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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:
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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).
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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
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Close all users of the device and backup device data as needed.
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Use umount to unmount any file systems that mounted the device.
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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.
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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.
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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.
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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.
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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-namesde, for example.
Another variation of this operation is echo 1 > /sys/class/scsi_device/h:c:t:l/device/delete, where hctl
The older form of these commands, echo "scsi remove-single-device 0 0 0 0" > /proc/scsi/scsi, is deprecated.
You can determine the device-namelsscsi, scsi_id, multipath -l, and ls -l /dev/disk/by-*.
Other procedures, such as the physical removal of the device, followed by a rescan of the SCSI bus (as described in Section 21.9, “Scanning Storage Interconnects”) to cause the operating system state to be updated to reflect the change, are not recommended. This will cause delays due to I/O timeouts, and devices may be removed unexpectedly. If it is necessary to perform a rescan of an interconnect, it must be done while I/O is paused, as described in Section 21.9, “Scanning Storage Interconnects”.
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