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   lvmraid    ( 7 )

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DEVICE FAILURE

Physical devices in a RAID LV can fail or be lost for multiple reasons. A device could be disconnected, permanently failed, or temporarily disconnected. The purpose of RAID LVs (levels 1 and higher) is to continue operating in a degraded mode, without losing LV data, even after a device fails. The number of devices that can fail without the loss of LV data depends on the RAID level: • RAID0 (striped) LVs cannot tolerate losing any devices. LV data will be lost if any devices fail. • RAID1 LVs can tolerate losing all but one device without LV data loss. • RAID4 and RAID5 LVs can tolerate losing one device without LV data loss. • RAID6 LVs can tolerate losing two devices without LV data loss. • RAID10 is variable, and depends on which devices are lost. It stripes across multiple mirror groups with raid1 layout thus it can tolerate losing all but one device in each of these groups without LV data loss.

If a RAID LV is missing devices, or has other device-related problems, lvs reports this in the health_status (and attr) fields:

lvs -o name,lv_health_status

partial Devices are missing from the LV. This is also indicated by the letter "p" (partial) in the 9th position of the lvs attr field.

refresh needed A device was temporarily missing but has returned. The LV needs to be refreshed to use the device again (which will usually require partial synchronization). This is also indicated by the letter "r" (refresh needed) in the 9th position of the lvs attr field. See Refreshing an LV. This could also indicate a problem with the device, in which case it should be be replaced, see Replacing Devices.

mismatches exist See Scrubbing.

Most commands will also print a warning if a device is missing, e.g. WARNING: Device for PV uItL3Z-wBME-DQy0-... not found or rejected ...

This warning will go away if the device returns or is removed from the VG (see vgreduce --removemissing).

Activating an LV with missing devices A RAID LV that is missing devices may be activated or not, depending on the "activation mode" used in lvchange:

lvchange -ay --activationmode complete|degraded|partial LV

complete The LV is only activated if all devices are present.

degraded The LV is activated with missing devices if the RAID level can tolerate the number of missing devices without LV data loss.

partial The LV is always activated, even if portions of the LV data are missing because of the missing device(s). This should only be used to perform extreme recovery or repair operations.

Default activation mode when not specified by the command: lvm.conf(5) activation/activation_mode

The default value is printed by: # lvmconfig --type default activation/activation_mode

Replacing Devices Devices in a RAID LV can be replaced by other devices in the VG. When replacing devices that are no longer visible on the system, use lvconvert --repair. When replacing devices that are still visible, use lvconvert --replace. The repair command will attempt to restore the same number of data LVs that were previously in the LV. The replace option can be repeated to replace multiple PVs. Replacement devices can be optionally listed with either option.

lvconvert --repair LV [NewPVs]

lvconvert --replace OldPV LV [NewPV]

lvconvert --replace OldPV1 --replace OldPV2 LV [NewPVs]

New devices require synchronization with existing devices. See Synchronization.

Refreshing an LV Refreshing a RAID LV clears any transient device failures (device was temporarily disconnected) and returns the LV to its fully redundant mode. Restoring a device will usually require at least partial synchronization (see Synchronization). Failure to clear a transient failure results in the RAID LV operating in degraded mode until it is reactivated. Use the lvchange command to refresh an LV:

lvchange --refresh LV

# lvs -o name,vgname,segtype,attr,size vg LV VG Type Attr LSize lv vg raid1 Rwi-a-r-r- 100.00g

# lvchange --refresh vg/lv

# lvs -o name,vgname,segtype,attr,size vg LV VG Type Attr LSize lv vg raid1 Rwi-a-r--- 100.00g

Automatic repair If a device in a RAID LV fails, device-mapper in the kernel notifies the dmeventd(8) monitoring process (see Monitoring). dmeventd can be configured to automatically respond using: lvm.conf(5) activation/raid_fault_policy

Possible settings are:

warn A warning is added to the system log indicating that a device has failed in the RAID LV. It is left to the user to repair the LV, e.g. replace failed devices.

allocate dmeventd automatically attempts to repair the LV using spare devices in the VG. Note that even a transient failure is treated as a permanent failure under this setting. A new device is allocated and full synchronization is started.

The specific command run by dmeventd(8) to warn or repair is: lvconvert --repair --use-policies LV

Corrupted Data Data on a device can be corrupted due to hardware errors without the device ever being disconnected or there being any fault in the software. This should be rare, and can be detected (see Scrubbing).

Rebuild specific PVs If specific PVs in a RAID LV are known to have corrupt data, the data on those PVs can be reconstructed with:

lvchange --rebuild PV LV

The rebuild option can be repeated with different PVs to replace the data on multiple PVs.