Difference between revisions of "Blocks, block devices and block sizes"

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== Disk 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:
 
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:
<source>
 
 
  tweedleburg:~ # dd if=/dev/sda1 of=/dev/null count=1
 
  tweedleburg:~ # dd if=/dev/sda1 of=/dev/null count=1
 
  1+0 records in
 
  1+0 records in
 
  1+0 records out
 
  1+0 records out
 
  512 bytes (512 B) copied, 1.8977e-05 s, 27.0 MB/s
 
  512 bytes (512 B) copied, 1.8977e-05 s, 27.0 MB/s
</source>
+
 
  
 
== File system block sizes ==
 
== File system block sizes ==
 
On the other hand, every file system needs to split up a partition into blocks to store [[file]]s and file parts. This is why there is a different block size for a file system as you can see here:
 
On the other hand, every file system needs to split up a partition into blocks to store [[file]]s and file parts. This is why there is a different block size for a file system as you can see here:
<source>
 
 
  tweedleburg:/mnt/sdb2 # stat -f .
 
  tweedleburg:/mnt/sdb2 # stat -f .
 
   File: "."
 
   File: "."
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  Blocks: Total: 76899893  Free: 8380419    Available: 4474114
 
  Blocks: Total: 76899893  Free: 8380419    Available: 4474114
 
  Inodes: Total: 39075840  Free: 38013010
 
  Inodes: Total: 39075840  Free: 38013010
</source>
+
 
 
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:
 
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:
<source>
 
 
  tweedleburg:/mnt/sdb2 # df .
 
  tweedleburg:/mnt/sdb2 # df .
 
  Filesystem          1K-blocks      Used Available Use% Mounted on
 
  Filesystem          1K-blocks      Used Available Use% Mounted on
Line 35: Line 32:
 
  4.0K    welt
 
  4.0K    welt
 
  4.0K    total
 
  4.0K    total
</source>
 
  
 
== Kernel block size ==
 
== Kernel block size ==
 
Also the kernel has its own block size. This is relevant e.g. for vmstat. In the [http://unixhelp.ed.ac.uk/CGI/man-cgi?vmstat vmstat man page] you find the statement
 
Also the kernel has its own block size. This is relevant e.g. for vmstat. In the [http://unixhelp.ed.ac.uk/CGI/man-cgi?vmstat vmstat man page] you find the statement
<source>
 
 
  All linux blocks are currently 1024 bytes.
 
  All linux blocks are currently 1024 bytes.
</source>
+
 
 
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.
 
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.
  
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== Find out the size of a block device ==
 
== Find out the size of a block device ==
 
To find out the size of the block device ''/dev/sda'':
 
To find out the size of the block device ''/dev/sda'':
<source>
 
 
  fdisk -l /dev/sda
 
  fdisk -l /dev/sda
</source>
+
 
 
or, a bit harder to read:
 
or, a bit harder to read:
<source>
 
 
  hwinfo --block
 
  hwinfo --block
</source>
 
  
 
== Find out the file system stored on a block device ==
 
== 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
 
A file system will typically be stored in a partition, not directly in a block device. To find all partitions on ''/dev/sda'' use
<source>
 
 
  fdisk -l /dev/sda
 
  fdisk -l /dev/sda
</source>
+
 
 
To find out what file system is stored on ''/dev/sda1'' use
 
To find out what file system is stored on ''/dev/sda1'' use
<source>
 
 
  file -s /dev/sda1
 
  file -s /dev/sda1
</source>
 
  
 
== Find out the elevator algorithm ==
 
== Find out the elevator algorithm ==
 
The elevator algorithm can be found like this:
 
The elevator algorithm can be found like this:
<source>
 
 
  cat /sys/devices/platform/host7/session1/target7\:0\:0/7\:0\:0\:0/block/sdi/queue/scheduler
 
  cat /sys/devices/platform/host7/session1/target7\:0\:0/7\:0\:0\:0/block/sdi/queue/scheduler
 
  noop anticipatory [deadline] cfq
 
  noop anticipatory [deadline] cfq
</source>
+
 
 
In this case, the deadline elevator algorithm is active.
 
In this case, the deadline elevator algorithm is active.
  
 
Or it may be found like this:
 
Or it may be found like this:
<source>
 
 
  cat /sys/class/block/sdb/queue/scheduler  
 
  cat /sys/class/block/sdb/queue/scheduler  
 
  noop deadline [cfq]
 
  noop deadline [cfq]
</source>
 
  
 
You can switch the scheduler like this:
 
You can switch the scheduler like this:
<source>
 
 
  # cat /sys/class/block/sdb/queue/scheduler  
 
  # cat /sys/class/block/sdb/queue/scheduler  
 
  noop deadline [cfq]  
 
  noop deadline [cfq]  
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  # cat /sys/class/block/sdb/queue/scheduler  
 
  # cat /sys/class/block/sdb/queue/scheduler  
 
  [noop] deadline cfq
 
  [noop] deadline cfq
</source>
 
  
 
= Associated commands =
 
= Associated commands =

Latest revision as of 07:23, 9 January 2021

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.

Applications

vmstat

vmstat uses 1k blocks

iostat

iostat uses 512B blocks

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.

Or it may be found like this:

cat /sys/class/block/sdb/queue/scheduler 
noop deadline [cfq]

You can switch the scheduler like this:

# cat /sys/class/block/sdb/queue/scheduler 
noop deadline [cfq] 
# echo "noop">/sys/class/block/sdb/queue/scheduler
# cat /sys/class/block/sdb/queue/scheduler 
[noop] deadline cfq

Associated commands

See also