snapshot操作在硬盘上形式：
/hbase/.snapshots
/.tmp <---- working directory
/[snapshot name] <----- completed snapshot
当snapshot完成时的形式展示：
/hbase/.snapshots/[snapshot name]
.snapshotinfo <--- Description of the snapshot
.tableinfo <--- Copy of the tableinfo
/.logs
/[server_name]
/... [log files]
...
/[region name] <---- All the region's information
.regioninfo <---- Copy of the HRegionInfo
/[column family name]
/[hfile name] <--- name of the hfile in the real region
...
...
snapshot基本步骤：
1.执行前会枷锁操作，不允许删除添加操作；
2.在hdfs在创建指定目录，写入相关的信息进去；
3.刷新memstore中的数据到hfile，
4.为hfile文件创建引用指针.
以下是大体的代码流程。
hbaseAdmin执行发起的snapshot：
public void snapshot(final String snapshotName, final TableName tableName, SnapshotDescription.Type type) throws IOException, SnapshotCreationException, IllegalArgumentException {
SnapshotDescription.Builder builder = SnapshotDescription.newBuilder();
builder.setTable(tableName.getNameAsString());
builder.setName(snapshotName);
builder.setType(type);
snapshot(builder.build());
}
执行快照并等待服务器完成该快照（阻止）。HBase实例一次只能有一个快照，或者结果可能是未定义（你可以告诉多个HBase集群同时快照，但只有一个在单个群集同时）。
public void snapshot(SnapshotDescription snapshot) throws IOException, SnapshotCreationException, IllegalArgumentException {
// actually take the snapshot
SnapshotResponse response = takeSnapshotAsync(snapshot);
MasterRpcService:异步触发并完成一次snapshot：
`master.snapshotManager.takeSnapshot(snapshot);`
SnapshotManager类：完成一次snapshot需要根据表的状态：disabled或者enabled
if (assignmentMgr.getTableStateManager().isTableState(snapshotTable, ZooKeeperProtos.Table.State.ENABLED)) {
LOG.debug("Table enabled, starting distributed snapshot.");
snapshotEnabledTable(snapshot);
LOG.debug("Started snapshot: " + ClientSnapshotDescriptionUtils.toString(snapshot));
}
// For disabled table, snapshot is created by the master
else if (assignmentMgr.getTableStateManager().isTableState(snapshotTable, ZooKeeperProtos.Table.State.DISABLED)) {
LOG.debug("Table is disabled, running snapshot entirely on master.");
snapshotDisabledTable(snapshot);
LOG.debug("Started snapshot: " + ClientSnapshotDescriptionUtils.toString(snapshot));
}
private synchronized void snapshotEnabledTable(SnapshotDescription snapshot) throws HBaseSnapshotException {
// setup the snapshot
prepareToTakeSnapshot(snapshot);
// Take the snapshot of the enabled table
EnabledTableSnapshotHandler handler = new EnabledTableSnapshotHandler(snapshot, master, this);
snapshotTable(snapshot, handler);
}
enabled状态下执行表的snapshot：
// setup the snapshot
准备工作
prepareToTakeSnapshot(snapshot);
// Take the snapshot of the enabled table
EnabledTableSnapshotHandler handler = new EnabledTableSnapshotHandler(snapshot, master, this);
开始执行snapshot
snapshotTable(snapshot, handler);
}
snapshot开始之前的设置准备：检查是否有一个在运行的snapshot工作以及还原snapshot工作的请求存在。#
// make sure we aren't already running a snapshot
if (isTakingSnapshot(snapshot)) {
SnapshotSentinel handler = this.snapshotHandlers.get(snapshotTable);
throw new SnapshotCreationException("Rejected taking " + ClientSnapshotDescriptionUtils.toString(snapshot) + " because we are already running another snapshot " + (handler != null ? ("on the same table " + ClientSnapshotDescriptionUtils.toString(handler.getSnapshot())) : "with the same name"), snapshot);
}
// make sure we aren't running a restore on the same table
if (isRestoringTable(snapshotTable)) {
SnapshotSentinel handler = restoreHandlers.get(snapshotTable);
throw new SnapshotCreationException("Rejected taking " + ClientSnapshotDescriptionUtils.toString(snapshot) + " because we are already have a restore in progress on the same snapshot " + ClientSnapshotDescriptionUtils.toString(handler.getSnapshot()), snapshot);
}
try {
// delete the working directory, since we aren't running the snapshot. Likely leftovers
// from a failed attempt.
