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Copy edited (e.g. ref. <https://en.wiktionary.org/wiki/let%27s#Contraction>). Removed meta information (this belongs in comments).
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edit approved Feb 22, 2015 at 8:48
Peter Mortensen
edit approved Feb 22, 2015 at 8:48
Peter Mortensen
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Why doesn't my exactly 100MiB100 MiB partition at 1KiB1 KiB block size doesn't have the corresponding available blocks/space?

Warning: no, this question is not yet another whining about 5% of root-reserved ext4 space. Please, read on carefully.

Okay, I have a very high density virtualized environment with containers, so I'm trying to make each container really small. "Really small" means 87M87 MB on base Ubuntu 14.04Ubuntu 14.04 (Trusty Tahr) without breaking up the package manager compatibility.

So I use LVMLVM as a backing storage for my containers and recently I found a very strange numbers. Here they are.

Let's create a 100MiB100 MiB (yeah, power of 2) logical volume.

Sweet, this is really 100MiB100 MiB.

Now let's make an ext4ext4 filesystem. And of course, we remember -m 0 parameter, which prevents space waste.

Sweet and clean. Mind the block size - our logical volume is small, so mkfs.ext4 decided to make a 1k1 KiB sized block, not the usual 4k4 KiB.

And letslet's call df without parameters (we would like to see 1K1 KiB-blocks)

We have 95054 blocks total. But the device itself has 102400 blocks of 1K1 KiB. We have only 92.8% of our storage. Where are my blocks, man?

Let's look at it on a real block device. A have a 16G16 GiB virtual disk, 16777216 blocks of 1K, but only 15396784 blocks are in df output. 91.7%, what is it?

Why my exactly 100MiB partition at 1KiB block size doesn't have the corresponding available blocks/space?

Warning: no, this question is not yet another whining about 5% of root-reserved ext4 space. Please, read on carefully.

Okay, I have a very high density virtualized environment with containers, so I'm trying to make each container really small. "Really small" means 87M on base Ubuntu 14.04 without breaking up the package manager compatibility.

So I use LVM as a backing storage for my containers and recently I found a very strange numbers. Here they are.

Let's create a 100MiB (yeah, power of 2) logical volume.

Sweet, this is really 100MiB

Now let's make an ext4 filesystem. And of course, we remember -m 0 parameter, which prevents space waste.

Sweet and clean. Mind the block size - our logical volume is small, so mkfs.ext4 decided to make a 1k sized block, not usual 4k.

And lets call df without parameters (we would like to see 1K-blocks)

We have 95054 blocks total. But the device itself has 102400 blocks of 1K. We have only 92.8% of our storage. Where are my blocks, man?

Let's look at it on a real block device. A have a 16G virtual disk, 16777216 blocks of 1K, but only 15396784 blocks are in df output. 91.7%, what is it?

Why doesn't my exactly 100 MiB partition at 1 KiB block size have the corresponding available blocks/space?

I have a very high density virtualized environment with containers, so I'm trying to make each container really small. "Really small" means 87 MB on base Ubuntu 14.04 (Trusty Tahr) without breaking up the package manager compatibility.

So I use LVM as a backing storage for my containers and recently I found very strange numbers. Here they are.

Let's create a 100 MiB (yeah, power of 2) logical volume.

Sweet, this is really 100 MiB.

Now let's make an ext4 filesystem. And of course, we remember -m 0 parameter, which prevents space waste.

Sweet and clean. Mind the block size - our logical volume is small, so mkfs.ext4 decided to make a 1 KiB sized block, not the usual 4 KiB.

And let's call df without parameters (we would like to see 1 KiB-blocks)

We have 95054 blocks total. But the device itself has 102400 blocks of 1 KiB. We have only 92.8% of our storage. Where are my blocks, man?

Let's look at it on a real block device. A have a 16 GiB virtual disk, 16777216 blocks of 1K, but only 15396784 blocks are in df output. 91.7%, what is it?

