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[dm-devel] block offset shift, mirroring bug resolved?



I've tried Kevin Corry's patches (with my extra modification to do_write, as quoted below), and the mirroring problem seems resolved. I might have a poke around and see if I can figure out why it was happening...

The setup I had was:

#!/bin/sh
dmsetup remove_all

dmsetup create mirror <<EOF
0 16777216 mirror core 1 2048 2 /dev/sdd 0 /dev/sdt 0
16777216 16777216 mirror core 1 2048 2 /dev/sde 0 /dev/sdu 0
33554432 16777216 mirror core 1 2048 2 /dev/sdf 0 /dev/sdv 0
50331648 16777216 mirror core 1 2048 2 /dev/sdg 0 /dev/sdw 0
67108864 16777216 mirror core 1 2048 2 /dev/sdp 0 /dev/sdae 0
83886080 16777216 mirror core 1 2048 2 /dev/sdq 0 /dev/sdaf 0
100663296 16777216 mirror core 1 2048 2 /dev/sdr 0 /dev/sdag 0
117440512 16777216 mirror core 1 2048 2 /dev/sds 0 /dev/sdah 0
EOF

dmsetup create reliable <<EOF
0 134217728 striped 8 512 /dev/mapper/mirror 0 /dev/mapper/mirror 16777216 /dev/mapper/mirror 33554432 /dev/mapper/mirror 50331648 /dev/mapper/mirror 67108864 /dev/mapper/mirror 83886080 /dev/mapper/mirror 100663296 /dev/mapper/mirror 117440512
EOF


Writing to /dev/mapper/reliable in the above configuration caused i/o to /dev/sd[defgpqrs] (the primary legs) and /dev/sdt but none of the other secondary mirror legs.

As I said before though, Kevin's patch + the extra bit fixes this. I'd post a patch but since I'm running an older version there's probably not much point.

I'm seeing kernel panics using the mirror target though - another post follows.

Cheers,
Tim

The extra change (haven't tested without, it just made sense to change this too - correct me if I'm wrong here!) was:

function do_write() in dm-raid1.c

Old:

        for (i = 0; i < ms->nr_mirrors; i++) {
                m = ms->mirror + i;

io[i].bdev = m->dev->bdev;
io[i].sector = m->offset + (bio->bi_sector - ms->ti->begin);
io[i].count = bio->bi_size >> 9;
}



New:


        for (i = 0; i < ms->nr_mirrors; i++) {
                m = ms->mirror + i;

                io[i].bdev = m->dev->bdev;
                io[i].sector = m->offset + bio->bi_sector;
                io[i].count = bio->bi_size >> 9;
        }



Tim Burgess wrote:

Re the patch from Kevin:

there looks like there is another reference to ti->begin in dm-raid1.c
that the patch does not remove (in do_write). I wasn't sure whether to leave it there or not, since you were talking about making each target unaware of its position within the overall mapped device...?


(note that my copy is not the latest - it's SUSE SLES SP1, so I
apologise if anything I say is not 100% true for the latest code :S)

Related:

I noticed that a similar collection of concatenated raid1 devices
(description below) was behaving strangely also, and splitting each
raid1 map into its own table fixed the problem...

For some reason, each of the mirror pairs was writing to its primary leg, but only the first one listed in the file was writing to its second leg... (note that this is before Kevin's patches - will try them in a moment!).



On Thursday 10 February 2005 11:18 am, Alasdair G Kergon wrote:

On Thu, Feb 10, 2005 at 04:02:28PM +1100, Tim Burgess wrote:
> However, dm appears to be trying
> to map the range 286749488-573498975 of the dm device to the same
> offsets in the sde/sdm device.
>
> Is this what was intended?

No.

In dm-mpath.c try adding to multipath_map() at the top of the function:

bio->bi_sector = (bio->bi_sector - ti->begin);


Actually, now that you point this out, I think this responsibility should
really be handled by the core driver's I/O path instead of each target
module. There's really no reason for the target modules to care or even
know about the presence of multiple targets within a device table. We can
move this line into the core's __map_bio() and get rid of a lot of
duplicate code. Here's a patch to demonstrate what I'm talking about.



-- -------------------------------------------------------------------------- ANU Supercomputer Facility tim burgess anu edu au and APAC National Facility Phone: +61 2 6125 1431 Leonard Huxley Bldg (No. 56) Fax: +61 2 6125 8199 Australian National University Canberra, ACT, 0200, Australia -------------------------------------------------------------------------- "Money can buy bandwidth, but latency is forever" -- John Mashey --------------------------------------------------------------------------


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