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help / color / mirror / Atom feedFrom: Heikki Linnakangas <[email protected]>
Subject: [PATCH v2 5/5] Allow pg_rewind to use a standby server as the source system.
Date: Wed, 19 Aug 2020 15:34:43 +0300
Using a hot standby server as the source has not been possible, because
pg_rewind creates a temporary table in the source system, to hold the
list of file ranges that need to be fetched. Refactor it to queue up the
file fetch requests in pg_rewind's memory, so that the temporary table
is no longer needed.
Also update the logic to compute 'minRecoveryPoint' correctly, when the
source is a standby server.
---
src/bin/pg_rewind/libpq_fetch.c | 279 ++++++++++++++++++++++----------
src/bin/pg_rewind/pg_rewind.c | 67 ++++++--
2 files changed, 252 insertions(+), 94 deletions(-)
diff --git a/src/bin/pg_rewind/libpq_fetch.c b/src/bin/pg_rewind/libpq_fetch.c
index 52c4e147e10..f61a4242999 100644
--- a/src/bin/pg_rewind/libpq_fetch.c
+++ b/src/bin/pg_rewind/libpq_fetch.c
@@ -15,39 +15,61 @@
#include "fetch.h"
#include "file_ops.h"
#include "filemap.h"
+#include "lib/stringinfo.h"
#include "pg_rewind.h"
#include "port/pg_bswap.h"
/*
- * Files are fetched max CHUNKSIZE bytes at a time.
- *
- * (This only applies to files that are copied in whole, or for truncated
- * files where we copy the tail. Relation files, where we know the individual
- * blocks that need to be fetched, are fetched in BLCKSZ chunks.)
+ * Files are fetched max CHUNK_SIZE bytes at a time, and with a
+ * maximum of MAX_CHUNKS_PER_QUERY chunks in a single query.
*/
-#define CHUNKSIZE 1000000
+#define CHUNK_SIZE (1024 * 1024)
+#define MAX_CHUNKS_PER_QUERY 1000
+
+/* represents the request to fetch a piece of a file from the source */
+typedef struct
+{
+ const char *path; /* path relative to data directory root */
+ uint64 offset;
+ uint32 length;
+} fetch_range_request;
typedef struct
{
rewind_source common; /* common interface functions */
PGconn *conn;
+
+ /*
+ * Queue of chunks that have been requested with the queue_fetch_range()
+ * function, but have not been fetched from the remote server yet.
+ */
+ int num_requests;
+ fetch_range_request request_queue[MAX_CHUNKS_PER_QUERY];
+
+ /* temporary space for process_queued_fetch_requests() */
+ StringInfoData paths;
+ StringInfoData offsets;
+ StringInfoData lengths;
} libpq_source;
static void init_libpq_conn(PGconn *conn);
static char *run_simple_query(PGconn *conn, const char *sql);
static void run_simple_command(PGconn *conn, const char *sql);
+static void appendArrayEscapedString(StringInfo buf, const char *str);
+
+static void process_queued_fetch_requests(libpq_source *src);
/* public interface functions */
static void libpq_traverse_files(rewind_source *source,
process_file_callback_t callback);
-static char *libpq_fetch_file(rewind_source *source, const char *path,
- size_t *filesize);
static void libpq_queue_fetch_range(rewind_source *source, const char *path,
uint64 off, size_t len);
static void libpq_finish_fetch(rewind_source *source);
-static void libpq_destroy(rewind_source *source);
+static char *libpq_fetch_file(rewind_source *source, const char *path,
+ size_t *filesize);
static XLogRecPtr libpq_get_current_wal_insert_lsn(rewind_source *source);
+static void libpq_destroy(rewind_source *source);
/*
* Create a new libpq source.
