linux kernel uses internally for caching and buffering. It is the most prominent of all block sizes.

Associated commands

 * dd
 * vmstat
 * iostat
 * stat
 * fdisk
 * hwinfo --block
 * hwinfo --partition