Re: (user) Help needed: mdadm seems to constantly touch my disks

[NeilBrown <neilb@suse.com>]

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On Tue, Dec 13 2016, Jure Erznožnik wrote:

> First of all, I apologise if this mail list is not intended for layman
> help, but this is what I am and I couldn't get an explanation
> elsewhere.
>
> My problem is that (as it seems) mdadm is touching HDD superblocks
> once per second, once at address 8 (sectors), next at address 16.
> Total traffic is kilobytes per second, writes only, no other
> detectable traffic.
>
> I have detailed the problem here:
> http://unix.stackexchange.com/questions/329477/
>
> Shortened:
> kubuntu 16.10 4.8.0-30-generic #32, mdadm v3.4 2016-01-28
> My configuration: 4 spinning platters (/dev/sd[cdef]) assembled into a
> raid5 array, then bcache set to cache (hopefully) everything
> (cache_mode = writeback, sequential_cutoff = 0). On top of bcache
> volume I have set up lvm.
>
> * iostat shows traffic on sd[cdef] and md0
> * iotop shows no traffic
> * iosnoop shows COMM=[idle, md0_raid5, kworker] as processes working
> on the disk. Blocks reported are 8, 16 (data size a few KB) and
> 18446744073709500000 (data size 0). That last one must be some virtual
> thingie as the disks are nowhere near that large.
> * enabling block_dump shows md0_raid5 process writing to block 8 (1
> sectors) and 16 (8 sectors)
>
> This touching is caused by any write into the array and goes on for
> quite a while after the write has been done (a couple of hours for
> 60GB of writes). When services actually work with the array, this
> becomes pretty much constant.
>
> What am I observing and is there any way of stopping it?

Start with the uppermost layer which has I/O that you cannot explain.
Presumably that is md0.
Run 'blktrace' on that device for a little while, then 'blkparse' to
look at the results.

 blktrace -w 10 md0
 blkparse *blktrace*

It will give the name of the process that initiated the request in [] at
the end of some lines.

NeilBrown

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