iscsi target Path configuration

momurda

Is there any real difference setting up iscsi targets using /dev/mapper/device as the iscsi target Path vs using /dev/vg/lv in the Path of ietd.conf. I have tried both and both work and point to the same device. I think one is just link to the other.
Is there any reason to not use either one?

scottalanmiller

Mapper is there to be theoretically more flexible. Both should be symlinks to the same dm device.

thwr

@scottalanmiller said in iscsi target Path configuration:

Mapper is there to be theoretically more flexible. Both should be symlinks to the same dm device.

@mormuda: ~~Just verifiy if /dev/vg/* is a symlink with something like~~

Like SAM said, they are supposed to be symlinks

I would use them anyway, because they are there for a purpose. Maybe the kernel device mapper gets replaced some day. In this case, LVM would point to the new "location". It may be not foreseeable as of today, but things like that already happened in the past.

Erm, just saw this:
http://unix.stackexchange.com/questions/53656/why-are-there-symlinks-in-dev-lvm

For convenience, udev creates a symlink in /dev/mapper with the device mapper name associated with it. And if that device mapper device also happens to be a LVM logical volume, then the LVM subsystem also adds a /dev/vg/lv symlink to it.

So it's exactly the other way around. Could you please check that with

ls -lsa /dev/mapper/*

Some of the links there should point to /dev/vg/...

scottalanmiller

Last system I checked LVM and mapper were equal symlinks pointing to the same thing. I checked it because I was documenting on CentOS.

thwr

@scottalanmiller said in iscsi target Path configuration:

Last system I checked LVM and mapper were equal symlinks pointing to the same thing. I checked it because I was documenting on CentOS.

Still interesting. Device mapper is just that, a mapper pointing to LVM (and LUKS) devices for example. So your mapper and LVM are both pointing to something "physical" like md devices or sd*?

momurda

Im just posting some stuff about iscsi as i test this new storage server i built.
I built an iscsi target initially with the type=fileio rather than blockio
During tests(moving vm disks between storage systems) i noticed incredibly slow write speeds. Hit a max of 20MB/sec with a 4 disk Raid10 software array, and was sad.

So then i thought about changing it to type=blockio in the target declaration file.
doing that doubled my write speeds to 40-45MB/sec. I think ive got an explanation why, but some here would know for sure.

scottalanmiller

@thwr said in iscsi target Path configuration:

@scottalanmiller said in iscsi target Path configuration:

Last system I checked LVM and mapper were equal symlinks pointing to the same thing. I checked it because I was documenting on CentOS.

Still interesting. Device mapper is just that, a mapper pointing to LVM (and LUKS) devices for example. So your mapper and LVM are both pointing to something "physical" like md devices or sd*?

Both pointing to the dm device, which is another layer of mapping.