Commit Graph

4 Commits

Author SHA1 Message Date
Cheng Chang
1a24f4d1d6 Track WAL in MANIFEST: add method to check WAL consistency (#7236)
Summary:
Add a method `CheckWals` in `WalSet` to check the logs on disk. See `CheckWals`'s comments.
This method will be used to check consistency of WALs during DB recovery.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7236

Test Plan: a set of tests are added to wal_edit_test.cc.

Reviewed By: riversand963

Differential Revision: D23036505

Pulled By: cheng-chang

fbshipit-source-id: 5b1d6857ac173429b00f950c32c4a5b8d063a732
2020-09-25 13:25:54 -07:00
Cheng Chang
00ee89b584 Track WAL in MANIFEST: update WalMetadata for WAL syncing (#7414)
Summary:
There are some tricky behaviors related to WAL sync:

- When creating a WAL, the WAL might not be synced, if the WAL directory is not synced, the WAL file's metadata may not even be synced to disk, so during recovery, when listing the WAL directory, the WAL may not even show up.
- During each DB::Write, the WriteOption can control whether the WAL should be synced, so a WAL previously not synced on creation can be synced during Write.

For each `SyncWAL`, we'll track the synced status and the current WAL size. Previously, we only track the WAL size on closing.
During recovery, we check that the on-disk WAL size is >= the last synced size.

So this PR introduces `synced_size` and `closed` to `WalMetadata` for the above design update.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7414

Test Plan:
- updated wal_edit_test
- updated version_edit_test

Reviewed By: riversand963

Differential Revision: D23796127

Pulled By: cheng-chang

fbshipit-source-id: 5498ab80f537c48a10157e71a4745716aef5cf30
2020-09-22 14:35:14 -07:00
Cheng Chang
ce4192375d Track WAL in MANIFEST: minor udpates (#7282)
Summary:
The updates resolve comments left from https://github.com/facebook/rocksdb/pull/7164.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7282

Test Plan: wal_edit_test

Reviewed By: ltamasi

Differential Revision: D23196824

Pulled By: cheng-chang

fbshipit-source-id: 797f3fef27fc72114c2be777d9eadd3429da5301
2020-08-20 15:12:00 -07:00
Cheng Chang
cd48ecaa1a Define WAL related classes to be used in VersionEdit and VersionSet (#7164)
Summary:
`WalAddition`, `WalDeletion` are defined in `wal_version.h` and used in `VersionEdit`.
`WalAddition` is used to represent events of creating a new WAL (no size, just log number), or closing a WAL (with size).
`WalDeletion` is used to represent events of deleting or archiving a WAL, it means the WAL is no longer alive (won't be replayed during recovery).

`WalSet` is the set of alive WALs kept in `VersionSet`.

1. Why use `WalDeletion` instead of relying on `MinLogNumber` to identify outdated WALs

On recovery, we can compute `MinLogNumber()` based on the log numbers kept in MANIFEST, any log with number < MinLogNumber can be ignored. So it seems that we don't need to persist `WalDeletion` to MANIFEST, since we can ignore the WALs based on MinLogNumber.

But the `MinLogNumber()` is actually a lower bound, it does not exactly mean that logs starting from MinLogNumber must exist. This is because in a corner case, when a column family is empty and never flushed, its log number is set to the largest log number, but not persisted in MANIFEST. So let's say there are 2 column families, when creating the DB, the first WAL has log number 1, so it's persisted to MANIFEST for both column families. Then CF 0 is empty and never flushed, CF 1 is updated and flushed, so a new WAL with log number 2 is created and persisted to MANIFEST for CF 1. But CF 0's log number in MANIFEST is still 1. So on recovery, MinLogNumber is 1, but since log 1 only contains data for CF 1, and CF 1 is flushed, log 1 might have already been deleted from disk.

We can make `MinLogNumber()` be the exactly minimum log number that must exist, by persisting the most recent log number for empty column families that are not flushed. But if there are N such column families, then every time a new WAL is created, we need to add N records to MANIFEST.

In current design, a record is persisted to MANIFEST only when WAL is created, closed, or deleted/archived, so the number of WAL related records are bounded to 3x number of WALs.

2. Why keep `WalSet` in `VersionSet` instead of applying the `VersionEdit`s to `VersionStorageInfo`

`VersionEdit`s are originally designed to track the addition and deletion of SST files. The SST files are related to column families, each column family has a list of `Version`s, and each `Version` keeps the set of active SST files in `VersionStorageInfo`.

But WALs are a concept of DB, they are not bounded to specific column families. So logically it does not make sense to store WALs in a column family's `Version`s.
Also, `Version`'s purpose is to keep reference to SST / blob files, so that they are not deleted until there is no version referencing them. But a WAL is deleted regardless of version references.
So we keep the WALs in `VersionSet`  for the purpose of writing out the DB state's snapshot when creating new MANIFESTs.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7164

Test Plan:
make version_edit_test && ./version_edit_test
make wal_edit_test && ./wal_edit_test

Reviewed By: ltamasi

Differential Revision: D22677936

Pulled By: cheng-chang

fbshipit-source-id: 5a3b6890140e572ffd79eb37e6e4c3c32361a859
2020-08-05 16:34:38 -07:00