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Braiam
Braiam
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ext4 filesystem Why my exactly 100MiB partition at 1KiB block size doesn't have the corresponding available blocks/space?

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maniaque
maniaque
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ext4 filesystem size

Warning: no, this question is not yet another whining about 5% of root-reserved ext4 space. Please, read on carefully.

Okay, I have a very high density virtualized environment with containers, so I'm trying to make each container really small. "Really small" means 87M on base Ubuntu 14.04 without breaking up the package manager compatibility.

So I use LVM as a backing storage for my containers and recently I found a very strange numbers. Here they are.

Let's create a 100MiB (yeah, power of 2) logical volume.

sudo lvcreate -L100M -n test1 /dev/purgatory

I'd like to check the size, so I issue sudo lvs --units k

test1 purgatory -wi-a---- 102400.00k

Sweet, this is really 100MiB

Now let's make an ext4 filesystem. And of course, we remember -m 0 parameter, which prevents space waste.

sudo mkfs.ext4 -m 0 /dev/purgatory/test1 mke2fs 1.42.9 (4-Feb-2014) Filesystem label= OS type: Linux Block size=1024 (log=0) Fragment size=1024 (log=0) Stride=0 blocks, Stripe width=0 blocks 25688 inodes, 102400 blocks 0 blocks (0.00%) reserved for the super user First data block=1 Maximum filesystem blocks=67371008 13 block groups 8192 blocks per group, 8192 fragments per group 1976 inodes per group Superblock backups stored on blocks: 8193, 24577, 40961, 57345, 73729 Allocating group tables: done Writing inode tables: done Creating journal (4096 blocks): done Writing superblocks and filesystem accounting information: done

Sweet and clean. Mind the block size - our logical volume is small, so mkfs.ext4 decided to make a 1k sized block, not usual 4k.

Now we will mount it.

sudo mount /dev/purgatory/test1 /mnt/test1

And lets call df without parameters (we would like to see 1K-blocks)

/dev/mapper/purgatory-test1 95054 1550 91456 2% /mnt/test1

Wait, oh shi~

We have 95054 blocks total. But the device itself has 102400 blocks of 1K. We have only 92.8% of our storage. Where are my blocks, man?

Let's look at it on a real block device. A have a 16G virtual disk, 16777216 blocks of 1K, but only 15396784 blocks are in df output. 91.7%, what is it?

Now follows the investigation (spoiler: no results)

  1. Filesystem could begin not at the beginning of the device. This is strange, but possible. Luckily, ext4 has magic bytes, let's check their presence.

    sudo hexdump -C /dev/purgatory/test1 | grep "53 ef"

This shows superblock:

00000430 a9 10 e7 54 01 00 ff ff 53 ef 01 00 01 00 00 00 |...T....S.......|

Hex 430 = Dec 1072, so somewhere after first kilobyte. Looks reasonable, ext4 skips first 1024 bytes for oddities like VBR, etc.

  1. This is journal!

No, it is not. Journal take space from Available if df output.

  1. Oh, we have dump2fs and could check the sizes there!

... a lot of greps ...

sudo dumpe2fs /dev/purgatory/test1 | grep "Free blocks"

Ouch.

Free blocks: 93504 Free blocks: 3510-8192 Free blocks: 8451-16384 Free blocks: 16385-24576 Free blocks: 24835-32768 Free blocks: 32769-40960 Free blocks: 41219-49152 Free blocks: 53249-57344 Free blocks: 57603-65536 Free blocks: 65537-73728 Free blocks: 73987-81920 Free blocks: 81921-90112 Free blocks: 90113-98304 Free blocks: 98305-102399

And we have another number. 93504 free blocks.

The question is: what is going on?

  • Block device: 102400k (lvs says)
  • Filesystem size: 95054k (df says)
  • Free blocks: 93504k (dumpe2fs says)
  • Available size: 91456k (df says)