cat /sys/block/md0/queue/physical_block_size; # what blocksize does my harddisk have?
512

stat -f /dev/md0; # alternatively

File: "/dev/md0"
    ID: 3d822af00fc67f8d Namelen: 255     Type: ext2/ext3 <- it should be actually EXT4!
Block size: 1024
Blocks: Total: 142799     Free: 16739      Available: 16739
Inodes: Total: 36864      Free: 30454


/sbin/fdisk -l /dev/md0 | grep Units
Units = cylinders of 8 * <span style="color: #ff0000;">512<span style="color: #000000;"> =</span></span> 4096 bytes

/sbin/fdisk -l /dev/sda3 | grep Units
Units = cylinders of 16065 * <span style="color: #ff0000;">512</span> = 8225280 bytes

Note the fdisk output says „cylinders of 16065 * 512“. This means that there are 512 bytes per block on the disk.

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

can also be read with:

tune2fs -l /dev/sda1
tune2fs 1.42.12 (29-Aug-2014)
Filesystem volume name:   
Last mounted on:          /
Filesystem UUID:          e9ca2a70-bbb4-4b00-aa8f-14ccdb1c7c3d
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
Filesystem flags:         signed_directory_hash
Default mount options:    user_xattr acl
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              8192000
Block count:              32768000
Reserved block count:     1638400
Free blocks:              31353633
Free inodes:              8051456
First block:              0
Block size:               4096 <----- HERE IT IS
Fragment size:            4096
Reserved GDT blocks:      1016
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         8192
Inode blocks per group:   512
Flex block group size:    16
Filesystem created:       Mon Apr 24 12:22:32 2017
Last mount time:          Fri May 19 10:03:13 2017
Last write time:          Fri May 19 10:03:55 2017
Mount count:              13
Maximum mount count:      -1
Last checked:             Fri May 12 10:11:37 2017
Check interval:           0 ()
Lifetime writes:          31 GB
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:               256
Required extra isize:     28
Desired extra isize:      28
Journal inode:            8
First orphan inode:       524303
Default directory hash:   half_md4
Directory Hash Seed:      cbc60d19-3f00-4607-9017-5098cb4b96f5
Journal backup:           inode blocks

source & creditz: http://wiki.linuxquestions.org/wiki/Block_devices_and_block_sizes

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