dm: add era target

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

         A simple cache policy that writes back all data to the

         origin.  Used when decommissioning a dm-cache.

config DM_ERA

       tristate "Era target (EXPERIMENTAL)"

       depends on BLK_DEV_DM

       default n

       select DM_PERSISTENT_DATA

       select DM_BIO_PRISON

       ---help---

         dm-era tracks which parts of a block device are written to

         over time.  Useful for maintaining cache coherency when using

         vendor snapshots.

config DM_MIRROR

       tristate "Mirror target"

       depends on BLK_DEV_DM

dm-cache-y	+= dm-cache-target.o dm-cache-metadata.o dm-cache-policy.o

dm-cache-mq-y   += dm-cache-policy-mq.o

dm-cache-cleaner-y += dm-cache-policy-cleaner.o

dm-era-y	+= dm-era-target.o

md-mod-y	+= md.o bitmap.o

raid456-y	+= raid5.o

obj-$(CONFIG_DM_CACHE)		+= dm-cache.o

obj-$(CONFIG_DM_CACHE_MQ)	+= dm-cache-mq.o

obj-$(CONFIG_DM_CACHE_CLEANER)	+= dm-cache-cleaner.o

obj-$(CONFIG_DM_ERA)		+= dm-era.o

ifeq ($(CONFIG_DM_UEVENT),y)

dm-mod-objs			+= dm-uevent.o