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Re: [SOLVED] Disk access every few seconds (was: Disk access every few seconds)

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David Brown <david@westcontrol.removethisbit.com> wrote:
> On 15/06/2011 08:51, Lasse Kliemann wrote:
>> David Brown<david@westcontrol.removethisbit.com>  wrote:
>>
>>>> There is one oddity with this array, maybe you have an idea for
>>>> that too. Even if no process is using the array (to the best of
>>>> my knowledge), there are accesses roughly every 3 seconds (LEDs
>>>> flash, drives make sound). This is as long as a filesystem is
>>>> mounted from the array. I use EXT4 and have already tried
>>>> different mount options, to no avail (read-only helps, but that's
>>>> not very useful).

[...]
>> I then switched to XFS, and the problem is gone. So it appears to
>> be related to the interplay of RAID-5, EXT4, and maybe the LVM
>> and encryption. I can live with XFS, but I wonder what's the
>> problem with EXT4. I use EXT4 as my standard filesystem almost
>> everywhere else, but this is the first time I use it on RAID-5.
> 
> I'm afraid I can't help you much.  The best I can do is say that I have 
> a set of hard disks that are in a raid10,far set, with an ext4 
> filesystem (no lvm on this disk set), mounted read-write on a directory 
> that is seldom used.  If I am not using files in that tree, the disks 
> are silent (in fact, they are set to spin down).
> 
> I am sure it is possible to trace what is going on, but it is beyond my 
> experience.

I've got it! From mke2fs's manual page:

  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 initial-
         ization.

There is also the init_itable mount option, explained in 
Documentation/filesystems/ext4.txt:

  init_itable=n         The lazy itable init code will wait n times the
                        number of milliseconds it took to zero out the
                        previous block group's inode table.  This
                        minimizes the impact on the systme performance
                        while file system's inode table is being initialized.

When I reduce its value, the interval between two accesses is 
reduced. So I am pretty sure that this is it.

I got onto the right trail by using the atop utility: 
  http://www.atoptool.nl/

It showed me an ext4lazyinit process running. (It also shows up 
in top, I just verified.)

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Thread

RAID: prevent out-of-sync with unstable connections Lasse Kliemann <lasse-usenet-2011@mail.plastictree.net> - 2011-06-12 13:30 +0200
  Re: RAID: prevent out-of-sync with unstable connections David Brown <david.brown@removethis.hesbynett.no> - 2011-06-12 15:33 +0200
    Re: RAID: prevent out-of-sync with unstable connections Lasse Kliemann <lasse-usenet-2011@mail.plastictree.net> - 2011-06-12 17:21 +0200
      Re: RAID: prevent out-of-sync with unstable connections David Brown <david.brown@removethis.hesbynett.no> - 2011-06-12 20:13 +0200
        Re: Disk access every few seconds (was: RAID: prevent out-of-sync with unstable connections) Lasse Kliemann <lasse-usenet-2011@mail.plastictree.net> - 2011-06-14 09:05 +0200
          Re: Disk access every few seconds David Brown <david@westcontrol.removethisbit.com> - 2011-06-14 13:39 +0200
            Re: Disk access every few seconds Lasse Kliemann <lasse-usenet-2011@mail.plastictree.net> - 2011-06-15 08:51 +0200
              Re: Disk access every few seconds David Brown <david@westcontrol.removethisbit.com> - 2011-06-15 11:25 +0200
                Re: Disk access every few seconds "Peter J. Holzer" <hjp-usenet2@hjp.at> - 2011-06-17 15:34 +0200
                Re: [SOLVED] Disk access every few seconds (was: Disk access every few seconds) Lasse Kliemann <lasse-usenet-2011@mail.plastictree.net> - 2011-06-26 19:11 +0200
                Re: [SOLVED] Disk access every few seconds Lasse Kliemann <lasse-usenet-2011@mail.plastictree.net> - 2011-06-27 22:24 +0200

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