Reserved blocks list in ext* filesystem

Reserved blocks list in ext* filesystem

[Jitendra Pawar]

Hi,

Below is snippet of superblock information of one of the block
device(say /dev/sda1)
Which mentions about the reserved blocks-
Reserved block count:     25600
Does anyone know about how to find out list of blocks which are
reserved(actual block numbers not count).

-------------------------------------------------
Filesystem volume name:   <none>
Last mounted on:          /boot
Filesystem UUID:          99ec51e5-8729-4cab-b2ea-6b55b302aae6
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 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:              128016
Block count:              512000
Reserved block count:     25600
Free blocks:              439769
Free inodes:              127977
First block:              1
Block size:               1024
Fragment size:            1024
Reserved GDT blocks:      256
Blocks per group:         8192
Fragments per group:      8192
Inodes per group:         2032
Inode blocks per group:   254
Flex block group size:    16
Filesystem created:       Thu Aug  8 06:35:16 2013
Last mount time:          Tue Nov 25 18:52:26 2014
Last write time:          Tue Nov 25 18:52:26 2014
Mount count:              12
Maximum mount count:      -1
Last checked:             Thu Aug  8 06:35:16 2013
Check interval:           0 (<none>)
Lifetime writes:          105 MB
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:              128
Journal inode:            8
Default directory hash:   half_md4
Directory Hash Seed:      4c6e20ac-c46d-4bf1-81c1-3c5ae77eaab7
Journal backup:           inode blocks
Journal features:         journal_incompat_revoke
Journal size:             8M
Journal length:           8192
Journal sequence:         0x00000028
Journal start:            0
---------------------------------------------

Thanks,
-Jitendra

Re: Reserved blocks list in ext* filesystem

[Lukáš Czerner]

On Mon, 15 Dec 2014, Jitendra Pawar wrote:

> Date: Mon, 15 Dec 2014 19:31:11 +0530
> From: Jitendra Pawar <jitendra.pawar@gmail.com>
> To: linux-ext4@vger.kernel.org
> Subject: Reserved blocks list in ext* filesystem
> 
> Hi,
> 
> Below is snippet of superblock information of one of the block
> device(say /dev/sda1)
> Which mentions about the reserved blocks-
> Reserved block count:     25600
> Does anyone know about how to find out list of blocks which are
> reserved(actual block numbers not count).

There are no statically reserved blocks. Reserved block count is a
number of free block bellow which a non-privileged processes are no
more allowed to allocate additional space from the file system.


In other words (see man mkfs.ext4):

Specify the percentage of the filesystem blocks reserved for the
super-user.  This avoids fragmentation,  and  allows root-owned  daemons,
such as syslogd(8), to continue to function correctly after non-privileged
processes are prevented from writing to the filesystem.  The default
percentage is 5%.

-Lukas

> 
> -------------------------------------------------
> Filesystem volume name:   <none>
> Last mounted on:          /boot
> Filesystem UUID:          99ec51e5-8729-4cab-b2ea-6b55b302aae6
> 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 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:              128016
> Block count:              512000
> Reserved block count:     25600
> Free blocks:              439769
> Free inodes:              127977
> First block:              1
> Block size:               1024
> Fragment size:            1024
> Reserved GDT blocks:      256
> Blocks per group:         8192
> Fragments per group:      8192
> Inodes per group:         2032
> Inode blocks per group:   254
> Flex block group size:    16
> Filesystem created:       Thu Aug  8 06:35:16 2013
> Last mount time:          Tue Nov 25 18:52:26 2014
> Last write time:          Tue Nov 25 18:52:26 2014
> Mount count:              12
> Maximum mount count:      -1
> Last checked:             Thu Aug  8 06:35:16 2013
> Check interval:           0 (<none>)
> Lifetime writes:          105 MB
> Reserved blocks uid:      0 (user root)
> Reserved blocks gid:      0 (group root)
> First inode:              11
> Inode size:              128
> Journal inode:            8
> Default directory hash:   half_md4
> Directory Hash Seed:      4c6e20ac-c46d-4bf1-81c1-3c5ae77eaab7
> Journal backup:           inode blocks
> Journal features:         journal_incompat_revoke
> Journal size:             8M
> Journal length:           8192
> Journal sequence:         0x00000028
> Journal start:            0
> ---------------------------------------------
> 
> Thanks,
> -Jitendra
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