jojochuang commented on code in PR #8496:
URL: https://github.com/apache/ozone/pull/8496#discussion_r2103598807
##########
hadoop-hdds/docs/content/feature/Snapshot.md:
##########
@@ -23,55 +23,260 @@ summary: Ozone Snapshot
limitations under the License.
-->
-## Introduction
+## **Introduction**
-Snapshot feature for Apache Ozone object store allows users to take
point-in-time consistent image of a given bucket. Snapshot feature enables
various use cases, including:
- * Backup and Restore: Create hourly, daily, weekly, monthly snapshots for
backup and recovery when needed.
- * Archival and Compliance: Take snapshots for compliance purpose and archive
them as required.
- * Replication and Disaster Recovery (DR): Snapshots provide frozen immutable
images of the bucket on the source Ozone cluster. Snapshots can be used for
replicating these immutable bucket images to remote DR sites.
- * Incremental Replication: DistCp with SnapshotDiff offers an efficient way
to incrementally sync up source and destination buckets.
+Snapshot feature for Apache Ozone object store allows users to take a
point-in-time consistent image of a given bucket. The snapshot is a read-only,
frozen image of the bucket’s state at the time of creation. Snapshot feature
enables various use cases, including:
-## Snapshot APIs
+* **Backup and Restore** – Create hourly, daily, weekly, monthly snapshots for
backup and recovery when needed.
+* **Archival and Compliance** – Take snapshots for compliance purposes and
archive them as required.
+* **Replication and Disaster Recovery (DR)** – Snapshots provide frozen,
immutable images of the bucket on the source Ozone cluster. These can be used
for replicating bucket images to remote DR sites.
+* **Incremental Replication** – DistCp with SnapshotDiff offers an efficient
way to incrementally sync up source and destination buckets.
-Snapshot feature is available through 'ozone fs' and 'ozone sh' CLI. This
feature can also be programmatically accessed from Ozone `ObjectStore` Java
client. The feature provides following functionalities:
-* Create Snapshot: Create an instantaneous snapshot for a given bucket
+## **Architecture**
+
+Ozone Snapshot architecture leverages the immutability of data blocks in
Ozone. Data blocks, once written, remain immutable for their lifetime and are
only reclaimed when the corresponding key metadata is removed from the
namespace. All Ozone metadata (volume, bucket, keys, directories) is stored in
the Ozone Manager (OM) metadata store (RocksDB). When a user takes a snapshot
of a bucket, the system internally creates a point-in-time copy of the bucket’s
namespace metadata on the OM. Since Ozone doesn’t allow in-place updates to
DataNode blocks, the integrity of data referenced by the snapshot is preserved.
The OM’s key deletion service is aware of snapshots: it will not permanently
delete any key as long as that key is still referenced by the active bucket or
any existing snapshot. When snapshots are deleted, a background
SnapshotDeletingService and garbage collector identify keys that are no longer
referenced by any snapshot or the live bucket, and reclaim those blocks.
+
+Ozone also provides a SnapshotDiff feature. When a user issues a SnapshotDiff
between two snapshots, the OM efficiently computes all the differences (added,
deleted, modified, or renamed keys) between the two snapshots and returns a
paginated list of changes. Snapshot diff results are cached to speed up
subsequent requests for the same snapshot pair.
+
+## **System Architecture Deep Dive**
+
+Internally, Ozone implements snapshots by **versioning the OM metadata for
each bucket** snapshot. The OM maintains a snapshot metadata table that records
the state of the bucket’s key directory tree at the moment of snapshot
creation. No data is physically copied at snapshot creation – the operation
simply marks a consistent snapshot of the OM’s RocksDB state (hence snapshots
are created instantaneously). Under the hood, Ozone relies on RocksDB’s
abilities (like snapshot and column family cloning) to preserve point-in-time
views of the metadata. Each snapshot is identified by a unique ID and name, and
each key entry in the OM DB carries information about which snapshots (if any)
it belongs to. This approach ensures that **common data is not duplicated**
across snapshots: if a key has not changed between two snapshots, both
snapshots reference the same underlying data blocks.