@@ -73,6 +95,10 @@ init_libpq_source(PGconn *conn)
src->conn = conn;
+ initStringInfo(&src->paths);
+ initStringInfo(&src->offsets);
+ initStringInfo(&src->lengths);
+
return &src->common;
}
@@ -90,7 +116,10 @@ init_libpq_conn(PGconn *conn)
run_simple_command(conn, "SET lock_timeout = 0");
run_simple_command(conn, "SET idle_in_transaction_session_timeout = 0");
- /* we don't intend do any updates. Put the connection in read-only mode to keep us honest */
+ /*
+ * We don't intend do any updates. Put the connection in read-only mode
+ * to keep us honest.
+ */
run_simple_command(conn, "SET default_transaction_read_only = off");
/* secure search_path */
@@ -100,17 +129,6 @@ init_libpq_conn(PGconn *conn)
PQresultErrorMessage(res));
PQclear(res);
- /*
- * Check that the server is not in hot standby mode. There is no
- * fundamental reason that couldn't be made to work, but it doesn't
- * currently because we use a temporary table. Better to check for it
- * explicitly than error out, for a better error message.
- */
- str = run_simple_query(conn, "SELECT pg_is_in_recovery()");
- if (strcmp(str, "f") != 0)
- pg_fatal("source server must not be in recovery mode");
- pg_free(str);
-
/*
* Also check that full_page_writes is enabled. We can get torn pages if
* a page is modified while we read it with pg_read_binary_file(), and we
@@ -121,15 +139,15 @@ init_libpq_conn(PGconn *conn)
pg_fatal("full_page_writes must be enabled in the source server");
pg_free(str);
- /*
- * First create a temporary table, and COPY to load it with the list of
- * blocks that we need to fetch.
- */
- run_simple_command(conn, "CREATE TEMPORARY TABLE fetchchunks(path text, begin int8, len int4)");
+ /* Prepare a statement we'll use to fetch files */
+ res = PQprepare(conn, "fetch_chunks_stmt",
+ "SELECT path, begin,\n"
+ " pg_read_binary_file(path, begin, len, true) AS chunk\n"
+ "FROM unnest ($1::text[], $2::int8[], $3::int4[]) as x(path, begin, len)",
+ 3, NULL);
- res = PQexec(conn, "COPY fetchchunks FROM STDIN");
- if (PQresultStatus(res) != PGRES_COPY_IN)
- pg_fatal("could not send file list: %s",
+ if (PQresultStatus(res) != PGRES_COMMAND_OK)
+ pg_fatal("could not prepare statement to fetch file contents: %s",
PQresultErrorMessage(res));
PQclear(res);
}
@@ -297,91 +315,143 @@ libpq_queue_fetch_range(rewind_source *source, const char *path, uint64 off,
size_t len)
{
libpq_source *src = (libpq_source *) source;
- uint64 begin = off;
- uint64 end = off + len;
/*
- * Write the file range to a temporary table in the server.
+ * Does this request happen to be a continuation of the previous chunk?
+ * If so, merge it with the previous one.
*
- * The range is sent to the server as a COPY formatted line, to be inserted
- * into the 'fetchchunks' temporary table. The libpq_finish_fetch() uses
- * the temporary table to actually fetch the data.
+ * XXX: We use pointer equality to compare the path. That's good enough
+ * for our purposes; the caller always passes the same pointer for the
+ * same filename. If it didn't, we would fail to merge requests, but it
+ * wouldn't affect correctness.
*/
-
- /* Split the range into CHUNKSIZE chunks */
- while (end - begin > 0)
+ if (src->num_requests > 0)
{
- char linebuf[MAXPGPATH + 23];
- unsigned int len;
+ fetch_range_request *prev = &src->request_queue[src->num_requests - 1];
- /* Fine as long as CHUNKSIZE is not bigger than UINT32_MAX */
- if (end - begin > CHUNKSIZE)
- len = CHUNKSIZE;
- else
- len = (unsigned int) (end - begin);
+ if (prev->offset + prev->length == off &&
+ prev->length < CHUNK_SIZE &&
+ prev->path == path)
+ {
+ /*
+ * Extend the previous request to cover as much of this new request
+ * as possible, without exceeding CHUNK_SIZE.
