Managing dynamic device naming

The coming increase in the size of dev_t adds to the urgency of the device naming problem. Even if device numbers remain entirely static, there will be management issues to deal with. Consider the case of SCSI disks, for example. The wider dev_t will make it possible to have thousands of disks on a single system, and the maximum number of partitions will be increased to 64. /dev is already a big directory on modern distributions - over 12,000 entries on a Red Hat Linux 7.3 system, 2000 in the cciss subdirectory alone. It is unwieldy to work with now, but consider what happens with the device names for all those new drives and partitions are added; now /dev has several hundred thousand entries. And we haven't even begun to look at all those new serial ports, tape drives, printers, and CueCat barcode readers we'll be able to add.

Richard Gooch beat the rush and started worrying about this problem some years ago; the result was devfs. The devfs code has been in the mainline kernel since the 2.3 days, but it is not heavily used. It puts naming policy firmly in the kernel itself (you get /dev/disc whether you like it or not), and it solves persistent permissions issues by way of a deamon process and a "make a tarball at shutdown" technique that strikes some as inelegant. Some kernel developers have also made a longstanding hobby of complaining about the quality of the devfs code.

The end result is that there would seem to be an opening for a different approach. One alternative began to come into focus this week with the release of udev 0.1. udev is an effort by Greg Kroah-Hartman (and others) to push the device naming issue completely into user space, with the result that the kernel hackers would be free to go off and argue about something else. The current udev implementation is a minimal demonstration of the concept, but the longer-term vision calls for three distinct components:

• "namedev" is a subsystem which has the job of coming up with useful names for devices. It could make use of whatever information is available: device numbers, hardware ID numbers, filesystem labels, etc.; it would then apply the site's particular policy to produce a suitable name. On simple systems, a simple flat file (or hardcoded names) would suffice; the 4000-disk monster system could dedicate one drive to a relational database for device naming.

• "libsysfs" would provide a common API for obtaining information about devices from sysfs.

• "udev" is a separate application which is run in response to hotplug events; it uses the above two modules to gather the information it needs, then creates or removes device nodes as appropriate.

In the current release, everything is bundled together into a single "udev" binary. It requires a series of patches on top of 2.5.67 to create hotplug events when kobjects are registered (these patches have been merged into Linus's BitKeeper repository, and thus will be unnecessary for 2.5.68 and later kernels), and, even then, can only work with devices which export their device number via sysfs. Still, your editor had no trouble making it work on his sacrificial system. Loading the simple block driver from the driver porting series caused a set of block device nodes to be created in /udev - with no changes to the driver required. The basic idea works.

A lot of work remains to be done before udev is ready for prime time, however. Some of the issues needing resolution are:

• Robust management of device events. The current hotplug mechanism creates a separate process for each event, each of which runs whatever program has been designated to handle those events. Among other things, this mechanism has race conditions; if a device is quickly attached and removed, the unplug event could end up being processed first. Attaching a large disk array could create an "event storm" that threatens to overwhelm the system. So there is a fair amount of interest in serializing events, but little agreement on how that should be done.

• A related issue is that multiple programs may want to receive hotplug events. One might load a driver, another runs udev, yet another mounts partitions on a newly-attached disk, etc. Possible solutions here include using Greg's /sbin/hotplug multiplexor, distributing events in user space with D-BUS, or distributing them in the kernel via a new event interface.

• How desirable is per-site device naming policy anyway? A world where each distribution, if not each installation, has its own device naming scheme does not look like an improvement to a lot of people. Vendors cringe at trying to support that sort of setup. So there is a need for some sort of common policy. The Linux Standard Base decrees that the LANANA devices.txt file is the definitive authority for standard device names, which is a start. But there is a strong desire for more flexible and generic naming (all disks under /dev/disk, for example, with no distinction between SCSI and IDE drives); the device list will probably have to be revised to fit the dynamic, very large systems of the future.

All of these issues should be solvable, of course, and the fact that they are being discussed indicates that people are getting serious about solving the problems. The 2.6 kernel will probably go out with the larger dev_t and, perhaps, some hooks for udev-like programs. Things could get more interesting once the 2.7 development series opens up, however.

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