diff --git a/doc/src/sgml/architecture.sgml b/doc/src/sgml/architecture.sgml index e547a87d08..0c663a9d80 100644 --- a/doc/src/sgml/architecture.sgml +++ b/doc/src/sgml/architecture.sgml @@ -19,19 +19,18 @@ In the case of PostgreSQL, the server launches a single process for each client connection, referred to as a Backend process. - Those Backend processes handle the client's requests by acting on the + Such a Backend process handles the client's requests by acting on the Shared Memory. This leads to other activities (file access, WAL, vacuum, ...) of the Instance. The Instance is a group of server-side processes acting on a common - Shared Memory. Notably, PostgreSQL does not utilize application - threading within its implementation. + Shared Memory. PostgreSQL does not utilize threading. - The first step in an Instance start is the start of the + The first step when an Instance starts is the start of the Postmaster. - He loads the configuration files, allocates Shared Memory, and + It loads the configuration files, allocates Shared Memory, and starts the other processes of the Instance: Background Writer, Checkpointer, @@ -66,32 +65,31 @@ When a client application tries to connect to a database, - this request is handled initially by the Postmaster. He - starts a new Backend process and instructs the client - application to connect to it. All further client requests - go to this process and are handled by it. + this request is handled initially by the Postmaster. It + starts a new Backend process, which handles all further + client's requests. Client requests like SELECT or UPDATE usually lead to the - necessity to read or write some data. This is carried out + necessity to read or write data. This is carried out by the client's backend process. Reads involve a page-level - cache housed in Shared Memory (for details see: + cache, located in Shared Memory (for details see: ) for the benefit of all processes - in the instance. Writes also involve this cache, in additional + in the instance. Writes also use this cache, in addition to a journal, called a write-ahead-log or WAL. - Shared Memory is limited in size. Thus, it becomes necessary + Shared Memory is limited in size and it can become necessary to evict pages. As long as the content of such pages hasn't changed, this is not a problem. But in Shared Memory also write actions take place. Modified pages are called dirty pages or dirty buffers and before they can be evicted they - must be written back to disk. This happens regularly by the - Background Writer and the Checkpointer process to ensure - that the disk version of the pages are kept up-to-date. + must be written to disk. This happens regularly by the + Checkpointer and the Background Writer processes to ensure + that the disk version of the pages are up-to-date. The synchronisation from RAM to disk consists of two steps. @@ -109,7 +107,7 @@ Shared Memory. The parallel running WAL Writer process reads them and appends them to the end of the current WAL file. - Such sequential writes are much faster than writes to random + Such sequential writes are faster than writes to random positions of heap and index files. All WAL records created out of one dirty page must be transferred to disk before the dirty page itself can be transferred to disk in the second step. @@ -119,35 +117,36 @@ Second, the transfer of dirty buffers from Shared Memory to files must take place. This is the primary task of the Background Writer process. Because I/O activities can block - other processes significantly, it starts periodically and + other processes, it starts periodically and acts only for a short period. Doing so, its extensive (and expensive) I/O activities are spread over time, avoiding - debilitating I/O peaks. Also, the Checkpointer process - transfers dirty buffers to file. + debilitating I/O peaks. The Checkpointer process + also transfers dirty buffers to file. - The Checkpointer creates + The Checkpointer process creates Checkpoints. A Checkpoint is a point in time when all older dirty buffers, all older WAL records, and finally a special Checkpoint record - have been written and flushed to disk. Heap and index files + are written and flushed to disk. Heap and index files on the one hand and WAL files on the other hand are in sync. Previous WAL is no longer required. In other words, a possibly occurring recovery, which integrates the delta information of WAL into heap and index files, will happen - by replaying only WAL past the last recorded checkpoint - on top of the current heap and files. This speeds up recovery. + by replaying only WAL past the last recorded checkpoint. + This limits the amount of WAL which needs to be replayed + during recovery in the event of a crash. - While the Checkpointer ensures that a running system can crash + While the Checkpointer ensures that the database system can crash and restart itself in a valid state, the administrator needs - to handle the case where the heap and files themselves become + to handle the case where the heap or other files become corrupted (and possibly the locally written WAL, though that is less common). The options and details are covered extensively in the backup and restore section (). - For our purposes here, note just that the WAL Archiver process + For our purposes here, just note that the WAL Archiver process can be enabled and configured to run a script on filled WAL files — usually to copy them to a remote location. @@ -159,7 +158,7 @@ --> - The Statistics Collector collects counters about accesses to + The Statistics Collector collects counters about access to SQL objects like tables, rows, indexes, pages, and more. It stores the obtained information