Re: Spin down

[NeilBrown <neilb@suse.de>]

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On Sat, 3 Dec 2011 12:23:37 +0100 Vincent Pelletier <plr.vincent@gmail.com>
wrote:

> Hi.
> 
> Short system description:
> up-to-date Debian sid, used on a NAS-ish machine (iscsi with
> open-iscsi's kernel module, nfs, dnsmask for netboot), ext4 FS over
> RAID1 on both system partition and exported data partitions.
> 
> I would like disks containing raid arrays to spin down, but there are
> periodic disk writes which prevent this.
> They do not show up in iotop (although it does show a write activity
> in the topmost line).
> Enabling /proc/sys/vm/block_dump generates the following output:
> 
> <7>[  918.078079] md0_raid1(304): WRITE block 8 on sda1 (2 sectors)
> <7>[  918.078169] md0_raid1(304): WRITE block 8 on sdc1 (2 sectors)
> <7>[  918.092124] md1_raid1(311): WRITE block 8 on sda5 (2 sectors)
> <7>[  918.092184] md1_raid1(311): WRITE block 8 on sdc5 (2 sectors)
> <7>[  918.292627] md0_raid1(304): WRITE block 8 on sda1 (2 sectors)
> <7>[  918.292714] md0_raid1(304): WRITE block 8 on sdc1 (2 sectors)
> <7>[  918.308500] md1_raid1(311): WRITE block 8 on sda5 (2 sectors)
> <7>[  918.308588] md1_raid1(311): WRITE block 8 on sdc5 (2 sectors)
> <7>[  923.113309] md0_raid1(304): WRITE block 8 on sda1 (2 sectors)
> <7>[  923.113397] md0_raid1(304): WRITE block 8 on sdc1 (2 sectors)
> <7>[  923.129330] md1_raid1(311): WRITE block 8 on sda5 (2 sectors)
> <7>[  923.129388] md1_raid1(311): WRITE block 8 on sdc5 (2 sectors)
> <7>[  923.332489] md0_raid1(304): WRITE block 8 on sda1 (2 sectors)
> <7>[  923.332545] md0_raid1(304): WRITE block 8 on sdc1 (2 sectors)
> <7>[  923.348507] md1_raid1(311): WRITE block 8 on sda5 (2 sectors)
> <7>[  923.348595] md1_raid1(311): WRITE block 8 on sdc5 (2 sectors)
> 
> Note the 5-seconds pause between each half of this output.
> Block 8 seems to be the md superblock (dixit hexdump + description of
> md superblock), and "mdadm --examine" shows the "Update time" is being
> increased.
> Those arrays are in clean state, no synchronisation is happening and
> no checking that I know of (I would expect a lot of disk activity).
> 
> Is there any reason why those writes are happening ?
> I can imagine a mechanism like filesystem superblock periodic
> flushing, but AFAIK it only happens when changes are happening (even
> if they are not flushed themselves), and I would expect the same to
> happen here.
> Is there any knob to control those writes ?
> 
> Regards,

5 seconds suggests a bitmap update.  Do you have an internal bitmap?

Try to remove it
   mdadm --grow /dev/md0 --bitmap=none

and see if they stop.
Then try adding it back with a longer delay
  mdadm --grow /dev/md0 --bitmap=internal --delay=60

What kernel version are you running?

NeilBrown

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