How file system block size works?

Question

All Linux file systems have 4kb block size. Let's say I have 10mb of hard disk storage. That means I have 2560 blocks available and let's say I copied 2560 files each having 1kb of size. Each 1 kb block will occupy 1 block though it is not filling entire block.

So my entire disk is now filled but still I have 2560x3kb of free space. If I want to store another file of say 1mb will the file system allow me to store? Will it write in the free space left in the individual blocks? Is there any concept addressing this problem?

I would appreciate some clarification. Thanks in advance.

Answer 1

It is true, you are in a way wasting disk space if you are storing a lot of files which are much smaller than the smallest block size of the file system.

The reason why the block size is around 4kb is the amount of metadata associated with blocks. Smaller the block size, more there is metadata about the locations of the blocks compared to the actual data and more fragmented is the worst case scenario.

However, there are filesystems with different block sizes, most filesystems let you define the block size, typically the minimum block size is 512 bytes. If you are storing a lot of very small files having a small block size might make sense.

http://www.tldp.org/LDP/sag/html/filesystems.html

XFS Filesystem documentation has some comments on how to select filesystem block size - it is also possible to defined the directory block size:

http://techpubs.sgi.com/library/tpl/cgi-bin/getdoc.cgi?coll=linux&db=bks&srch=&fname=/SGI_Admin/LX_XFS_AG/sgi_html/ch02.html

You should consider setting a logical block size for a filesystem directory that is greater than the logical block size for the filesystem if you are supporting an application that reads directories (with the readdir(3C) or getdents(2) system calls) many times in relation to how much it creates and removes files. Using a small filesystem block size saves on disk space and on I/O throughput for the small files.