Blocks, block devices and block sizes
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: <source>
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
</source>
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: <source>
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
</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: <source>
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
</source>
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 <source>
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.
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: <source>
fdisk -l /dev/sda
</source> or, a bit harder to read: <source>
hwinfo --block
</source>
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 <source>
fdisk -l /dev/sda
</source> To find out what file system is stored on /dev/sda1 use <source>
file -s /dev/sda1
</source>
Find out the elevator algorithm
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 noop anticipatory [deadline] cfq
</source> In this case, the deadline elevator algorithm is active.
Or it may be found like this: <source>
cat /sys/class/block/sdb/queue/scheduler noop deadline [cfq]
</source>
You can switch the scheduler like this: <source>
# 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
</source>
Associated commands
- dd
- vmstat
- iostat
- stat
- fdisk
- hwinfo --block
- hwinfo --partition
- file -s /dev/sda : finds out the filesystem type of /dev/sda