+
+When keys are modified or deleted in the active bucket, Ozone checks for
snapshots: if a snapshot exists that references the old version, the key’s data
blocks are retained until the snapshot is deleted. The reference counting in
metadata ensures that deleting a snapshot will mark any keys that were
exclusively held by that snapshot as reclaimable, triggering block cleanup in
the background.
+
+**SnapshotDiff Implementation:** Ozone computes snapshot diffs efficiently by
leveraging RocksDB key range comparisons and a directed acyclic graph (DAG) of
compaction history. For a configurable time window after snapshot creation (by
default 30 days), OM maintains a *compaction DAG* that allows computing diffs
in time proportional to the number of changed keys. If snapshots are relatively
recent, OM can determine differences by examining only the metadata changes
captured in the DAG (e.g., RocksDB SST files differences) rather than scanning
the entire key space. The parameter
`ozone.om.snapshot.compaction.dag.max.time.allowed` controls this window
(default 30 days). For older snapshots beyond this window (or if the efficient
diff data has been compacted away), OM falls back to a full metadata scan to
compute the diff. In the worst case, the cost of diff is proportional to
iterating over all keys in the bucket (if nearly the entire namespace changed
or if using full scan). The Snap
shotDiff results (a list of keys with indicators for created (`+`), deleted
(`-`), modified (`M`), or renamed (`R`)) are stored in a temporary on-disk
cache so that subsequent requests for the same diff can be served quickly
without re-computation.
+
+**Snapshot Data Storage:** Snapshot metadata resides on the OM in the same
RocksDB as the live metadata, but separated by snapshot-specific prefixes or
tables. The OM persists snapshots such that each snapshot’s metadata can be
treated as a read-only view. Additionally, OM stores snapshot-related info such
as the mapping of snapshot names to snapshot IDs and the list of snapshot diff
jobs. By default, temporary data for snapshot diff computations is stored under
the OM metadata directory, but this location can be configured via
`ozone.om.snapshot.diff.db.dir` (a dedicated directory for snapshot diff
scratch space).
+
+For a more in-depth discussion of the snapshot design and its evolution, refer
to Prashant Pogde’s introduction of Apache Ozone snapshots (the first in a
series of blog posts). This Medium post covers the motivation and high-level
design of Ozone snapshots, and subsequent posts delve further into the
technical implementation.
+
+## **User Tutorial**
+
+In this section, we demonstrate how to create and use Ozone snapshots via
command-line and programmatically.
+
+### **Using Snapshots via CLI**
+
+The Ozone shell provides convenient commands to manage snapshots. Snapshots
can be created and manipulated either through the **`ozone sh`** subcommands or
the **`ozone fs`** Hadoop-compatible filesystem commands:
+
+* **Creating a Snapshot:** As shown earlier, use `ozone sh snapshot create
<bucket> [snapshotName]` to create a snapshot. For example, to snapshot a
bucket named `bucket1` in volume `vol1` with an optional name:
```shell
-ozone sh snapshot create [-hV] <bucket> [<snapshotName>]
+ozone sh snapshot create /vol1/bucket1 finance_backup_2025
```
-* List Snapshots: List all snapshots of a given bucket
+ This captures an instantaneous image of all keys under `/vol1/bucket1`. The
operation requires you to be the bucket owner or volume owner (admin
privilege). If no snapshot name is provided, Ozone will auto-generate a name
(often using a timestamp).
+ Alternatively, you can create a snapshot using the Hadoop FS interface:
```shell
-ozone sh snapshot list [-hV] <bucket>
+ozone fs -createSnapshot ofs://om-service/vol1/bucket1 finance_backup_2025
```
-* Delete snapshot: Delete a given snapshot for a given bucket
+ (Here, `ofs://om-service/vol1/bucket1` is the URI of the bucket in the Ozone
FileSystem; see the *Ozone File System API* reference.)
