109 lines
3.1 KiB
Plaintext
109 lines
3.1 KiB
Plaintext
|
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
|