1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
|
/*! @class doc_bulk_durability
Bulk-loads are not commit-level durable, that is, the creation and
bulk-load of an object will not appear in the database log files.\ For
this reason, applications doing incremental backups after a full backup
should repeat the full backup step after doing a bulk-load to make the
bulk-load durable.\ In addition, incremental backups after a bulk-load
can cause recovery to report errors because there are log records that
apply to data files which don't appear in the backup.
*/
/*! @m_page{{c,java},backup,Backups}
WiredTiger cursors provide access to data from a variety of sources.
One of these sources is the list of files required to perform a backup
of the database. The list may be the files required by all of the
objects in the database, or a subset of the objects in the database.
WiredTiger backups are "on-line" or "hot" backups, and applications may
continue to read and write the databases while a snapshot is taken.
@section backup_process Backup from an application
1. Open a cursor on the \c "backup:" data source, which begins the
process of a backup.
2. Copy each file returned by the WT_CURSOR::next method to the backup
location, for example, a different directory. Do not reuse backup
locations unless all files have first been removed from them, in
other words, remove any previous backup information before using a
backup location.
3. Close the cursor; the cursor must not be closed until all of the
files have been copied.
The directory into which the files are copied may subsequently be
specified as an directory to the ::wiredtiger_open function and
accessed as a WiredTiger database home.
Copying the database files for a backup does not require any special
alignment or block size (specifically, Linux or Windows filesystems that
do not support read/write isolation can be safely read for backups).
The database file may grow in size during the copy, and the file copy
should not consider that an error. Blocks appended to the file after the
copy starts can be safely ignored, that is, it is correct for the copy
to determine an initial size of the file and then copy that many bytes,
ignoring any bytes appended after the backup cursor was opened.
The cursor must not be closed until all of the files have been copied,
however, there is no requirement the files be copied in any order or in
any relationship to the WT_CURSOR::next calls, only that all files have
been copied before the cursor is closed. For example, applications might
aggregate the file names from the cursor and then list the file names as
arguments to a file archiver such as the system tar utility.
During the period the backup cursor is open, database checkpoints can
be created, but no checkpoints can be deleted. This may result in
significant file growth.
Additionally, if a crash occurs during the period the backup cursor is open and
logging is disabled, then the system will be restored to the most recent
checkpoint prior to the opening of the backup cursor, even if later database
checkpoints were created.
The following is a programmatic example of creating a backup:
@snippet ex_all.c backup
In cases where the backup is desired for a checkpoint other than the
most recent, applications can discard all checkpoints subsequent to the
checkpoint they want using the WT_SESSION::checkpoint method. For
example:
@snippet ex_all.c backup of a checkpoint
@section backup_util Backup from the command line
The @ref_single util_backup command may also be used to create backups:
@code
rm -rf /path/database.backup &&
mkdir /path/database.backup &&
wt -h /path/database.source backup /path/database.backup
@endcode
@section backup_incremental Incremental backup
Once a backup has been done, it can be rolled forward incrementally by
adding log files to the backup copy. Adding log files to the copy
decreases potential data loss from switching to the copy, but increases
the recovery time necessary to switch to the copy. To reset the
recovery time necessary to switch to the copy, perform a full backup of
the database. For example, an application might do a full backup of the
database once a week during a quiet period, and then incrementally copy
new log files into the backup directory for the rest of the week.
Incremental backups may also save time when the tables are very large.
@copydoc doc_bulk_durability
By default, WiredTiger automatically removes log files no longer
required for recovery. Applications wanting to use log files for
incremental backup must first disable automatic log file removal using
the \c log=(archive=false) configuration to ::wiredtiger_open.
The following is the procedure for incrementally backing up a database
and removing log files from the original database home:
1. Perform a full backup of the database (as described above).
2. Open a cursor on the \c "backup:" data source, configured with the
\c "target=(\"log:\\")" target specified, which begins the process
of an incremental backup.
3. Copy each log file returned by the WT_CURSOR::next method to the backup
directory. It is not an error to copy a log file which has been copied
before, but care should be taken to ensure each log file is completely copied
as the most recent log file may grow in size while being copied.
4. If all log files have been successfully copied, archive the log
files by calling the WT_SESSION::truncate method with the URI
<code>log:</code> and specifying the backup cursor as the start
cursor to that method. (Note there is no requirement backups be
coordinated with database checkpoints, however, an incremental backup
will repeatedly copy the same files, and will not make additional log
files available for archival, unless there was a checkpoint after the
previous incremental backup.)
5. Close the backup cursor.
Steps 2-5 can be repeated any number of times before step 1 is repeated.
Full and incremental backups may be repeated as long as the backup
database directory has not been opened and recovery run. Once recovery
has run in a backup directory, you can no longer back up to that
database directory.
An example of opening the backup data source for an incremental backup:
@snippet ex_all.c incremental backup
@section backup_o_direct Backup and O_DIRECT
Many Linux systems do not support mixing \c O_DIRECT and memory mapping
or normal I/O to the same file. If \c O_DIRECT is configured for data
or log files on Linux systems (using the wiredtiger_open \c direct_io
configuration), any program used to copy files during backup should also
specify \c O_DIRECT when configuring its file access. Likewise, when
\c O_DIRECT is not configured by the database application, programs
copying files should not configure \c O_DIRECT.
*/
|