+* **Listing Snapshots:** To list all snapshots of a bucket, use:
```shell
-ozone sh snapshot delete [-hV] <bucket> <snapshotName>
+ozone sh snapshot list /vol1/bucket1
```
-* Snapshot Diff: Given two snapshots, list all the keys that are different
between the them.
+ This will display all snapshot names for that bucket and their creation
times. You can achieve the same via the filesystem interface by listing the
hidden `.snapshot` directory:
```shell
-ozone sh snapshot diff [-chV] [-p=<pageSize>] [-t=<continuation-token>]
<bucket> <fromSnapshot> <toSnapshot>
+ozone fs -ls /vol1/bucket1/.snapshot
```
-SnapshotDiff CLI/API is asynchronous. The first time the API is invoked, OM
starts a background thread to calculate the SnapshotDiff, and returns "Retry"
with suggested duration for the retry operation. Once the SnapshotDiff is
computed, this API returns the diffs in multiple Pages. Within each Diff
response, OM also returns a continuation token for the client to continue from
the last batch of Diff results. This API is safe to be called multiple times
for a given snapshot source and destination pair. Internally each Ozone Manager
computes Snapdiff only once and stores it for future invocations of the same
Snapshot Diff API.
+ As soon as a snapshot is created, it appears under the bucket’s `.snapshot`
directory. (Note: The `.snapshot` path is a reserved, read-only namespace that
exposes snapshots; you cannot create or modify files inside `.snapshot`.)
+* **Reading from a Snapshot:** A snapshot presents a read-only view of the
bucket’s data at the snapshot time. You can list keys within a snapshot or read
a specific key’s content:
+ *List keys in snapshot:*
+```shell
+ozone sh key list /vol1/bucket1/.snapshot/finance_backup_2025
+```
+ or equivalently
+```shell
+ozone fs -ls /vol1/bucket1/.snapshot/finance_backup_2025
-* List SnapshotDiff Jobs: List all snapshotDiff jobs of a given bucket
+```
+ This will list all keys and directories that were in the bucket at the
moment of that snapshot. To read a file from the snapshot, for example:
```shell
-ozone sh snapshot listDiff [-ahV] [-s=<jobStatus>] <bucket>
+ozone sh key get /vol1/bucket1/.snapshot/finance_backup_2025/reports/Q1.csv
./Q1_snapshot.csv
```
+ which downloads the key as it existed at snapshot time. (Using `ozone fs`,
one could likewise use `-get` on the `.snapshot/<snapshotName>/<key>` path.)
**Note:** Accessing data in a snapshot requires read privileges on the bucket,
just as reading the current data does.
-* Snapshot Info: Returns information about an existing snapshot
+**Snapshot Diff operations:** Snapshot diff allows you to see what changed
between two snapshots (or between a snapshot and the live bucket). For example:
```shell
-ozone sh snapshot info [-hV] <bucket> <snapshotName>
+ozone sh snapshot diff /vol1/bucket1 finance_backup_2025 finance_backup_2026
+```
+This will initiate or retrieve a diff report between the two snapshots
`finance_backup_2025` and `finance_backup_2026`. The output will list keys with
a prefix indicating addition (`+`), deletion (`-`), modification (`M`), or
rename (`R`). For instance, you might see:
+```shell
++ Q2.csv
+- temp/dataset_old.csv
+M reports/summary.txt
+R projectX -> projectY
+```
+ indicating a new file `Q2.csv` added, an old file deleted, a file modified,
and a directory renamed from `projectX` to `projectY`. SnapshotDiff results may
span multiple pages; use the `-p` and `-t` options for pagination if the result
is large. While a SnapshotDiff job is running, you can list active jobs with
`ozone sh snapshot listDiff /vol1/bucket1` and even cancel one with `ozone sh
snapshot cancelDiff <jobId>` (if needed for administrative reasons).
