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Repairer documentation improvement.

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Summary: Adding verbosity to existing comments.

Test Plan: None

Reviewers: sdong

CC: leveldb

Task ID: #6718960

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krad 2015-04-09 16:11:15 -07:00
parent 2f66d7f925
commit 697380f3d7
1 changed files with 41 additions and 12 deletions
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53
db/repair.cc
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@ -7,24 +7,53 @@
// Use of this source code is governed by a BSD-style license that can be // 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. // 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. // Repairer does best effort recovery to recover as much data as possible after
// (1) Any log files are first converted to tables // a disaster without compromising consistency. It does not guarantee bringing
// (2) We scan every table to compute // the database to a time consistent state.
// (a) smallest/largest for the table //
// (b) largest sequence number in the table // Repair process is broken into 4 phases:
// (3) We generate descriptor contents: // (a) Find files
// - log number is set to zero // (b) Convert logs to tables
// - next-file-number is set to 1 + largest file number we found // (c) Extract metadata
// - last-sequence-number is set to largest sequence# found across // (d) Write Descriptor
// all tables (see 2c) //
// - compaction pointers are cleared // (a) Find files
// - every table file is added at level 0 //
// 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: // Possible optimization 1:
// (a) Compute total size and use to pick appropriate max-level M // (a) Compute total size and use to pick appropriate max-level M
// (b) Sort tables by largest sequence# in the table // (b) Sort tables by largest sequence# in the table
// (c) For each table: if it overlaps earlier table, place in level-0, // (c) For each table: if it overlaps earlier table, place in level-0,
// else place in level-M. // 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: // Possible optimization 2:
// Store per-table metadata (smallest, largest, largest-seq#, ...) // Store per-table metadata (smallest, largest, largest-seq#, ...)
// in the table's meta section to speed up ScanTable. // in the table's meta section to speed up ScanTable.