UdevLvm

Revision 2 as of 2006-11-09 23:24:18

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Please check the status of this specification in Launchpad before editing it. If it is Approved, contact the Assignee or another knowledgeable person before making changes.

Summary

This specification details how to make udev and LVM play nicely together, in particular ensuring that udev events are issued for LVM volumes and that UUIDs are correctly exported and do not conflict.

Rationale

LVM is used by system administrators to collect block devices together into Volume Groups and then split them into Logical Volumes which can be readily resized and adjusted within the group without needing complicated work. In order to support event-based mounting of these filesystems, we need reliable events from the block subsystem and no race conditions.

Use cases

  • Fabio uses a combination of LVM and RAID for his root filesystem, he would like this to continue to be supported.

Scope

The scope of this specification is limited to the interaction between udev and LVM2; other specifications address similar concerns with device-mapper (on which LVM is based), RAID, etc.

Design

vgchange -ay is the command run to iterate all block devices on the system (filtered by CD-ROM, etc. devices) and combine them into volume groups, and active logical volumes. If the necessary components of a group cannot be found, it is not activated.

This can be run directly from a udev rule whenever an LVM block device is added to the system. vol_id can be used to determine whether a block device is an LVM Physical Volume or not.

As LVM builds Logical Volumes using device-mapper, a further block event will be issued when the logical volume has been activated.

There is a race between vgchange creating the /dev/VGNAME/LVNAME symlinks and udev receiving the device-mapper block event for the device that those should point to. For this reason, vgchange will be instructed not to create these symlinks and instead vgmknodes will be called from a udev rule for device-mapper block devices.

When mirroring is involved, it's possible for a logical volume to be mounted even though it's not yet complete. We will offer the option of forcing a partial volume group mount after a timeout if it has not yet been activated.

As with device-mapper, the Volume Group and Logical Volume name, and thus the device path, consitutes a unique identifier; there is no need for UUID or LABEL support for these block devices. We will continue to ignore them.

Implementation

  • Patch vgchange to accept an option to inhibit device symlink creation.

  • Add a udev rule to call vgchange when block devices are added: 
    • SUBSYSTEM=="block", RUN+="watershed /sbin/vgchange -ay --no-symlinks"
  • Add a udev rule to call vgmknodes when device-mapper block devices are added: 
    • SUBSYSTEM=="block", KERNEL=="dm-[0-9]*", RUN+="watershed /sbin/vgmknodes"

  • Modify the mountroot script to give the option of attempting vgchange -P after a timeout.

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