+* **Renaming a Snapshot:** To rename an existing snapshot (e.g., give it a
more meaningful name):
+```shell
+ozone sh snapshot rename /vol1/bucket1 finance_backup_2025 finance_backup_Q1
+```
+ This requires the bucket owner or admin privilege, since it changes snapshot
metadata. After renaming, the snapshot will appear under the new name in
`.snapshot` (e.g., `.snapshot/finance_backup_Q1`). Any references to the old
snapshot name (such as for diff or info) should use the new name going forward.
+* **Snapshot Info:** You can fetch detailed metadata about a snapshot:
+```shell
+ozone sh snapshot info /vol1/bucket1 finance_backup_Q1
+```
+ The output will include information like the snapshot ID, creation time, the
status (whether the snapshot is active or scheduled for deletion), and space
usage metrics such as **Reference Size** and **Exclusive Size**. *Reference
size* is the total size of data that the snapshot can see (i.e., all keys in
the snapshot, counting shared data), whereas *Exclusive size* is the size of
data unique to that snapshot (data that would be freed if this snapshot were
deleted).
+
+All the above CLI operations correspond to underlying RPC calls to Ozone
Manager. They enforce the same security checks described in the
**Authorization** section.
+
+### **Programmatic Access via Java**
+
+In addition to the CLI, Ozone snapshots can be managed and accessed using Java
APIs:
+
+**Using the Hadoop Compatible FileSystem (HCFS) Interface:** The Ozone
FileSystem (OFS) API presents Ozone storage as a Hadoop `FileSystem`. For
example:
+
+```java
+Configuration conf = new OzoneConfiguration();
+FileSystem fs = FileSystem.get(new
Path("ofs://om-service/vol1/bucket1").toUri(), conf);
+
+Path bucketPath = new Path("/vol1/bucket1");
+String snapshotName = "finance_backup_2025";
+
+// Create a snapshot
+fs.createSnapshot(bucketPath, snapshotName);
+
+// List snapshots (by listing .snapshot directory)
+FileStatus[] snapshots = fs.listStatus(new Path(bucketPath, ".snapshot"));
+for (FileStatus snap : snapshots) {
+System.out.println("Snapshot: " + snap.getPath().getName());
+}
+
+// Read from snapshot (e.g., list files inside the snapshot)
+Path snapshotPath = new Path(bucketPath, ".snapshot/" + snapshotName);
+FileStatus[] files = fs.listStatus(snapshotPath);
+for (FileStatus file : files) {
+System.out.println("Snapshot content: " + file.getPath().toString());
+}
+
+// Rename snapshot (rename directory under .snapshot)
+Path oldSnap = new Path(bucketPath, ".snapshot/" + snapshotName);
+Path newSnap = new Path(bucketPath, ".snapshot/finance_backup_Q1");
+fs.rename(oldSnap, newSnap);
+
+// Delete snapshot
+fs.deleteSnapshot(bucketPath, "finance_backup_Q1");
```
-## Architecture
+The above code demonstrates the Hadoop Compatible File System Snapshot APIs.
Under the hood, these calls translate to the same OM actions as the CLI. The
Java API will throw exceptions if the user lacks privileges or if the snapshot
does not exist, etc. Always wrap these calls in try-catch and handle
`OMException` or `IOException` as appropriate.
+
+Snapshots are visible in the File System interface under the special
`.snapshot` directory of each bucket. For example, an application can list and
read snapshot contents using standard Hadoop file system calls:
+
+```java
+OzoneConfiguration conf = new OzoneConfiguration();
+FileSystem fs = FileSystem.get(new URI("ofs://ozone1.vol1.bucket1"), conf);
+
+// List all snapshots in the bucket by listing the .snapshot directory
+FileStatus[] snapshots = fs.listStatus(new Path("/vol1/bucket1/.snapshot"));
+for (FileStatus snap : snapshots) {
+ System.out.println("Snapshot name: " + snap.getPath().getName());
+}
+
+// Read a file from a snapshot
+FSDataInputStream in = fs.open(new
Path("/vol1/bucket1/.snapshot/finance_backup_Q1/reports/Q1.csv"));
+// ... (read from 'in' as usual, then close it)
+in.close();
+
+```
+
+In the code above, the `ofs://ozone1.vol1.bucket1` URI is using the **Ozone
File System (OFS)** scheme. The `.snapshot/finance_backup_Q1` path behaves like
a read-only directory. Standard file system operations (e.g., `open`,
`listStatus`) can be used on snapshot paths. For reference, see the [Ozone File
System API guide](https://ozone.apache.org/docs/edge/interface/ofs.html) for
details on the `ofs://` scheme.
