Installation guide

Chapter 21. Online Storage Management
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• File system label
These identifiers are persistent, and based on metadata written on the device by certain applications.
They may also be used to access the device using the symlinks maintained by the operating system
in the /dev/disk/by-label/ (e.g. boot -> ../../sda1 ) and /dev/disk/by-uuid/ (e.g.
f8bf09e3-4c16-4d91-bd5e-6f62da165c08 -> ../../sda1) directories.
md and LVM write metadata on the storage device, and read that data when they scan devices. In
each case, the metadata contains a UUID, so that the device can be identified regardless of the
path (or system) used to access it. As a result, the device names presented by these facilities are
persistent, as long as the metadata remains unchanged.
21.4. Removing a Storage Device
Before removing access to the storage device itself, it is advisable to back up data from the device
first. Afterwards, flush I/O and remove all operating system references to the device (as described
below). If the device uses multipathing, then do this for the multipath "pseudo device" (Section 21.3.1,
“WWID”) and each of the identifiers that represent a path to the device. If you are only removing a
path to a multipath device, and other paths will remain, then the procedure is simpler, as described in
Section 21.6, “Adding a Storage Device or Path”.
Removal of a storage device is not recommended when the system is under memory pressure,
since the I/O flush will add to the load. To determine the level of memory pressure, run the command
vmstat 1 100; device removal is not recommended if:
• Free memory is less than 5% of the total memory in more than 10 samples per 100 (the command
free can also be used to display the total memory).
• Swapping is active (non-zero si and so columns in the vmstat output).
The general procedure for removing all access to a device is as follows:
Procedure 21.1. Ensuring a Clean Device Removal
1. Close all users of the device and backup device data as needed.
2. Use umount to unmount any file systems that mounted the device.
3. Remove the device from any md and LVM volume using it. If the device is a member of an LVM
Volume group, then it may be necessary to move data off the device using the pvmove command,
then use the vgreduce command to remove the physical volume, and (optionally) pvremove to
remove the LVM metadata from the disk.
4. If the device uses multipathing, run multipath -l and note all the paths to the device.
Afterwards, remove the multipathed device using multipath -f device.
5. Run blockdev –flushbufs device to flush any outstanding I/O to all paths to the device.
This is particularly important for raw devices, where there is no umount or vgreduce operation to
cause an I/O flush.
6. Remove any reference to the device's path-based name, like /dev/sd, /dev/disk/by-path or
the major:minor number, in applications, scripts, or utilities on the system. This is important in
ensuring that different devices added in the future will not be mistaken for the current device.
7. Finally, remove each path to the device from the SCSI subsystem. To do so, use the command
echo 1 > /sys/block/device-name/device/delete where device-name may be sde,
for example.