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While poking around trying to figure out why a Linux -> Linux file transfer is running slower than I think it should, I stumbled across something I'm not familiar with. /dev/dm-0 seems to be my bottleneck, but I have no idea what it is.

On my destination server, the iostat command shows a device at the bottom, /dev/dm-0, as being 100% utilized. This server has 6 disks in a mdadm raid5 set, with LVM running on top of it. Each of the underlying disks are sitting around 50% util. The transfer is writing to a logical volume located on this raidset.

What is this /dev/dm-0 thing? Once I know what it is, maybe I can find how to increase its speed, or at least understand why its the speed that it is.

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It turns out that the dm-0 device wasn't causing the slowness, I had the partitions aligned wrong. On a 4k sector drive, you need to manually align the partitions when you create them. Dumb, but easy to fix. –  Jeff Shattock Apr 16 '10 at 21:15

4 Answers 4

up vote 46 down vote accepted

It's part of the device mapper in the kernel, used by LVM. Use dmsetup ls to see what is behind it.

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pvs displays the physical volumes that underpin the LVM volume group. A number of physical devices can be grouped together to from one volume group. The group is then re-split into logical volumes as required treating the physical disks as a single resource to be chopped up and re-sized at will. Each time you slice of part of the volume group as a logical volume it is presented as a dm- device

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sudo lvdisplay

showed the mapping of mounted virtual partitions to "volume groups".

When I typed sudo pvs I saw the mapping of a volume group to a "physical volume" that looked like a familiar /dev/sdaN notation. (My looking at the whole disk with fdisk -l /dev/sda showed another partition).

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Those are LVM logical "devices"

You can map them using:

# sudo lvdisplay|awk  '/LV Name/{n=$3} /Block device/{d=$3; sub(".*:","dm-",d); print d,n;}'
dm-0 /dev/SysVolGroup/LogVolRoot
dm-1 /dev/SysVolGroup/xen
dm-2 /dev/SysVolGroup/db1-2
dm-3 /dev/SysVolGroup/db1-2swap
dm-4 /dev/SysVolGroup/python1
dm-5 /dev/SysVolGroup/python1swap
dm-6 /dev/SysVolGroup/db1-2snap

source: http://www.linuxquestions.org/questions/linux-newbie-8/dm0-in-iostat-652771/

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