ext4lazyinit

• ​Ubuntu日本語フォーラム / HDDに書き込み続ける
• ​https://twitter.com/__nlkl/status/108910649106644992

  HDDを増設したら何かがHDDにアクセスし続けてる。原因はext4lazyinitというカーネルスレッドのようだ。2.6.37から入った機能で、inodeテーブルの作成を遅延させてmkfsを高速化するらしい。確かにmkfsが異様にはやかった気がする。

• ​ext4lazyinit?
  • ​ext4lazyinit?

    So when you have millions of files the backup takes many hours.

• ​[ubuntu Constant Hard Drive Access - Ubuntu Forums]
• ​Gentoo Forums :: View topic - ext4-dio-unwrit / ext4lazyinit processes

  ext4 has the feature of "lazy init" which basically means formatting doesn't complete when mkfs finishes, but that some of the formatting gets done upon first mount.

• Ubuntu 12.04.1 x86_64で作ったext4のfeature
  • sudo mkfs.ext4 /dev/md0 -m 1
  • sudo tune2fs -l /dev/md0

    Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
    Filesystem flags:         signed_directory_hash
    Default mount options:    user_xattr acl
    

• man mke2fs

         -E extended-options
                     lazy_itable_init[= <0 to disable, 1 to enable>]
                            If enabled and the uninit_bg feature is enabled, the
                            inode table will not be fully initialized by mke2fs.
                            This speeds up filesystem initialization noticeably,
                            but it requires the kernel  to  finish  initializing
                            the filesystem in the background when the filesystem
                            is first mounted.  If the option value  is  omitted,
                            it defaults to 1 to enable lazy inode table zeroing.
  
                     lazy_journal_init[= <0 to disable, 1 to enable>]
                            If  enabled,  the  journal  inode  will not be fully
                            zeroed out by mke2fs.   This  speeds  up  filesystem
                            initialization  noticeably,  but  carries some small
                            risk if the system crashes before  the  journal  has
                            been  overwritten  entirely one time.  If the option
                            value is omitted, it defaults to 1  to  enable  lazy
                            journal inode zeroing.
  
  (snip)
  
         -O feature[,...]
                     uninit_bg
                            Create a filesystem without initializing all of  the
                            block  groups.   This feature also enables checksums
                            and highest-inode-used  statistics  in  each  block‐
                            group.   This  feature  can speed up filesystem cre‐
                            ation  time  noticeably  (if   lazy_itable_init   is
                            enabled),  and  can also reduce e2fsck time dramati‐
                            cally.  It is only supported by the ext4  filesystem
                            in recent Linux kernels.
  

• /etc/mke2fs.conf

  [defaults]
          base_features = sparse_super,filetype,resize_inode,dir_index,ext_attr
          default_mntopts = acl,user_xattr
          enable_periodic_fsck = 0
          blocksize = 4096
          inode_size = 256
          inode_ratio = 16384
  
  [fs_types]
          ext3 = {
                  features = has_journal
          }
          ext4 = {
                  features = has_journal,extent,huge_file,flex_bg,uninit_bg,dir_nlink,extra_isize
                  auto_64-bit_support = 1
                  inode_size = 256
          }
          ext4dev = {
                  features = has_journal,extent,huge_file,flex_bg,uninit_bg,dir_nlink,extra_isize
                  inode_size = 256
                  options = test_fs=1
          }
  

• ​lvm - mkfs.ext4 is taking hours to complete on 4TB raid 5 - Server Fault

  Note that lazy_itable_init is turned on by default if the kernel you are running supports automatic background initialization of the inode table ( 2.6.37+ ). Without that support, enabling this option is unsafe as it leaves the inode tables uninitialized, so they may contain garbage that fsck will mistake as valid inodes under certain circumstances. Also uninit_bg is enabled by default for ext4.