in system tables. @@ -234,13 +233,13 @@ Every database must contain at least one schema because all SQL Objects - are contained in a schema. - Schemas are namespaces for their SQL objects and ensure - (with one exception) that within their scope names are used - only once across all types of SQL objects. E.g., it is not possible + must be contained in a schema. + Schemas are namespaces for SQL objects and ensure + (with one exception) that the SQL object names are used only once within + their scope across all types of SQL objects. E.g., it is not possible to have a table employee and a view employee within the same schema. But it is - possible to have two tables employee in + possible to have two tables employee in two different schemas. In this case, the two tables are separate objects and independent of each other. The only exception to this cross-type uniqueness is that @@ -273,7 +272,7 @@ Global SQL Objects, are outside of the strict hierarchy: All database names, all tablespace names, and all - role names are automatically known and + role names are automatically available throughout the cluster, independent from the database or schema in which they where defined originally. @@ -302,7 +301,7 @@ The physical Perspective: Directories and Files - PostgreSQL organizes long-lasting + PostgreSQL organizes long-lasting (persistent) data as well as volatile state information about transactions or replication actions in the file system. Every has its root directory @@ -352,27 +351,26 @@ every table and every index to store heap and index data. Those files are accompanied by files for the Free Space Maps - (extension _fsm) and + (suffixed _fsm) and Visibility Maps - (extension _vm), which contain optimization information. + (suffixed _vm), which contain optimization information. - Another subdirectory is global. - In analogy to the database-specific - subdirectories, there are files containing information about + Another subdirectory is global which + contains files with information about Global SQL Objects. One type of such Global SQL Objects are tablespaces. In global there is information about - the tablespaces, not the tablespaces themselves. + the tablespaces; not the tablespaces themselves. The subdirectory pg_wal contains the WAL files. - They arise and grow parallel to data changes in the - cluster and remain alive as long as + They arise and grow in parallel with data changes in the + cluster and remain as long as they are required for recovery, archiving, or replication. @@ -385,18 +383,18 @@ In pg_tblspc, there are symbolic links - that point to directories containing such SQL objects - that are created within tablespaces. + that point to directories containing SQL objects + that exist within a non-default tablespace. In the root directory data there are also some files. In many cases, the configuration - files of the cluster are stored here. As long as the + files of the cluster are stored here. If the instance is up and running, the file postmaster.pid exists here and contains the process ID (pid) of the - Postmaster which has started the instance. + Postmaster which started the instance. @@ -411,7 +409,7 @@ In most cases, PostgreSQL databases - support many clients at the same time. Therefore, it is necessary to + support many clients at the same time which makes it necessary to protect concurrently running requests from unwanted overwriting of other's data as well as from reading inconsistent data. Imagine an online shop offering the last copy of an article. Two clients have the @@ -425,18 +423,18 @@ A first approach to implement protections against concurrent - accesses to the same data may be the locking of critical + access to the same data may be the locking of critical rows. Two such techniques are: Optimistic Concurrency Control (OCC) and Two Phase Locking (2PL). PostgreSQL implements a third, more sophisticated technique: Multiversion Concurrency Control (MVCC). The crucial advantage of MVCC - over other technologies gets evident in multiuser OLTP + over other technologies becomes evident in multiuser OLTP environments with a massive number of concurrent write actions. There, MVCC generally performs better than solutions using locks. In a PostgreSQL - database reading never blocks writing and writing never + database, reading never blocks writing and writing never blocks reading, even in the strictest level of transaction isolation. @@ -444,14 +442,14 @@ Instead of locking rows, the MVCC technique creates a new version of the row when a data-change takes place. To - distinguish between these two versions and to track the timeline + distinguish between these two versions, and to track the timeline of the row, each of the versions contains, in addition to their user-defined columns, two special system columns, which are not visible for the usual SELECT * FROM ... command. The column xmin contains the transaction ID (xid) - of the transaction, which created this version of the row. Accordingly, - xmax contains the xid of the transaction, which has - deleted this version, or zero, if the version is not + of the transaction which created this version of the row. + xmax contains the xid of the transaction which has + deleted this version, or zero if the version is not deleted. You can read both with the command SELECT xmin, xmax, * FROM ... . @@ -461,7 +459,7 @@ sequences. Every new transaction receives the next number as its ID. Therefore, this flow of xids represents the flow of transaction start events over time. But keep in mind that xids are independent of - any time measurement — in milliseconds or whatever. If you dive + any time measurement — in milliseconds or otherwise. If you dive deeper