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Use vectored I/O to fill new WAL segments. 

Instead of making many block-sized write() calls to fill a new WAL file
with zeroes, make a smaller number of pwritev() calls (or various
emulations).  The actual number depends on the OS's IOV_MAX, which
PG_IOV_MAX currently caps at 32.  That means we'll write 256kB per call
on typical systems.  We may want to tune the number later with more
experience.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKGJA%2Bu-220VONeoREBXJ9P3S94Y7J%2BkqCnTYmahvZJwM%3Dg%40mail.gmail.com
This commit is contained in:
Thomas Munro 2021-01-11 14:41:39 +13:00
parent 13a021f3e8
commit ce6a71fa53
1 changed files with 22 additions and 6 deletions
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28
src/backend/access/transam/xlog.c
View File

@ -48,6 +48,7 @@
#include "pg_trace.h"
#include "pgstat.h"
#include "port/atomics.h"
#include "port/pg_iovec.h"
#include "postmaster/bgwriter.h"
#include "postmaster/startup.h"
#include "postmaster/walwriter.h"
@ -3270,7 +3271,6 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
XLogSegNo installed_segno;
XLogSegNo max_segno;
int fd;
int nbytes;
int save_errno;
XLogFilePath(path, ThisTimeLineID, logsegno, wal_segment_size);
@ -3317,6 +3317,9 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
save_errno = 0;
if (wal_init_zero)
{
struct iovec iov[PG_IOV_MAX];
int blocks;
/*
* Zero-fill the file. With this setting, we do this the hard way to
* ensure that all the file space has really been allocated. On
@ -3326,15 +3329,28 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
* indirect blocks are down on disk. Therefore, fdatasync(2) or
* O_DSYNC will be sufficient to sync future writes to the log file.
*/
for (nbytes = 0; nbytes < wal_segment_size; nbytes += XLOG_BLCKSZ)
/* Prepare to write out a lot of copies of our zero buffer at once. */
for (int i = 0; i < lengthof(iov); ++i)
{
errno = 0;
if (write(fd, zbuffer.data, XLOG_BLCKSZ) != XLOG_BLCKSZ)
iov[i].iov_base = zbuffer.data;
iov[i].iov_len = XLOG_BLCKSZ;
}
/* Loop, writing as many blocks as we can for each system call. */
blocks = wal_segment_size / XLOG_BLCKSZ;
for (int i = 0; i < blocks;)
{
int iovcnt = Min(blocks - i, lengthof(iov));
off_t offset = i * XLOG_BLCKSZ;
if (pg_pwritev_with_retry(fd, iov, iovcnt, offset) < 0)
{
/* if write didn't set errno, assume no disk space */
save_errno = errno ? errno : ENOSPC;
save_errno = errno;
break;
}
i += iovcnt;
}
}
else