+ */
+ int32 thislen;
+
+ thislen = Min(len, CHUNK_SIZE - prev->length);
+ src->request_queue[src->num_requests - 1].length += thislen;
- begin += len;
+ off += thislen;
+ len -= thislen;
- snprintf(linebuf, sizeof(linebuf), "%s\t" UINT64_FORMAT "\t%u\n", path, begin, len);
+ /*
+ * Fall through to create new requests for any remaining 'len' that
+ * didn't fit in the previous chunk.
+ */
+ }
+ }
+
+ /* Divide the request into pieces of CHUNK_SIZE bytes each */
+ while (len > 0)
+ {
+ int32 thislen;
+
+ /* if the queue is full, perform all the work queued up so far */
+ if (src->num_requests == MAX_CHUNKS_PER_QUERY)
+ process_queued_fetch_requests(src);
+
+ thislen = Min(len, CHUNK_SIZE);
+ src->request_queue[src->num_requests].path = path;
+ src->request_queue[src->num_requests].offset = off;
+ src->request_queue[src->num_requests].length = thislen;
+ src->num_requests++;
- if (PQputCopyData(src->conn, linebuf, strlen(linebuf)) != 1)
- pg_fatal("could not send COPY data: %s",
- PQerrorMessage(src->conn));
+ off += thislen;
+ len -= thislen;
}
}
-
-/*----
- * Runs a query, which returns pieces of files from the remote source data
- * directory, and overwrites the corresponding parts of target files with
- * the received parts. The result set is expected to be of format:
- *
- * path text -- path in the data directory, e.g "base/1/123"
- * begin int8 -- offset within the file
- * chunk bytea -- file content
- *----
+/*
+ * Fetche all the queued chunks and writes them to the target data directory.
*/
+static void
+libpq_finish_fetch(rewind_source *source)
+{
+ process_queued_fetch_requests((libpq_source *) source);
+}
+
/*
* Receive all the queued chunks and write them to the target data directory.
*/
static void
-libpq_finish_fetch(rewind_source *source)
+process_queued_fetch_requests(libpq_source *src)
{
- libpq_source *src = (libpq_source *) source;
+ const char *params[3];
PGresult *res;
- const char *sql;
+ int chunkno;
- if (PQputCopyEnd(src->conn, NULL) != 1)
- pg_fatal("could not send end-of-COPY: %s",
- PQerrorMessage(src->conn));
+ if (src->num_requests == 0)
+ return;
- while ((res = PQgetResult(src->conn)) != NULL)
+ pg_log_debug("getting %d file chunks", src->num_requests);
+
+ /*
+ * The prepared statement, 'fetch_chunks_stmt', takes three arrays
+ * with the same length as parameters: paths, offsets and lengths.
+ * Construct the string representations of the parameter arrays.
+ */
+ resetStringInfo(&src->paths);
+ resetStringInfo(&src->offsets);
+ resetStringInfo(&src->lengths);
+
+ appendStringInfoChar(&src->paths, '{');
+ appendStringInfoChar(&src->offsets, '{');
+ appendStringInfoChar(&src->lengths, '{');
+ for (int i = 0; i < src->num_requests; i++)
{
- if (PQresultStatus(res) != PGRES_COMMAND_OK)
- pg_fatal("unexpected result while sending file list: %s",
- PQresultErrorMessage(res));
- PQclear(res);
+ fetch_range_request *rq = &src->request_queue[i];
+
+ if (i > 0)
+ {
+ appendStringInfoChar(&src->paths, ',');
+ appendStringInfoChar(&src->offsets, ',');
+ appendStringInfoChar(&src->lengths, ',');
+ }
+
+ appendArrayEscapedString(&src->paths, rq->path);
+ appendStringInfo(&src->offsets, INT64_FORMAT, rq->offset);
+ appendStringInfo(&src->lengths, "%d", rq->length);
}
+ appendStringInfoChar(&src->paths, '}');
+ appendStringInfoChar(&src->offsets, '}');
+ appendStringInfoChar(&src->lengths, '}');
/*
- * We've now copied the list of file ranges that we need to fetch to the
- * temporary table. Now, actually fetch all of those ranges.
