Repairer documentation improvement.

Summary: Adding verbosity to existing comments.

Test Plan: None

Reviewers: sdong

CC: leveldb

Task ID: #6718960

Blame Rev:

db/repair.cc

@@ -7,24 +7,53 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file. See the AUTHORS file for names of contributors.
 //
-// We recover the contents of the descriptor from the other files we find.
-// (1) Any log files are first converted to tables
-// (2) We scan every table to compute
-//     (a) smallest/largest for the table
-//     (b) largest sequence number in the table
-// (3) We generate descriptor contents:
-//      - log number is set to zero
-//      - next-file-number is set to 1 + largest file number we found
-//      - last-sequence-number is set to largest sequence# found across
-//        all tables (see 2c)
-//      - compaction pointers are cleared
-//      - every table file is added at level 0
+// Repairer does best effort recovery to recover as much data as possible after
+// a disaster without compromising consistency. It does not guarantee bringing
+// the database to a time consistent state.
+//
+// Repair process is broken into 4 phases:
+// (a) Find files
+// (b) Convert logs to tables
+// (c) Extract metadata
+// (d) Write Descriptor
+//
+// (a) Find files
+//
+// The repairer goes through all the files in the directory, and classifies them
+// based on their file name. Any file that cannot be identified by name will be
+// ignored.
+//
+// (b) Convert logs to table
+//
+// Every log file that is active is replayed. All sections of the file where the
+// checksum does not match is skipped over. We intentionally give preference to
+// data consistency.
+//
+// (c) Extract metadata
+//
+// We scan every table to compute
+// (1) smallest/largest for the table
+// (2) largest sequence number in the table
+//
+// If we are unable to scan the file, then we ignore the table.
+//
+// (d) Write Descriptor
+//
+// We generate descriptor contents:
+//  - log number is set to zero
+//  - next-file-number is set to 1 + largest file number we found
+//  - last-sequence-number is set to largest sequence# found across
+//    all tables (see 2c)
+//  - compaction pointers are cleared
+//  - every table file is added at level 0
 //
 // Possible optimization 1:
 //   (a) Compute total size and use to pick appropriate max-level M
 //   (b) Sort tables by largest sequence# in the table
 //   (c) For each table: if it overlaps earlier table, place in level-0,
 //       else place in level-M.
+//   (d) We can provide options for time consistent recovery and unsafe recovery
+//       (ignore checksum failure when applicable)
 // Possible optimization 2:
 //   Store per-table metadata (smallest, largest, largest-seq#, ...)
 //   in the table's meta section to speed up ScanTable.