Blocks, block devices and block sizes

From Linuxintro

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.

I/O

There is

  • buffered I/O and direct I/O
  • async I/O and sync I/O

The difference between buffered I/O and direct I/O is that direct I/O does not use the operating system cache. During the open syscall, you can say that you want direct I/O. More info...

The difference between async I/O and sync I/O is that async I/O reads/writes from/to a file without waiting for the data to actually arrive, but rather having the data sent to a network socket. You must use libaio for this. More info...

How to...

Find out the size of a block device

To find out the size of the block device /dev/sda: 

fdisk -l /dev/sda

or, a bit harder to read: 

hwinfo --block

Find out the file system stored on a block device

A file system will typically be stored in a partition, not directly in a block device. To find all partitions on /dev/sda use 

fdisk -l /dev/sda

To find out what file system is stored on /dev/sda1 use 

file -s /dev/sda1

Find out the elevator algorithm

The elevator algorithm can be found like this: 

cat /sys/devices/platform/host7/session1/target7\:0\:0/7\:0\:0\:0/block/sdi/queue/scheduler
noop anticipatory [deadline] cfq

In this case, the deadline elevator algorithm is active.

Associated commands

See also

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