+ * Execute the prepared statement.
*/
- sql =
- "SELECT path, begin,\n"
- " pg_read_binary_file(path, begin, len, true) AS chunk\n"
- "FROM fetchchunks\n";
+ params[0] = src->paths.data;
+ params[1] = src->offsets.data;
+ params[2] = src->lengths.data;
- if (PQsendQueryParams(src->conn, sql, 0, NULL, NULL, NULL, NULL, 1) != 1)
+ if (PQsendQueryPrepared(src->conn, "fetch_chunks_stmt", 3, params, NULL, NULL, 1) != 1)
pg_fatal("could not send query: %s", PQerrorMessage(src->conn));
- pg_log_debug("getting file chunks");
-
if (PQsetSingleRowMode(src->conn) != 1)
pg_fatal("could not set libpq connection to single row mode");
+ /*----
+ * The result set is of format:
+ *
+ * path text -- path in the data directory, e.g "base/1/123"
+ * begin int8 -- offset within the file
+ * chunk bytea -- file content
+ *----
+ */
+ chunkno = 0;
while ((res = PQgetResult(src->conn)) != NULL)
{
+ fetch_range_request *rq = &src->request_queue[chunkno];
char *filename;
int filenamelen;
int64 chunkoff;
@@ -402,6 +472,9 @@ libpq_finish_fetch(rewind_source *source)
PQresultErrorMessage(res));
}
+ if (chunkno > src->num_requests)
+ pg_fatal("received more data chunks than requested");
+
/* sanity check the result set */
if (PQnfields(res) != 3 || PQntuples(res) != 1)
pg_fatal("unexpected result set size while fetching remote files");
@@ -446,9 +519,7 @@ libpq_finish_fetch(rewind_source *source)
* If a file has been deleted on the source, remove it on the target
* as well. Note that multiple unlink() calls may happen on the same
* file if multiple data chunks are associated with it, hence ignore
- * unconditionally anything missing. If this file is not a relation
- * data file, then it has been already truncated when creating the
- * file chunk list at the previous execution of the filemap.
+ * unconditionally anything missing.
*/
if (PQgetisnull(res, 0, 2))
{
@@ -463,14 +534,54 @@ libpq_finish_fetch(rewind_source *source)
pg_log_debug("received chunk for file \"%s\", offset %lld, size %d",
filename, (long long int) chunkoff, chunksize);
- open_target_file(filename, false);
+ if (strcmp(filename, rq->path) != 0)
+ {
+ pg_fatal("received data for file \"%s\", when requested for \"%s\"",
+ filename, rq->path);
+ }
+ if (chunkoff != rq->offset)
+ pg_fatal("received data at offset " UINT64_FORMAT" of file \"%s\", when requested for offset " UINT64_FORMAT,
+ chunkoff, rq->path, rq->offset);
+ if (chunksize > rq->length)
+ {
+ pg_fatal("received more than requested for file \"%s\"",
+ rq->path);
+ /* receiving less is OK, though */
+ }
+ open_target_file(filename, false);
write_target_range(chunk, chunkoff, chunksize);
pg_free(filename);
PQclear(res);
+ chunkno++;
+ }
+ if (chunkno != src->num_requests)
+ pg_fatal("unexpected number of data chunks received");
+
+ src->num_requests = 0;
+}
+
+/*
+ * Escape a string to be used as element in a text array constant
+ */
+static void
+appendArrayEscapedString(StringInfo buf, const char *str)
+{
+ appendStringInfoCharMacro(buf, '\"');
+ while (*str)
+ {
+ char ch = *str;
+
+ if (ch == '"' || ch == '\\')
+ appendStringInfoCharMacro(buf, '\\');
+
+ appendStringInfoCharMacro(buf, ch);
+
+ str++;
}
+ appendStringInfoCharMacro(buf, '\"');
}
/*
diff --git a/src/bin/pg_rewind/pg_rewind.c b/src/bin/pg_rewind/pg_rewind.c
index 9e04a085226..e9a3481be58 100644
--- a/src/bin/pg_rewind/pg_rewind.c
+++ b/src/bin/pg_rewind/pg_rewind.c
@@ -45,6 +45,7 @@ static void disconnect_atexit(void);
static ControlFileData ControlFile_target;
static ControlFileData ControlFile_source;
+static ControlFileData ControlFile_source_after;
const char *progname;
int WalSegSz;
@@ -446,30 +447,76 @@ main(int argc, char **argv)
progress_report(true);
+ /*
+ * Fetch the control file from the source last. This ensures that the
+ * minRecoveryPoint is up-to-date.