into PostgreSQL, you will recognize parameters with names such as 'xxx_age'. Despite their names, these '_age' parameters do not specify a period of time but represent @@ -469,7 +467,7 @@ - The description in this chapter simplifies by omitting some details. + The description in this chapter simplifies by omitting details. When many transactions are running simultaneously, things can get complicated. Sometimes transactions get aborted via ROLLBACK immediately or after a lot of other activities, sometimes @@ -481,7 +479,7 @@ - So, what's going on in detail when write accesses take place? + So, what's going on in detail when write access takes place? shows details concerning xmin, xmax, and user data. @@ -516,36 +514,38 @@ executes an UPDATE of this row by changing the user data from 'x' to 'y'. According to the MVCC principles, - the data in the old version of the row does not change! + the data in the old version of the row is not changed! The value 'x' remains as it was before. Only xmax changes to 135. Now, this version is treated as valid exclusively for transactions with xids from 123 to - 134. As a substitute for the non-occurring + 134. In addition to the non-occurring data change in the old version, the UPDATE creates a new version of the row with its xid in xmin, 0 in xmax, and 'y' in the - user data (plus all the other user data from the old version). - This version is now valid for all coming transactions. + user data (plus all other user data from the old version). + This new row version is visible to all future transactions. + (Internally, an UPDATE command acts + as a DELETE command followed by + an INSERT command.) All subsequent UPDATE commands behave - in the same way as the first one: they put their xid to + in the same way as the first one: they put their xid in xmax of the current version, create - the next version with their xid in xmin, - 0 in xmax, and the - new user data. + a new version with their xid in xmin and + 0 in xmax. Finally, a row may be deleted by a DELETE command. Even in this case, all versions of the row remain as - before. Nothing is thrown away so far! Only xmax - of the last version changes to the xid of the DELETE - transaction, which indicates that it is only valid for - transactions with xids older than its own (from + before. Nothing is thrown away! Only xmax + of the last version is set to the xid of the DELETE + transaction, which indicates that (if committed) it is only visible to + transactions with xids older than that (from 142 to 820 in this example). @@ -555,10 +555,10 @@ of the same row in the table's heap file and leaves them there, even after a DELETE command. Only the youngest version is relevant for all future transactions. But the - system must also preserve some of the older ones for a - certain amount of time because the possibility exists that - they are or could become relevant for any pending - transactions. Over time, also the older ones get out of scope + system must also preserve some of the older ones for + awhile, because they could still be needed by + transactions which started before the deleting transaction commits. + Over time, also the older ones get out of scope for ALL transactions and therefore become unnecessary. Nevertheless, they do exist physically on the disk and occupy space. @@ -573,13 +573,13 @@ xmin and xmax indicate the range from where to where - row versions are valid (visible) for transactions. + row versions are valid (visible) for transactions. This range doesn't imply any direct temporal meaning; the sequence of xids reflects only the sequence of transaction begin events. As xids grow, old row versions get out of scope over time. - If an old row version is no longer valid for ALL existing - transactions, it's called dead. The + If an old row version is no longer relevant for ANY existing + transactions, it can be marked dead. The space occupied by dead row versions is part of the bloat. @@ -612,21 +612,21 @@ As we have seen in the previous chapter, the database - tends to occupy more and more disk space, the + tends to occupy more and more disk space, caused by bloat. This chapter explains how the SQL command VACUUM and the automatically running Autovacuum processes clean up - by eliminating bloat. + and avoid continued growth. Autovacuum runs automatically by default. Its default parameters as well as such for - VACUUM fit well for most standard + VACUUM are appropriate for most standard situations. Therefore a novice database manager can - easily skip the rest of this chapter which explains + skip the rest of this chapter which explains a lot of details. @@ -637,8 +637,8 @@ special situations, or they start it in batch jobs which run periodically. Autovacuum processes run as part of the Instance at the server. - There is a constantly running Autovacuum daemon. It permanently - controls the state of all databases based on values that are collected by the + There is a constantly running Autovacuum daemon. It continuously + monitors the state of all databases based on values that are collected by the Statistics Collector and starts Autovacuum processes whenever it detects certain situations. Thus, it's a dynamic behavior of @@ -659,9 +659,9 @@ - Freeze: Mark the youngest row version - as frozen. This means that the version - is always treated as valid (visible) independent from + Freeze: Mark certain row versions + as frozen. This means that they + are always treated as valid (visible) independent from the wraparound problem (see below). @@ -686,22 +686,22 @@ - The eagerness — you can call it 'aggression' — of the - operations eliminating bloat and + The eagerness — you can call it 'aggressiveness' — of the + operations