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design: adjust description of snapshot diffs

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Signed-off-by: Stephen J Day <stephen.day@docker.com>
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Stephen J Day 2016-11-29 11:47:34 -08:00
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design/snapshots.md
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@ -48,14 +48,15 @@ The _Snapshot Manager_ provides an API for allocating, snapshotting and mounting
abstract, layer-based filesytems. The model works by building up sets of abstract, layer-based filesytems. The model works by building up sets of
directories with parent-child relationships, known as _Snapshots_. directories with parent-child relationships, known as _Snapshots_.
Creating _snapshots_ is a transactional operation. Each _Snapshot_ may have a Every snapshot is represented by an opaque `diff` directory, which acts as a
parent snapshot. When one starts a transaction on an existing snapshot, the handle to the snapshot. It may contain driver specifc data, including changeset
result may only be used as a parent _after_ being committed. data, parent information and arbitrary metadata.
Every snapshot has an associated `diff` directory, which contains driver The `diff` directory for a _snapshot_ is created with a transactional
specific data. This may include parent information and changeset data, operation. Each _snapshot_ may have one parent snapshot. When one starts a
depending on the implementation. We define the empty string as the ancestor of transaction on an existing snapshot, the result may only be used as a parent
all snapshots, which corresponds to the empty snapshot. _after_ being committed. The empty string `diff` directory is a handle to the
empty snapshot, which is the ancestor of all snapshots.
The `target` directory represents the active snapshot location. The driver may The `target` directory represents the active snapshot location. The driver may
maintain internal metadata associated with the `target` but the contents is maintain internal metadata associated with the `target` but the contents is
@ -95,6 +96,16 @@ As snapshots are imported into the container system, a "graph" of snapshots and
their parents will form. Queries over this graph must be a supported operation. their parents will form. Queries over this graph must be a supported operation.
Subsequently, each snapshot ends up representing Subsequently, each snapshot ends up representing
### Path Management
No path layout for snapshot locations is imposed on the caller. The paths used
by the snapshot drivers are largely under control of the caller. This provides
the most flexibility in using the snapshot system but requires discipline when
deciding which paths to use and which ones to avoid.
We may provide a helper component to manage `diff` path layout when working
with OCI and docker images.
## How snapshots work ## How snapshots work
To bring the terminology of _snapshots_, we are going to demonstrate the use of To bring the terminology of _snapshots_, we are going to demonstrate the use of