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Re: [dm-devel] Determine partition prefix used for mpath devices



Hannes Reinecke wrote:
David Huffman wrote:
Hannes Reinecke wrote:
David Huffman wrote:
In a script, I need to somehow determine what prefix is being used for
partitions as set by UDEV. It seems that the naming convention is in
flux and could change frequently. When using user_friendly_names the
prefix is set using kpartx within udev rules. My problem is I need to
know what the prefix is so my scripts know what device naming convention
to look for.

Here are the different iterations of naming schemes:

mpath0p1
mpath0-part1
mpathap1
mpatha-part1

Is there a way other than grepping through the udev rules to determine
what the prefix is? The partitions may not exist so I cannot just look
to see what they are. I need to know BEFORE I create partitions. I know
this is a strange request, but I am working with a lot of code that was
designed before multipath was available and using the sys naming is not
an option (i.e. /disk/by-id/wwid).

I wouldn't rely on the device-mapper table names, as they could be pretty
much everything.

To handle this situation I've patches up multipath and kpartx to provide
UUID prefixes, which normally can't be influenced from userspace.
Multipath devices carry the prefix 'mpath-', kpartx generated devices
carry the prefix 'part<num>-'.
So by checking the prefix you pretty much know which program generated
this particular device.

Cheers,

Hannes
Humm.. Maybe I did not explain myself well enough because I do not
understand your response. I am trying to determine what prefix is
currently being used for the partitions by kpartx.

Example:
If I am on a SLES9 SP4 system there is a udev rule that states to use
kpartx -a -p -part so that the prefix is "-part". Partitions that are
created later are named [alias|mpath|uuid]a-part1.
If I am on a RHEL 5.1 system, there is a udev rule that states to use
kpartx -a -p p so that the prefix is "p".
Partitions that are created later are named [alias|uuid|mpath]0p1

Right now, the only way I can find out what prefix is used for
partitions is to grep and awk through the udev rule (which is
unreliable). I am looking for a better way to see what prefix is
currently being used by kpartx.

Ah. Leads to the question: Why?
What's the whole point of the exercise?
I am working on scripts that query disk and partition information that is later used to recreate the devices during system recovery. To determine if a device is supported, I have a list of "known" naming schemes to look for. For example sd[a-z][a-z][1-15]. The user is required to use "user_friendly_names" in multipath.conf. This reduces the naming schemes to something a bit more more predictable.

When the partition is created, I need to be able to "find" that partition. Otherwise, it ignores devices that are unknown/unsupported (such as powerpath devices). The original system might have used internal scsi disks, but the restored system could be using mpath devices. This prevents me from relying on what was used before. At the time I am creating a partition table, I may know the disk name, but the partition names could be different. Because this is used in customer environments, we do not want to force them to re-write their udev rules to use a specific partition prefix with kpartx. It's bad enough we enforce "user_friendly_names".

I am working with legacy code that is not easily changed (supporting systems back to 2.4 kernels). This is all happening in a minimal environment much like an initrd and needs to be non-interactive.
When you state that you have a patch to provide UUID prefixes, I do not
understand. If you are not using "user_friendly_names yes" in
multipath.conf, isn't it already the uuid? How does this change the
naming of the partitions used? Right now it could be "-part" or "p".

Nothing to do with multipath.

If you do a 'dmsetup info' you'll see something like:
Name:              3600508b40008ddd70000500000250000
State:             ACTIVE
Read Ahead:        1024
Tables present:    LIVE
Open count:        1
Event number:      19
Major, minor:      253, 11
Number of targets: 1
UUID: mpath-3600508b40008ddd70000500000250000

The last line is the UUID I'm talking about.

This may come in handy to not force "user_friendly_names" in the future. As long as there is SOMETHING that I can use to determine if a device is a multipath device.
Sorry but my limited knowledge of the naming schemes is probably
confusing the issue. It appears that there are too many players on the
field when it comes to naming these devices. (UDEV, multipath,
device-mapper)

Again, what do you need it for?
The whole exercise is already quite cumbersome if
you have only _one_ device-mapper device for a partition;
however, nothing prevents you on calling kpartx (or
some other tools) again with a different prefix
and hence a different name.
So you'll end up with several device-mapper tables
with identical definitions but different names.
Which one will you pick then?

Cheers,

Hannes
I am just trying to pick the one that is used by default by udev. If there are multiple udev rules that contradict each other, then I guess I can address that. However, I have not seen any distros that do this by default and the majority of users will never do that. I have to first address the inconsistencies between distros on what partition prefix is used. So to sum it up, if I am going to create a partition on an mpath device, is there a way (other than grepping through udev rules) to determine what prefix will be used? That way I can add that to my "known" list of naming schemes so I do not ignore the device.

Thanks,
David


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