diff options
Diffstat (limited to 'Documentation/device-mapper/thin-provisioning.txt')
| -rw-r--r-- | Documentation/device-mapper/thin-provisioning.txt | 34 |
1 files changed, 31 insertions, 3 deletions
diff --git a/Documentation/device-mapper/thin-provisioning.txt b/Documentation/device-mapper/thin-provisioning.txt index 8a7a3d4..05a27e9 100644 --- a/Documentation/device-mapper/thin-provisioning.txt +++ b/Documentation/device-mapper/thin-provisioning.txt @@ -116,6 +116,35 @@ Resuming a device with a new table itself triggers an event so the userspace daemon can use this to detect a situation where a new table already exceeds the threshold. +A low water mark for the metadata device is maintained in the kernel and +will trigger a dm event if free space on the metadata device drops below +it. + +Updating on-disk metadata +------------------------- + +On-disk metadata is committed every time a FLUSH or FUA bio is written. +If no such requests are made then commits will occur every second. This +means the thin-provisioning target behaves like a physical disk that has +a volatile write cache. If power is lost you may lose some recent +writes. The metadata should always be consistent in spite of any crash. + +If data space is exhausted the pool will either error or queue IO +according to the configuration (see: error_if_no_space). If metadata +space is exhausted or a metadata operation fails: the pool will error IO +until the pool is taken offline and repair is performed to 1) fix any +potential inconsistencies and 2) clear the flag that imposes repair. +Once the pool's metadata device is repaired it may be resized, which +will allow the pool to return to normal operation. Note that if a pool +is flagged as needing repair, the pool's data and metadata devices +cannot be resized until repair is performed. It should also be noted +that when the pool's metadata space is exhausted the current metadata +transaction is aborted. Given that the pool will cache IO whose +completion may have already been acknowledged to upper IO layers +(e.g. filesystem) it is strongly suggested that consistency checks +(e.g. fsck) be performed on those layers when repair of the pool is +required. + Thin provisioning ----------------- @@ -258,10 +287,9 @@ ii) Status should register for the event and then check the target's status. held metadata root: - The location, in sectors, of the metadata root that has been + The location, in blocks, of the metadata root that has been 'held' for userspace read access. '-' indicates there is no - held root. This feature is not yet implemented so '-' is - always returned. + held root. discard_passdown|no_discard_passdown Whether or not discards are actually being passed down to the |