fs.delete(workingDir, true);
// recreate the working directory for the snapshot
if (!fs.mkdirs(workingDir)) {
throw new SnapshotCreationException("Couldn't create working directory (" + workingDir + ") for snapshot", snapshot);
}
设置准备工作完成就开始进行snapshot用指定的handler进行snapshot工作：
handler.prepare();
this.executorService.submit(handler);
this.snapshotHandlers.put(TableName.valueOf(snapshot.getTable()), handler);
...
TakeSnapshotHandler真正开始处理snapshot操作：
1.将snapshot描述信息写入.snapshotinfo目录
FsPermission perms = FSUtils.getFilePermissions(fs, fs.getConf(), HConstants.DATA_FILE_UMASK_KEY);
Path snapshotInfo = new Path(workingDir, SnapshotDescriptionUtils.SNAPSHOTINFO_FILE);
try {
FSDataOutputStream out = FSUtils.create(fs, snapshotInfo, perms, true);
try {
snapshot.writeTo(out);
} finally {
out.close();
}
}
2.复制表的信息：
snapshotManifest.addTableDescriptor(this.htd);
3.获取hregionserver上的regions以及位置信息 ##：
List<Pair<HRegionInfo, ServerName>> regionsAndLocations;
if (TableName.META_TABLE_NAME.equals(snapshotTable)) {
regionsAndLocations = new MetaTableLocator().getMetaRegionsAndLocations(server.getZooKeeper());
} else {
regionsAndLocations = MetaTableAccessor.getTableRegionsAndLocations(server.getZooKeeper(), server.getConnection(), snapshotTable, false);
}
4.开始执行snapshot操作，上面获取到的region信息及位置信息
// run the snapshot
snapshotRegions(regionsAndLocations);
启动snapshot程序：：：
在regionserver上开始snapshot // start the snapshot on the RS所有的snapshot操作的具体细节
Procedure proc = coordinator.startProcedure(this.monitor, this.snapshot.getName(), this.snapshot.toByteArray(),
Lists.newArrayList(regionServers));
if (proc == null) {
String msg = "Failed to submit distributed procedure for snapshot '" + snapshot.getName() + "'";
LOG.error(msg);
throw new HBaseSnapshotException(msg);
}
等待snapshot完成：
proc.waitForCompleted();
将下线的region作为disabled处理
// Take the offline regions as disabled
for (Pair<HRegionInfo, ServerName> region : regions) {
HRegionInfo regionInfo = region.getFirst();
if (regionInfo.isOffline() && (regionInfo.isSplit() || regionInfo.isSplitParent())) {
LOG.info("Take disabled snapshot of offline region=" + regionInfo);
snapshotDisabledRegion(regionInfo);
}
}
5.相关region信息以及servername，用来验证snapshot的有效性
// extract each pair to separate lists
Set<String> serverNames = new HashSet<String>();
for (Pair<HRegionInfo, ServerName> p : regionsAndLocations) {
if (p != null && p.getFirst() != null && p.getSecond() != null) {
HRegionInfo hri = p.getFirst();
if (hri.isOffline() && (hri.isSplit() || hri.isSplitParent()))
continue;
serverNames.add(p.getSecond().toString());
}
}
6.刷新内存状态，写snapshot-mnifest信息到目录
// flush the in-memory state, and write the single manifest
status.setStatus("Consolidate snapshot: " + snapshot.getName());
snapshotManifest.consolidate();
7.开始验证snapshot的有效性
// verify the snapshot is valid
status.setStatus("Verifying snapshot: " + snapshot.getName());
verifier.verifySnapshot(this.workingDir, serverNames);
8.完成snapshot，转移目录等
// complete the snapshot, atomically moving from tmp to .snapshot dir.
completeSnapshot(this.snapshotDir, this.workingDir, this.fs);
msg = "Snapshot " + snapshot.getName() + " of table " + snapshotTable + " completed";
status.markComplete(msg);
LOG.info(msg);
metricsSnapshot.addSnapshot(status.getCompletionTimestamp() - status.getStartTime());
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