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

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

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