+ */
+ buffer = source->fetch_file(source, "global/pg_control", &size);
+ digestControlFile(&ControlFile_source_after, buffer, size);
+ pg_free(buffer);
+
+ /*
+ * Sanity check: If the source is a local system, the control file should
+ * not have changed since we started.
+ *
+ * XXX: We assume it hasn't been modified, but actually, what could go
+ * wrong? The logic handles a libpq source that's modified concurrently,
+ * why not a local datadir?
+ */
+ if (datadir_source &&
+ memcmp(&ControlFile_source, &ControlFile_source_after,
+ sizeof(ControlFileData)) != 0)
+ {
+ pg_fatal("source system was modified while pg_rewind was running");
+ }
+
if (showprogress)
pg_log_info("creating backup label and updating control file");
createBackupLabel(chkptredo, chkpttli, chkptrec);
/*
* Update control file of target. Make it ready to perform archive
- * recovery when restarting.
+ * recovery when restarting, starting from the last common checkpoint.
*
- * minRecoveryPoint is set to the current WAL insert location in the
- * source server. Like in an online backup, it's important that we recover
- * all the WAL that was generated while we copied the files over.
+ * Like in an online backup, it's important that we replay all the WAL
+ * that was generated while we copied the files over. To enforce that,
+ * set 'minRecoveryPoint' in the control file.
*/
- memcpy(&ControlFile_new, &ControlFile_source, sizeof(ControlFileData));
-
if (connstr_source)
{
- endrec = source->get_current_wal_insert_lsn(source);
- endtli = ControlFile_source.checkPointCopy.ThisTimeLineID;
+ if (ControlFile_source_after.state == DB_IN_ARCHIVE_RECOVERY)
+ {
+ /*
+ * Source is a standby server. We must replay to its
+ * minRecoveryPoint.
+ */
+ endrec = ControlFile_source_after.minRecoveryPoint;
+ endtli = ControlFile_source_after.minRecoveryPointTLI;
+ }
+ else
+ {
+ /*
+ * Source is a production, non-standby, server. We must recover up
+ * to the last WAL insert location.
+ */
+ if (ControlFile_source_after.state != DB_IN_PRODUCTION)
+ pg_fatal("source system was in unexpected state at end of rewind");
+
+ endrec = source->get_current_wal_insert_lsn(source);
+ endtli = ControlFile_source_after.checkPointCopy.ThisTimeLineID;
+ }
}
else
{
- endrec = ControlFile_source.checkPoint;
- endtli = ControlFile_source.checkPointCopy.ThisTimeLineID;
+ /*
+ * Source is a local data directory. It should've shut down cleanly,
+ * and we must to the latest shutdown checkpoint.
+ */
+ endrec = ControlFile_source_after.checkPoint;
+ endtli = ControlFile_source_after.checkPointCopy.ThisTimeLineID;
}
+
+ memcpy(&ControlFile_new, &ControlFile_source_after, sizeof(ControlFileData));
ControlFile_new.minRecoveryPoint = endrec;
ControlFile_new.minRecoveryPointTLI = endtli;
ControlFile_new.state = DB_IN_ARCHIVE_RECOVERY;
--
2.20.1
--------------BF34D0120055DC3839060F92--
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Subject: Re: [PATCH v2 5/5] Allow pg_rewind to use a standby server as the source system.
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