+
+**Using the Ozone ObjectStore Client API:** Ozone also provides a native RPC
client interface. Applications can use `OzoneClient` and the `ObjectStore` API
to programmatically create, list, and delete snapshots:
+
+```Java
+// Initialize Ozone RPC client
+OzoneClient ozClient = OzoneClientFactory.getRpcClient(conf);
+ObjectStore store = ozClient.getObjectStore();
+
+// Create a snapshot
+store.createSnapshot("vol1", "bucket1", "finance_backup_2025");
+
+// List snapshots
+Iterator<OzoneSnapshot> snapshotIter = store.listSnapshot("vol1", "bucket1",
null, null);
+while (snapshotIter.hasNext()) {
+ OzoneSnapshot snap = snapshotIter.next();
+ System.out.printf("Snapshot %s (ID=%s) created at %s%n",
+ snap.getName(), snap.getSnapshotId(), snap.getCreationTime());
+}
+
+// Get snapshot info
+OzoneSnapshot snapInfo = store.getSnapshotInfo("vol1", "bucket1",
"finance_backup_2025");
+long refBytes = snapInfo.getReferencedSize();
+long exclBytes = snapInfo.getExclusiveSize();
+System.out.println("Snapshot exclusive size = " + exclBytes + " bytes,
referenced size = " + refBytes + " bytes");
+
+// Rename snapshot
+store.renameSnapshot("vol1", "bucket1", "finance_backup_2025",
"finance_backup_Q1");
+
+// Delete snapshot
+store.deleteSnapshot("vol1", "bucket1", "finance_backup_Q1");
+```
+
+The above code snippet demonstrates the Ozone ObjectStore Client Java API for
snapshot operations. Again under the hood, these calls translate to the same OM
actions as the CLI. The Java API will throw exceptions if the user lacks
privileges or if the snapshot does not exist, etc. Always wrap these calls in
try-catch and handle `OMException` or `IOException` as appropriate.
+
+**HTTP REST API Access:** For applications that cannot use the Java API
directly, Ozone offers an HttpFS Gateway which exposes a WebHDFS-compatible
REST API. Through HttpFS, you can perform filesystem operations (including
reading from `.snapshot` paths) over HTTP. This can be useful for remote access
to snapshot data. Snapshot management operations like create/delete/rename via
HttpFS are supported, but getSnapshotDiff is not yet supported.
+
+## **System Administration How-To**
+
+This section summarizes the configuration parameters and administrative
considerations for Ozone snapshots. Administrators can tune these settings in
**ozone-site.xml** to control snapshot behavior and resource usage.
+
+**Snapshot-Related Configuration Parameters:**
+
+| Configuration Key | Default Value | Description |
+| ----- | ----- | ----- |
+| **ozone.om.fs.snapshot.max.limit** | **10000** | Maximum number of snapshots
that Ozone Manager (OM) will allow per bucket (or overall). This is a safety
limit on how many snapshots can be created. Earlier versions defaulted to 1000,
but it has been increased to 10,000 in Ozone 2.1.0. |
+| **ozone.om.snapshot.compaction.dag.max.time.allowed** | **2592000000 ms**
(30 days) | Time window for which OM retains compaction DAG information to
enable efficient SnapshotDiff calculations. Snapshots older than this age will
still support diff, but the diff will fall back to a full metadata scan
(slower). Administrators can increase this if longer retention of fast-diff
data is needed (at the cost of more metadata storage). |
+| **ozone.om.snapshot.diff.db.dir** | *(empty)* | Directory on the OM node for
storing SnapshotDiff job data and scratch files. By default, if not set, it
uses the same location as OM metadata (`ozone.metadata.dirs`). An admin may
point this to a location with ample space, especially if large diffs are
expected. |
+| **ozone.om.snapshot.rocksdb.metrics.enabled** | **false** | Controls whether
to collect detailed RocksDB metrics for the snapshot metadata store. By default
this is disabled to avoid overhead. Enable (`true`) only if you need detailed
RocksDB performance stats for snapshots (for debugging or monitoring). |
+| **ozone.om.snapshot.load.native.lib** | **true** | When true, Ozone will use
the native RocksDB library for certain snapshot operations (like loading native
RocksDB checkpoints for OM). In some environments, this may cause issues.
