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-rw-r--r--Documentation/device-mapper/thin-provisioning.txt34
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