for eliminating bloat and freeze is controlled by configuration parameters, runtime flags, and in extreme situations by the processes themselves. Because vacuum operations typically are I/O intensive, which can hinder other activities, Autovacuum avoids performing many vacuum operations in bulk. Instead, - it carries out many small actions with time gaps in between. + it carries out many small actions with delay points in between. The SQL command VACUUM runs immediately - and without any time gaps. + and without any time delay. Eliminate Bloat - To determine which of the row versions are superfluous, the + To determine which of the row versions are no longer needed, the elimination operation must evaluate xmax against several criteria which all must apply: @@ -742,14 +742,13 @@ - After the vacuum operation detects a superfluous row version, it + After the vacuum operation detects an unused row version, it marks its space as free for future use of writing actions. Only in rare situations (or in the case of VACUUM FULL), - this space is released to the operating system. In most cases, + is this space released to the operating system. In most cases, it remains occupied by PostgreSQL and will be used by future INSERT or - UPDATE commands concerning this row or a - completely different one. + UPDATE commands to this table. @@ -762,10 +761,10 @@ When a client issues the SQL command VACUUM in its default format, i.e., without any option. To boost performance, in this and the next case VACUUM does not - read and act on all pages of the heap. - The Visibility Map, which is very compact and therefore has a small - size, contains information about pages, where bloat-candidates might - be found. Only such pages are processed. + read and act on all pages of the heap file. + The Visibility Map, which is very compact and therefore fast to read, + contains information about which pages have no deleted row versions, and + can be skipped by VACUUM. @@ -773,7 +772,7 @@ When a client issues the SQL command VACUUM with the option FREEZE. (In this case, - it undertakes much more actions, see + it undertakes many more actions, see Freeze Row Versions.) @@ -782,12 +781,11 @@ When a client issues the SQL command VACUUM with the option FULL. - Also, in this mode, the bloat disappears, but the strategy used - is very different: In this case, the complete table is copied - to a different file skipping all outdated row versions. This - leads to a significant reduction of used disk space because - the new file contains only the actual data. The old file - is deleted. + In this mode, an exclusive lock is taken, and + the whole table is copied to a different file, skipping all outdated row + versions. All bloat is thereby eliminated, which + may lead to a significant reduction of used disk space. + The old file is deleted. @@ -809,17 +807,17 @@ This logic only applies to row versions of the heap. Index entries don't use xmin/xmax. Nevertheless, such index - entries, which would lead to outdated row versions, are released + entries, which would lead to outdated row versions, are cleaned up accordingly. The above descriptions omit the fact that xids on a real computer - have a limited size. They count up in the same way as sequences, and after - a certain number of new transactions they are forced to restart + have a limited size, and after + a certain number of transactions they are forced to restart from the beginning, which is called wraparound. Therefore the terms 'old transaction' / 'young transaction' does - not always correlate with low / high values of xids. Near to the + not always correlate with low / high values of xids. Near to wraparound point, there are cases where xmin has a higher value than xmax, although their meaning is said to be older than xmax. @@ -858,7 +856,7 @@ and the corresponding transactions of xmin and xmax must be committed. However, PostgreSQL has to consider the - possibility of wraparounds. + possibility of wraparound. Therefore the decision becomes more complex. The general idea of the solution is to use the 'between xmin and xmax' @@ -885,7 +883,7 @@ With each newly created transaction the two split-points - move forward. When 'txid_current + 2^31' would reach a + move forward. If 'txid_current + 2^31' reached a row version with xmin equal to that value, it would immediately jump from 'past' to 'future' and would be no longer visible! @@ -894,11 +892,11 @@ - To avoid this unacceptable extinction of data, the vacuum - operation freeze clears the situation - long before the split-point is reached. It sets a flag - in the header of the row version, which completely eliminates - the future use of xmin/xmax and indicates + If not handled in some way, data inserted many transactions ago + would become invisibile. The vacuum operation freeze + avoids this long before the split-point is reached by setting + a flag in the header of the row version which avoids + future comparison of its xmin/xmax and indicates that the version is valid not only in the 'past'-half but also in the 'future'-half as well as in all coming epochs. @@ -945,19 +943,19 @@ When a client issues the SQL command VACUUM with its FREEZE option. In this case, all pages are processed that are marked in the Visibility Map - to potentially have unfrozen rows. + as potentially having unfrozen rows. When a client issues the SQL command VACUUM without - any options but finds that there are xids older than + any options but there are xids older than (default: 150 million) minus (default: 50 million). As before, all pages are processed that are - marked in the Visibility Map to potentially have unfrozen + marked in the Visibility Map to potentially having unfrozen