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Using UDEV to do Persistent storage device naming
for large numbers of storage devices
3/16/2004
Here are some lessons we learned at OSDL recently on how to use
UDEV (version 021) to do persistent device naming for lots of storage
devices. We used what was available in udev for scsi devices. Here is
an outline of this report:
Background information - a list of resources we needed to get
started.
Setup - what we needed to create the right enviroment (kernel,
patches, drivers)
How udev works to assign persistent storage device names -
what the documentation didn't tell us.
Performance - A sanity test we ran to compare with and without
persistent naming.
BACKGROUND INFORMATION
To get started, here are some references. Review the overview
articles so that the rest of the information makes sense.
Download the latest udev stuff from:
http://www.kernel.org/pub/linux/utils/kernel/hotplug/
mailing list:
linux-hotplug-devel@lists.sourceforge.net
Here is a nice overview article to get started (warning, this is from
summer 2003 so many items indicated as "todo" have been done and
configuration file name references have sometime changed):
http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf
(also included when you download udev)
More general info (also included in the udev package):
http://kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ
UDEV version 021 Announcement:
http://marc.theaimsgroup.com/?l=linux-hotplug-devel&m=107827264803336&w=2
"Managing Dynamic Naming"
http://lwn.net/Articles/28897/
If you are a fan of devfs, whatever you do, don't complain until you
read everything you possibly can about udev. This for example:
http://kernel.org/pub/linux/utils/kernel/hotplug/udev_vs_devfs
You will need to create udev.rules to supply consistent names. (See
etc/udev/udev.rules in the download). This article gives you some
background about udev.rules, but avoids describing the "PROGRAM"
key which is needed for our work. Read it for background:
writing udev rules (current as of udev 018)
http://www.reactivated.net/udevrules.php
bitkeeper tree:
bk://kernel.bkbits.net/gregkh/udev
Libsysfs (used to get sysfs information):
http://www-124.ibm.com/linux/papers/libsysfs/libsysfs-linuxconfau2004.pdf
UDEV works using the way hotplug events are handled by the kernel.
Several overview articles about hotplug include:
Hotplug events
http://lwn.net/Articles/52621/
Overview of Hotplug
http://linux-hotplug.sourceforge.net/
Gentoo centric install info:
http://webpages.charter.net/decibelshelp/LinuxHelp_UDEVPrimer.html
rpms built against Red Hat FC2-test1 may be available at:
http://kernel.org/pub/linux/utils/kernel/hotplug/udev-021-1.i386.rpm
with the source rpm at:
http://kernel.org/pub/linux/utils/kernel/hotplug/udev-021-1.src.rpm
SETUP
Here is a brief checklist of what you need on your system for this to
work:
Kernel must be a 2.6 kernel
Must use CONFIG_HOTPLUG kernel config option, since the solution
is based on hotplug capabilities.
To test more than 256 scsi devices you need a patch to the scsi driver
to support that many (available from IBM or SuSE). To see the patch
we used, see this link:
http://developer.osdl.org/maryedie/DCL/PSDN/lotsofdisks.patch
Your storage device must support (via the driver) a unique identifier for
persistent device naming. (Adaptec RAID device does not, for
example.)
Your device driver must support sysfs (new in 2.6 kernel). This is
already done for scsi devices and most if not all block devices.
A program (scsi_id) exists in the udev download
( extras/scsi_id/scsi_id.c) for scsi devices. It can read the identifier and
is needed for persistent naming.
HOW UDEV WORKS TO ASSIGN PERSISTENT NAMES:
There are three places where device information is stored that udev
uses:
(1) /sys maintained by sysfs
(2) /etc/udev/udev.rules - where you can store the identifier to NAME
mapping information.
(3) The tdb (udev-021/tdb/tdb.c), trivial data base, that is held in
memory and holds the valid system configuration. It is not saved
between one boot to the next. It is constructed at boot time and
updated with configuration changes.
The persistent names are kept (at least this is one way to do it) in
udev.rules (uuid and NAME), one entry per device. If you want to
initially give your 1000 disk devices a default name and then make
sure those names are preserved, here is how :
Start with no special entry in udev.rules when do you an initial boot of
your system with disks in place. Udev will assign default names (there
are ways to control what you want for default too).
Once the names are assigned, use a script supplied for scsi devices -
udev-021/extras/scsi_id/gen_scsi_id_udev_rules.sh
to generate the lines needed for udev.rules, one per device. Each line
indicates the identifier and the NAME it was assigned. You could
optionally create this manually if you prefer other names .
[example entries in udev.rules for scsi disks]
BUS="scsi", PROGRAM="scsi_id", RESULT="<uuid1>",NAME="<name1>"
BUS="scsi", RESULT="<uuid2>",NAME="<name2>"
...
BUS="scsi", RESULT="<uuid1000>",NAME="<name1000>"
(The actual file we used is the file udev.rules_1000_scsi_debug in this
directory )
Upon reboot, for each device a hotplug event occurs. The udev.rules
file is scanned looking for the device type (BUS) in this case for "scsi".
The first entry generated by the above program references a
PROGRAM in the key field (scsi_id) which is called to probe the device
and determine the unique identifier. sysfs is used to determine the
major/minor number for the device. The result of the program
execution (the uuid) is compared with the RESULT entry in the same
udev.rules line.
-If it matches, then the NAME entered on this line is used. The uuid
and major/minor number is saved in tdb (newly recreated upon boot).
That device is created in /udev (the target directory name is
configurable) with the assigned NAME.
-If it doesn't match, the RESULT (uuid) is preserved for use on the next
udev.rules line as long as the bus type (scsi) is the same. So the result
(the uuid) is compared on the next line, and the next until a match
occurs.
-If no match occurs, the device will be assigned a default name.
-Tdb is updated with the resulting name assignment.
Thus if the uuid and names are enumerated, they will be found,
assigned, and are therefore permanent.
If the device is removed from a live system, a hotplug event occurs,
and it is removed from tdb and the /udev entry disappears.
If it is re-inserted at a new location, the udev.rules file is scanned as
above. The new major/minor number goes in tdb with the uuid , the
name in udev.rules is found again, and the /udev name re-appears.
PERFORMANCE
Now the question becomes, how much longer does it take to scan the
udev.rules table once there are 1000 entries?
To test this, we created 1000 "scsi " devices using the scsi debug
device driver supplied in the kernel. When this device driver is loaded
you can specify how many fake scsi devices to create. There is no
real I/O involved but it does respond to some scsi commands. It
simulates the uuid by using the device number assigned when the
device is created.
Then we auto-generated entries into udev.rules with
gen_scsi_id_udev_rules.sh. We then removed the devices and
reassigned them to simulate a reboot. The delta between assigning
defaults and assigning the names enumerated in the udev.rules file
was 7 seconds (that's for 1000 drives).
Scripts utilized the feature (described above) that saves the "RESULT"
key after one scsi-id program call for later reference with other
udev.rules entries (so only have one PROGRAM key is the moral of
the story). If you repeated the PROGRAM key, you would
unnecessarily call the program up to 999 times!
The script that creates udev.rules did not work for 1000 drives (the
input line is too long). We determined that a patch for this already
existed but had not yet been checked in.
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