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I've got a question concerning the block size and cluster size. Regarding to what I have read about that I assume the following:

  • The block size is the physical size of a block, mostly 512 bytes. There is no way to change this.
  • The cluster size is the minimal size of a block that is read and writable by the OS. If I create a new filesystem e.g. ext3, I can specify this minimal block size with the switch -b. Almost all programs like dumpe2fs, mke2fs use block size as an name for cluster size.

If I have got the following output:

$ stat test
File: `test'
Size: 13            Blocks: 4          IO Block: 2048   regular file
Device: 700h/1792d  Inode: 15          Links: 1

Is it correct that the size is the actual space in bytes, blocks are the physically used blocks (512 bytes for each) and IO block relates to the block size specified when creating the FS?

hippietrail
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pluckyDuck
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3 Answers3

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I think you're confused, possibly because you've read several documents that use different terminology. Terms like “block size” and “cluster size” don't have a universal meaning, even within the context of filesystem literature.

Filesystems

For ext2 or ext3, the situation is relatively simple: each file occupies a certain number of blocks. All blocks on a given filesystem have the same size, usually one of 1024, 2048 or 4096 bytes. A file¹ whose size is between N blocks plus one byte and N+1 blocks occupies N+1 blocks. That block size is what you specify with mke2fs -b. There is no separate notion of clusters.

The FAT filesystem used in particular by MS-DOS and early versions of Windows has a similarly simple space allocation. What ext2 calls blocks, FAT calls clusters; the concept is the same.

Some filesystems have a more sophisticated allocation scheme: they have fixed-size blocks, but can use the same block to store the last few bytes of more than one file. This is known as block suballocation; Reiserfs and Btrfs do it, but not ext3 or even ext4.

Utilities

Unix utilities often use the word “block” to mean an arbitrarily-sized unit, typically 512 bytes or 1kB. This usage is unrelated to any particular filesystem or disk hardware. Historically, the 512B block did come about because disks and filesystems at the time often operated in 512B chunks, but the modern usage is just arbitrary. Traditional unix utilities and interfaces still use 512B blocks sometimes, though 1kB blocks are now often preferred. You need to check the documentation of each utility to know what size of block it's using (some have a switch, e.g. du -B or df -B on Linux).

In the GNU/Linux stat utility, the blocks figure is the number of 512B blocks used by the file. The IO Block figure is the preferred size for file input-output, which is in principle unrelated but usually an indication of the underlying filesystem's block size (or cluster size if that's what you want to call it). Here, you have a 13-byte file, which is occupying one block on the ext3 filesystem with a block size of 2048; therefore the file occupies 4 512-byte units (called “blocks” by stat).

Disks

Most disks present an interface that shows the disk as a bunch of sectors. The disk can only write or read a whole sector, not individual bits or bytes. Most hard disks have 512-byte sectors, though 4kB-sector disks started appearing a couple of years ago.

The disk sector size is not directly related to the filesystem block size, but having a block be a whole number of sectors is better for performance.

¹ Exception: sparse files save space .

Gilles 'SO- stop being evil'
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    Thanks for your clarification. I arrived here due to such confusion caused by the ext2fsd tool for windows. Their command line options start: ´Usage: mke2fs [-c|-l filename] [-b block-size] [-C cluster-size]...´ Well, you see, both block and cluster size... – Bernhard Hiller Sep 07 '16 at 13:16
  • I'm also interested to know what's the difference between mke2fs' block-size and cluster-size. Does anyone know ? – X.LINK Feb 09 '17 at 01:28
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    @X.LINK It's explained in the man page. The cluster size is used by the bigalloc feature, which if I understand correctly makes access to large files faster at the expense of more wasted storage. – Gilles 'SO- stop being evil' Feb 09 '17 at 10:24
  • In effect a very clear and much needed 'disambiguation` of the term 'block'. – darbehdar Aug 18 '21 at 08:27
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The very importnat point it needs to understand that if your file system has for instance 4KB unut\cluster\extent\whatever size it does NOT mean that OS reads and writes data on hdd with 4KB in size. it does not matter what is FS extent\cluster\whatever size. OS can read and write data from hdd with 512B granularity. where 512B is the physycal size of a block on hdd. you can check it easyly. launch ioemeter and perfon on windows. and see it on yourself

suppose we have ntfs with 4k cluster size\unit allocation size and hdd has 512B sector size. we fire iometer with the pattern of 512B rand write. you will see in perfmon that OS without any problem writes 512B at a time and there is no any read IO's as you would expect. OS just directly writes 512B on to FS. it does NOT happen like OS reads 4k changes 512B in it and writes back 4K. NOPE.

Alex
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to think decivering the word "block" may grants access to an (but maybe only one of probably >1) answer to that question: in former times whilst 'floppy discs' existed, some times the question was if i store mostly SMAL or mostly BIG files on a 'harddisk' (e.g. floppy disks), because then maybe i waste some mathematially possible 'space'... because ONE FILE needs DEFINATELY one block (thats a 'rule'/'cost'). i would think "sectors" are more HARDWARE & firmware of a device related than 'software blocks'. (~because the TOC of a file-system also needs 'physical' space, so the smaler the block-size the bigger the 'TOC' (or 'journal' is also a name that i think)).

user531331
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