rows. @@ -983,7 +981,7 @@ The process switches to an aggressive mode if it recognizes - that for the processed table their oldest xid exceeds + that for the processed table the oldest xid exceeds (default: 200 million). The value of the oldest unfrozen xid is stored per table in pg_class.relfrozenxid. @@ -1039,29 +1037,28 @@ The Visibility Map (VM) contains two flags — stored as - two bits — for each page of the heap. If the first bit - is set, that indicates that the associated page does not - contain any bloat. If the second one is set, that indicates - that the page contains only frozen rows. + two bits — for each page of the heap file. The + first bit indicates that the associated page does + not contain any bloat. The second bit indicates + that the page contains only frozen row versions. Please consider two details. First, in most cases a page - contains many rows, some of them in many versions. + contains many rows or row versions. However, the flags are associated with the page, - not with a row or a row version. The flags are set + not with an individual row version. The flags are set only under the condition that they are valid for ALL row versions of the page. Second, since there are only two bits per page, the VM is considerably - smaller than the heap. Therefore it is buffered - in RAM in almost all cases. + smaller than the heap. The setting of the flags is silently done by VACUUM and Autovacuum during their bloat and freeze operations. This is done to speed up future vacuum actions, - regular accesses to heap pages, and some accesses to + regular access to heap pages, and some access to the index. Every data-modifying operation on any row version of the page clears the flags. @@ -1070,7 +1067,7 @@ The Free Space Map (FSM) tracks the amount of free space per page. It is organized as a highly condensed b-tree of (rounded) sizes. - As long as VACUUM or Autovacuum change + Whenever VACUUM or Autovacuum changes the free space on any processed page, they log the new values in the FSM in the same way as all other writing processes. @@ -1079,18 +1076,19 @@ Statistics - Statistic information helps the Query Planner to make optimal decisions for the generation of execution plans. This information can be gathered with the SQL commands ANALYZE or VACUUM ANALYZE. - But also Autovacuum processes gather + But Autovacuum processes also gather such information. Depending on the percentage of changed rows - per table , + , + and minimum number of changed rows , the Autovacuum daemon starts Autovacuum processes to collect - statistics per table. This dynamic invocation of analyze - operations allows PostgreSQL to - adopt queries to changing circumstances. + statistics per table. The automatic analysis + allows PostgreSQL to + adapt query execution to changing circumstances. @@ -1143,7 +1141,7 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; atomicity: either all or none of its operations succeed, regardless of the fact that it may consist of a lot of different write-operations, and each such operation may - affect thousands or millions of rows. As soon as one of the + affect many rows. As soon as one of the operations fails, all previous operations fail also, which means that all modified rows retain their values as of the beginning of the transaction. @@ -1151,20 +1149,20 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; The atomicity also affects the visibility of changes. No - connection running simultaneously to a data modifying + connection running simultaneously with a data modifying transaction will ever see any change before the transaction successfully executes a COMMIT — even in the lowest isolation level of transactions. PostgreSQL - does never show uncommitted changes to other connections. + never shows uncommitted changes to other connections. The situation regarding visibility is somewhat different from the point of view of the modifying transaction. - SELECT commands issued inside a - transaction delivers all changes done so far by this + A SELECT command issued inside a + transaction shows all changes done so far by this transaction. @@ -1230,9 +1228,9 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; Transactions ensure that the - consistency - of the complete database always keeps valid. Declarative - rules like + database always remains + consistent. + Declarative rules like primary- or foreign keys, checks, @@ -1241,13 +1239,6 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; are part of the all-or-nothing nature of transactions. - - Also, all self-evident — but possibly not obvious - — low-level demands on the database system are - ensured; e.g. index entries for rows must become - visible at the same moment as the rows themselves. - - There is the additional feature 'isolation level', @@ -1257,11 +1248,11 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; - Lastly, it is worth to notice that changes done by a - committed transaction will survive all future application, - instance, or hardware failures. The next chapter - explains this - durability. + Lastly, it is worth noticing that changes done by a + committed transaction will survive all failures in the application or + database cluster. The next chapter explains the + durability + guarantees. @@ -1287,7 +1278,7 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; a severe software error like a null pointer exception. Because PostgreSQL uses a client/server architecture, no direct problem for the - database will occur. In all of this cases, the + database will occur. In all of these cases, the Backend process, which is the client's counterpart at the server-side, may recognize that the network connection is no