Setting this to false forces a pure-Java path for loading snapshot data, which
can be used as a workaround if you encounter RocksDB native library problems. |
+| **ozone.om.snapshot.diff.concurrent.max** | **10** | The maximum number of
SnapshotDiff jobs that can run concurrently on one OM node (default is 10). If
users request more diffs at once, additional requests will be queued or
rejected until running jobs complete. An administrator can increase this limit
to allow more parallel diff computations if OM has sufficient memory and CPU.
(The config property name for this setting may be
`ozone.om.snapshot.diff.thread.pool.size` in some versions.) |
+
+In addition to the above, snapshot operations will consume memory proportional
to the number of active snapshots and ongoing diff jobs. Administrators should
monitor OM heap usage if many snapshots are kept or many large diffs are run
simultaneously. It is also advisable to enable **Ozone Native ACLs or Ranger**
for fine-grained access control on snapshot operations (see **Authorization**
below).
+
+**Monitoring Snapshots:** Ozone exposes metrics and JMX information for
snapshot operations. For example, there are OM metrics for the number of
snapshots, SnapshotDiff operation counts, etc., and these can be observed via
the OM’s Prometheus metrics or RPC metrics. Administrators should also pay
attention to the OM logs for snapshot-related messages – e.g., when a
SnapshotDiff job starts or finishes, or if a snapshot delete triggers a large
number of keys to be purged, etc.
+
+## **Authorization**
+
+Snapshot operations in Ozone are subject to access control checks. Not all
users can create or delete snapshots by default, to prevent abuse or unintended
usage. Below are the requirements and ACLs for each operation:
+
+* **Create Snapshot / Delete Snapshot / Rename Snapshot:** These operations
**require admin or owner privileges** on the bucket. For example, only the
bucket’s owner (or a cluster admin) can create a snapshot on that bucket.
Similarly, deleting or renaming a snapshot is restricted to the bucket owner or
admin. If a non-owner user attempts these operations, OM will reject it with an
**Access Denied** error.
+* **List Snapshots / Get Snapshot Info:** These read-only operations perform
ACL checks to ensure the user has **read/list access** on the bucket. In
general, a user who has permission to list the bucket’s contents can also list
its snapshots. The Ozone native ACL type that governs listing child objects
(like snapshots) is “l” (list). In practice, having *READ* access on the bucket
implicitly grants the ability to see snapshot listings and info. The
documentation often phrases this as requiring *read privilege on the bucket*.
+* **SnapshotDiff / Cancel SnapshotDiff / List SnapshotDiff Jobs:** These
operations also perform ACL checks on the bucket. Since a snapshot diff reveals
information about keys (names and whether they changed), the user must have
**read access** on the bucket to run a diff or view diff jobs. In Ozone’s
native ACLs, that means the user needs either the “l” (list) permission to list
the keys or “r” (read) to read the key metadata. In practice, if a user can
read the bucket’s data, they can compare its snapshots. The
`listSnapshotDiffJobs` and `cancelSnapshotDiff` administrative calls typically
require the same level of access (or owner/admin privileges if attempting to
cancel someone else’s job). Generally, a non-admin user will only see and
manage their own snapshot diff jobs for buckets they have access to.
