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Commit3bdbdf5

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Introduce pg_pwrite_zeros() in fileutils.c
This routine is designed to write zeros to a file using vectored I/O,for a size given by its caller, being useful when it comes toinitializing a file with a final size already known.XLogFileInitInternal() in xlog.c is changed to use this new routine wheninitializing WAL segments with zeros (wal_init_zero enabled). Note thatthe aligned buffers used for the vectored I/O writes have a size ofXLOG_BLCKSZ, and not BLCKSZ anymore, as pg_pwrite_zeros() relies onPGAlignedBlock while xlog.c originally used PGAlignedXLogBlock.This routine will be used in a follow-up patch to do the pre-padding ofWAL segments for pg_receivewal and pg_basebackup when these are notcompressed.Author: Bharath RupireddyReviewed-by: Nathan Bossart, Andres Freund, Thomas Munro, MichaelPaquierDiscussion:https://www.postgresql.org/message-id/CALj2ACUq7nAb7%3DbJNbK3yYmp-SZhJcXFR_pLk8un6XgDzDF3OA%40mail.gmail.com
1 parentd7744d5 commit3bdbdf5

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3 files changed

+80
-28
lines changed

3 files changed

+80
-28
lines changed

‎src/backend/access/transam/xlog.c‎

Lines changed: 5 additions & 28 deletions
Original file line numberDiff line numberDiff line change
@@ -2921,7 +2921,6 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
29212921
bool*added,char*path)
29222922
{
29232923
chartmppath[MAXPGPATH];
2924-
PGAlignedXLogBlockzbuffer;
29252924
XLogSegNoinstalled_segno;
29262925
XLogSegNomax_segno;
29272926
intfd;
@@ -2965,14 +2964,11 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
29652964
(errcode_for_file_access(),
29662965
errmsg("could not create file \"%s\": %m",tmppath)));
29672966

2968-
memset(zbuffer.data,0,XLOG_BLCKSZ);
2969-
29702967
pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_WRITE);
29712968
save_errno=0;
29722969
if (wal_init_zero)
29732970
{
2974-
structioveciov[PG_IOV_MAX];
2975-
intblocks;
2971+
ssize_trc;
29762972

29772973
/*
29782974
* Zero-fill the file. With this setting, we do this the hard way to
@@ -2983,29 +2979,10 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
29832979
* indirect blocks are down on disk. Therefore, fdatasync(2) or
29842980
* O_DSYNC will be sufficient to sync future writes to the log file.
29852981
*/
2982+
rc=pg_pwrite_zeros(fd,wal_segment_size);
29862983

2987-
/* Prepare to write out a lot of copies of our zero buffer at once. */
2988-
for (inti=0;i<lengthof(iov);++i)
2989-
{
2990-
iov[i].iov_base=zbuffer.data;
2991-
iov[i].iov_len=XLOG_BLCKSZ;
2992-
}
2993-
2994-
/* Loop, writing as many blocks as we can for each system call. */
2995-
blocks=wal_segment_size /XLOG_BLCKSZ;
2996-
for (inti=0;i<blocks;)
2997-
{
2998-
intiovcnt=Min(blocks-i,lengthof(iov));
2999-
off_toffset=i*XLOG_BLCKSZ;
3000-
3001-
if (pg_pwritev_with_retry(fd,iov,iovcnt,offset)<0)
3002-
{
3003-
save_errno=errno;
3004-
break;
3005-
}
3006-
3007-
i+=iovcnt;
3008-
}
2984+
if (rc<0)
2985+
save_errno=errno;
30092986
}
30102987
else
30112988
{
@@ -3014,7 +2991,7 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
30142991
* enough.
30152992
*/
30162993
errno=0;
3017-
if (pg_pwrite(fd,zbuffer.data,1,wal_segment_size-1)!=1)
2994+
if (pg_pwrite(fd,"\0",1,wal_segment_size-1)!=1)
30182995
{
30192996
/* if write didn't set errno, assume no disk space */
30202997
save_errno=errno ?errno :ENOSPC;

‎src/common/file_utils.c‎

Lines changed: 73 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -527,3 +527,76 @@ pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, off_t offset)
527527

528528
returnsum;
529529
}
530+
531+
/*
532+
* pg_pwrite_zeros
533+
*
534+
* Writes zeros to file worth "size" bytes, using vectored I/O.
535+
*
536+
* Returns the total amount of data written. On failure, a negative value
537+
* is returned with errno set.
538+
*/
539+
ssize_t
540+
pg_pwrite_zeros(intfd,size_tsize)
541+
{
542+
PGAlignedBlockzbuffer;/* worth BLCKSZ */
543+
size_tzbuffer_sz;
544+
structioveciov[PG_IOV_MAX];
545+
intblocks;
546+
size_tremaining_size=0;
547+
inti;
548+
ssize_twritten;
549+
ssize_ttotal_written=0;
550+
551+
zbuffer_sz=sizeof(zbuffer.data);
552+
553+
/* Zero-fill the buffer. */
554+
memset(zbuffer.data,0,zbuffer_sz);
555+
556+
/* Prepare to write out a lot of copies of our zero buffer at once. */
557+
for (i=0;i<lengthof(iov);++i)
558+
{
559+
iov[i].iov_base=zbuffer.data;
560+
iov[i].iov_len=zbuffer_sz;
561+
}
562+
563+
/* Loop, writing as many blocks as we can for each system call. */
564+
blocks=size /zbuffer_sz;
565+
remaining_size=size %zbuffer_sz;
566+
for (i=0;i<blocks;)
567+
{
568+
intiovcnt=Min(blocks-i,lengthof(iov));
569+
off_toffset=i*zbuffer_sz;
570+
571+
written=pg_pwritev_with_retry(fd,iov,iovcnt,offset);
572+
573+
if (written<0)
574+
returnwritten;
575+
576+
i+=iovcnt;
577+
total_written+=written;
578+
}
579+
580+
/* Now, write the remaining size, if any, of the file with zeros. */
581+
if (remaining_size>0)
582+
{
583+
/* We'll never write more than one block here */
584+
intiovcnt=1;
585+
586+
/* Jump on to the end of previously written blocks */
587+
off_toffset=i*zbuffer_sz;
588+
589+
iov[0].iov_len=remaining_size;
590+
591+
written=pg_pwritev_with_retry(fd,iov,iovcnt,offset);
592+
593+
if (written<0)
594+
returnwritten;
595+
596+
total_written+=written;
597+
}
598+
599+
Assert(total_written==size);
600+
601+
returntotal_written;
602+
}

‎src/include/common/file_utils.h‎

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -44,4 +44,6 @@ extern ssize_t pg_pwritev_with_retry(int fd,
4444
intiovcnt,
4545
off_toffset);
4646

47+
externssize_tpg_pwrite_zeros(intfd,size_tsize);
48+
4749
#endif/* FILE_UTILS_H */

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