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Update comment on max_valid_backups_to_open (#6105)

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Summary:
To reflect changes in PR https://github.com/facebook/rocksdb/issues/6072

This comment also implies that a seemingly valid use-case for
max_valid_backups_to_open is flawed: even if you only want to add a new
backup without trying to delete, you might need to clean up after a
backup creation that never finished. To clean up properly requires
opening all backups to get proper ref counts on shared files.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6105

Test Plan: code comment only

Differential Revision: D18736716

Pulled By: pdillinger

fbshipit-source-id: 2447c0000eefe3a4ca606926bfe922a8456b0cb7
This commit is contained in:
Peter Dillinger 2019-11-27 15:05:32 -08:00 committed by Facebook Github Bot
parent 09fcf4fb6b
commit e8f997ca59
1 changed files with 6 additions and 6 deletions
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12
include/rocksdb/utilities/backupable_db.h
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@ -104,13 +104,13 @@ struct BackupableDBOptions {
// Default: 4194304
uint64_t callback_trigger_interval_size;
// When Open() is called, it will open at most this many of the latest
// non-corrupted backups.
// For BackupEngineReadOnly, Open() will open at most this many of the
// latest non-corrupted backups.
//
// Note setting this to a non-default value prevents old files from being
// deleted in the shared directory, as we can't do proper ref-counting. If
// using this option, make sure to occasionally disable it (by resetting to
// INT_MAX) and run GarbageCollect to clean accumulated stale files.
// Note: this setting is ignored (behaves like INT_MAX) for any kind of
// writable BackupEngine because it would inhibit accounting for shared
// files for proper backup deletion, including purging any incompletely
// created backups on creation of a new backup.
//
// Default: INT_MAX
int max_valid_backups_to_open;