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Introduction
============

dm-era is a target that behaves similar to the linear target.  In
addition it keeps track of which blocks were written within a user
defined period of time called an 'era'.  Each era target instance
maintains the current era as a monotonically increasing 32-bit
counter.

Use cases include tracking changed blocks for backup software, and
partially invalidating the contents of a cache to restore cache
coherency after rolling back a vendor snapshot.

Constructor
===========

 era <metadata dev> <origin dev> <block size>

 metadata dev    : fast device holding the persistent metadata
 origin dev	 : device holding data blocks that may change
 block size      : block size of origin data device, granularity that is
		     tracked by the target

Messages
========

None of the dm messages take any arguments.

checkpoint
----------

Possibly move to a new era.  You shouldn't assume the era has
incremented.  After sending this message, you should check the
current era via the status line.

take_metadata_snap
------------------

Create a clone of the metadata, to allow a userland process to read it.

drop_metadata_snap
------------------

Drop the metadata snapshot.

Status
======

<metadata block size> <#used metadata blocks>/<#total metadata blocks>
<current era> <held metadata root | '-'>

metadata block size	 : Fixed block size for each metadata block in
			     sectors
#used metadata blocks	 : Number of metadata blocks used
#total metadata blocks	 : Total number of metadata blocks
current era		 : The current era
held metadata root	 : The location, in blocks, of the metadata root
			     that has been 'held' for userspace read
			     access. '-' indicates there is no held root

Detailed use case
=================

The scenario of invalidating a cache when rolling back a vendor
snapshot was the primary use case when developing this target:

Taking a vendor snapshot
------------------------

- Send a checkpoint message to the era target
- Make a note of the current era in its status line
- Take vendor snapshot (the era and snapshot should be forever
  associated now).

Rolling back to an vendor snapshot
----------------------------------

- Cache enters passthrough mode (see: dm-cache's docs in cache.txt)
- Rollback vendor storage
- Take metadata snapshot
- Ascertain which blocks have been written since the snapshot was taken
  by checking each block's era
- Invalidate those blocks in the caching software
- Cache returns to writeback/writethrough mode

Memory usage
============

The target uses a bitset to record writes in the current era.  It also
has a spare bitset ready for switching over to a new era.  Other than
that it uses a few 4k blocks for updating metadata.

   (4 * nr_blocks) bytes + buffers

Resilience
==========

Metadata is updated on disk before a write to a previously unwritten
block is performed.  As such dm-era should not be effected by a hard
crash such as power failure.

Userland tools
==============

Userland tools are found in the increasingly poorly named
thin-provisioning-tools project:

    https://github.com/jthornber/thin-provisioning-tools