longer @@ -1310,7 +1301,7 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; automatically recognizes that the last shutdown of the instance did not happen as expected: files might not be closed properly and the postmaster.pid - file exists. PostgreSQL + file unexpectedly exists. PostgreSQL tries to clean up the situation. This is possible because all changes in the database are stored twice. First, the WAL files contain them as a chronology of @@ -1318,7 +1309,7 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; which include the new data values and information about commit actions. The WAL records are written first. Second, the data itself shall exist in the heap and index files. - In opposite to the WAL records, this part may or may + In constrast with the WAL records, this part may or may not have been transferred entirely from Shared Memory to the files. @@ -1328,13 +1319,13 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; checkpoint. This checkpoint signals that the database files are in a consistent state, especially that all WAL records up to - this point were successfully stored in heap and index. Starting - here, the recovery process copies the following WAL records - to heap and index. As a result, the files contain all - changes and reach a consistent state. Changes of committed - transactions are visible; those of uncommited transactions + this point were successfully stored in heap and index files. Starting + here, the recovery process copies the remaining WAL records + to heap and index. As a result, the heap files contain all + changes recorded to the WAL and reach a consistent state. Changes of committed + transactions are visible; those of uncommitted transactions are also in the files, but - as usual - they are never seen - by any of the following transactions because uncommited + by any of the following transactions because uncommitted changes are never shown. Such recovery actions run completely automatically, it is not necessary that a database administrator configure or start anything by @@ -1344,13 +1335,13 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; Disk crash If a disk crashes, the course of action described previously - cannot work. It is likely that the WAL files and/or the + cannot work: it is likely that the WAL files and/or the data and index files are no longer available. The database administrator must take special actions to - overcome such situations. + prepare for such a situation. - He obviously needs a backup. How to take such a backup + They obviously needs a backup. How to take such a backup and use it as a starting point for a recovery of the cluster is explained in more detail in the next chapter. @@ -1362,11 +1353,11 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; and there is no room for additional data. In this case, PostgreSQL stops accepting data-modifying commands or even terminates completely. - No data loss or data corruption will occur. + Committed data is neither lost nor corrupted. - To come out of such a situation, the administrator should - remove unused files from this disk. But he should never + To recover from such a situation, the administrator should + remove unused files from this disk. But they should never delete files from the data directory. Nearly all of them are necessary for the consistency @@ -1427,7 +1418,7 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; The obvious disadvantage of this method is that there - is a downtime where no user interaction is possible. + is a downtime. The other two strategies run during regular operating times. @@ -1437,9 +1428,9 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; The tool pg_dump is able to take a copy of the complete cluster or certain parts of it. It stores - the copy in the form of SQL CREATE and - INSERT commands. It runs in - parallel to other processes in its own transaction. + the copy in the form of SQL commands like CREATE + and COPY. It runs in + parallel with other processes in its own transaction. The output of pg_dump may be used as @@ -1456,17 +1447,17 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; Continuous archiving based on pg_basebackup and WAL files This method - is the most sophisticated and complex one. It + is the most sophisticated and most complex one. It consists of two phases. - First, you need to create a so called + First, you need to create a so-called basebackup with the tool pg_basebackup. The result is a - directory structure plus files which contains a + directory structure plus files which contain a consistent copy of the original cluster. pg_basebackup runs in - parallel to other processes in its own transaction. + parallel with other processes in its own transaction. The second step is recommended but not necessary. All @@ -1484,7 +1475,7 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; Archiver process will automatically copy every single WAL file to a save location. Its configuration - consists mainly of a string, which contains a copy command + consists mainly of a string that contains a copy command in the operating system's syntax. In order to protect your data against a disk crash, the destination location of a basebackup as well as of the @@ -1492,9 +1483,8 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob'; disk which is different from the data disk. - If it gets necessary to restore the cluster, you have to - copy the basebackup and the - archived WAL files to + If it becomes necessary to restore the cluster, you have to + copy the basebackup and the archived WAL files to their original directories. The configuration of this recovery procedure contains a string with the reverse copy command: from