ZanderXu created HDFS-17479:
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Summary: [FGL] Snapshot related operations still use global lock
Key: HDFS-17479
URL: https://issues.apache.org/jira/browse/HDFS-17479
Project: Hadoop HDFS
Issue Type: Sub-task
Reporter: ZanderXu
Snapshot feature is a very useful feature in certain scenarios. As far as I
know, very few companies use this feature on the prod environment. The
implementation is complex and it is difficult to support FGL with only a minor
modifications.
So we can still use the Global lock to make snapshot-related operations
thread-safe.
Snapshot has some access modules, let's analyze them and find a way to still
use GlobalLock.
!image-2024-04-18-10-31-34-458.png|width=288,height=219!
The above picture shows a simple case, we can access the iNode foo through the
following paths:
# /abc/foo
# /abc/.snapshot/s1/foo
If we want to delete the iNode foo, we need to lock /abc and /abc/.snapshot/s1
(DirectoryWithSnapshotFeature on iNode abc).
If we want to change permission of the iNode foo, we need to lock /abc/foo and
/abc/.snapshot/s1/foo (DirectoryWithSnapshotFeature on the iNode foo)
For this case, we can directly acquire the global lock when resolving the IIPs
for the input path if there is an iNode that has DirectorySnapshottableFeature.
!image-2024-04-18-10-48-08-773.png|width=368,height=383!
After /abc/foo is renamed to /xyz/bar, the access modules will be changed, as
the above picture shows. We can access this bar through the following path:
# /abc/.snapshot/s1/bar
# /xyz/bar
For /abc/.snapshot/s1/bar, since the iNode abc has
DirectorySnapshottableFeature, so we can identify it and acquire the global
lock.
For /xyz/bar, we can identify it through Reference flag, since the iNode bar is
a DstReference Node.
So we can use DirectorySnapshottableFeature and Reference to determine if we
need to acquire the Global lock when resolving the IIPs for input path.
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