+
+It’s worth noting that Ozone supports two kinds of authorization models for
snapshots: **Ozone native ACLs** and optionally **Ranger policies** (if Ranger
is integrated). The above describes native ACL behavior. If using Ranger or a
similar authorizer, ensure that appropriate permissions are granted for the
`snapshot` operations (Ranger defines separate permissions for volume, bucket,
and key actions, and snapshot create/delete would fall under bucket admin
operations).
+
+## **Comparison to HDFS Snapshots**
+
+Apache Ozone’s snapshot feature is conceptually similar to HDFS snapshots, but
there are important differences:
+
+* **Granularity:** Ozone snapshots are **bucket-level only**. You can snapshot
an entire bucket at once (which may contain a hierarchy of directories and
keys), but you cannot snapshot a sub-directory of a bucket in isolation. By
contrast, HDFS supports snapshots at any directory level (you must designate a
directory as *snapshottable* and then you can snapshot that directory and its
subtree). In Ozone, the bucket is the smallest unit of snapshot; if you need
snapshot-like behavior on a subset of data, you would need to organize that
data into its own bucket.
+* **Metadata vs. Data Changes:** Ozone snapshots primarily track **key-level
changes** (file creations, deletions, modifications, and renames) within the
bucket. They do not capture changes to bucket metadata or properties. For
example, if you change a bucket’s quota or ACLs, those changes are not
versioned via snapshots – a snapshot only reflects the state of the filesystem
namespace (keys and directories). HDFS snapshots are similar in that they track
file/directory changes and not higher-level constructs (like no snapshot of an
HDFS mount point’s permission outside the snapshotted dir).
+* **Access and Restore:** Reading from an Ozone snapshot is done through the
special `.snapshot` path in the bucket, analogous to HDFS where snapshots
appear in a `.snapshot` directory under the snapshottable directory. Restoring
data from a snapshot in Ozone is a manual process of copying data out of the
`.snapshot` directory back into the active area if needed (or using DistCp for
large-scale copy). There is no automatic “rollback” of a bucket to a snapshot,
since Ozone treats snapshots as read-only backups (similar approach in HDFS –
HDFS snapshots can be used to restore files but not instant revert of the live
state without manual intervention).
+* **Implementation:** Internally, Ozone snapshots leverage the OM’s key-value
store and share data blocks between snapshots, which is analogous to HDFS’s
copy-on-write approach for metadata. One notable difference is that Ozone being
object-store, does not have an equivalent of HDFS block tracking for snapshots
at the DataNode level – all the intelligence is in the OM. HDFS NameNode also
handles snapshot metadata, but Ozone’s design (with its RocksDB metadata store)
means snapshot creation is O(1) and implemented via metadata pointers rather
than duplication. Both systems do not copy actual file data on snapshot – they
rely on not deleting underlying blocks.
+
+In summary, Ozone snapshots give similar benefits for Ozone buckets as HDFS
snapshots do for directories, but Ozone’s approach is bucket-scoped and
oriented toward object store semantics. If coming from HDFS, just note that you
cannot snapshot volumes or the entire namespace at once (you snapshot each
bucket individually), and you cannot snapshot sub-folders of a bucket.
+
+## **Known Issues and Limitations**
+
+While Apache Ozone’s snapshot feature is powerful, there are a few current
limitations and caveats to be aware of:
+
+* **S3 Interface Support:** Snapshot operations are *not* supported via the S3
API (`s3://` or `s3a://` access). Ozone’s S3 Gateway does not expose snapshot
functionality, since Amazon S3 has no direct concept of snapshots. This means
you cannot create or list snapshots through the S3 Gateway or the `s3a` Hadoop
FileSystem connector. Snapshots must be managed via the Ozone RPC interface
(ozone shell, ozone fs, or Ozone Client API). However, snapshotted data can be
read via the S3 Gateway using the special `.snapshot` key prefix. For example,
a key `key1` in the snapshot `snap1` can be read using the path
`.snapshot/snap1/key1`.
Review Comment:
Sure but S3 API does not have the notion of snapshots
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