MDEV-38507: fadvise64() called on socat pipe by Mariabackup is useless overhead (ESPIPE)

Currently, posix_fadvise(..., 0, 0, POSIX_FADV_DONTNEED) is executed repeatedly after each my_write. In case of pipes, it evaluates to ESPIPE and fails. Because it executes after every write, this results in millions of redundant syscalls, introducing massive overhead.

As proposed by Daniel Black, posix_fadvise with a length/size of 0 (which equates to EOF) should be invoked once when the file is opened, rather than repetitively after every write. This change moves the fadvise call to the respective _open functions for ds_local, ds_stdout and ds_tmpfile datasinks. It fails fast on pipes early on and eliminates the redundant syscall overhead for subsequent write operations.

Author: kjarir

extra/mariabackup/ds_local.cc

@@ -136,6 +136,7 @@ local_open(ds_ctxt_t *ctxt, const char *path,
 	local_file = (ds_local_file_t *) (file + 1);
 
 	local_file->fd = fd;
+	posix_fadvise(local_file->fd, 0, 0, POSIX_FADV_DONTNEED);
 	local_file->init_ibd_done = 0;
 	local_file->is_ibd = (path_len > 5) && !strcmp(fullpath + path_len - 5, ".ibd");
 	local_file->compressed = 0;
@@ -166,9 +167,7 @@ static int write_compressed(File fd,  uchar *data, size_t len, size_t pagesize)
 		size_t n_bytes =  MY_MIN(pagesize, len - written);
 		size_t datasize= trim_binary_zeros(ptr,n_bytes);
 		if (datasize > 0) {
-			if (!my_write(fd, ptr, datasize, MYF(MY_WME | MY_NABP)))
-				posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED);
-			else
+			if (my_write(fd, ptr, datasize, MYF(MY_WME | MY_NABP)))
 				return 1;
 		}
 		if (datasize < n_bytes) {
@@ -242,11 +241,10 @@ local_write(ds_file_t *file, const uchar *buf, size_t len)
 		return write_compressed(fd, b, len, local_file->pagesize);
 	}
 
-	if (!my_write(fd, b , len, MYF(MY_WME | MY_NABP))) {
-		posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED);
-		return 0;
+	if (my_write(fd, b , len, MYF(MY_WME | MY_NABP))) {
+		return 1;
 	}
-	return 1;
+	return 0;
 }
 
 static

extra/mariabackup/ds_stdout.cc

@@ -84,6 +84,7 @@ stdout_open(ds_ctxt_t *ctxt __attribute__((unused)),
 #endif
 
 	stdout_file->fd = my_fileno(stdout);
+	posix_fadvise(stdout_file->fd, 0, 0, POSIX_FADV_DONTNEED);
 
 	file->path = (char *) stdout_file + sizeof(ds_stdout_file_t);
 	memcpy(file->path, fullpath, pathlen);
@@ -99,12 +100,11 @@ stdout_write(ds_file_t *file, const uchar *buf, size_t len)
 {
 	File fd = ((ds_stdout_file_t *) file->ptr)->fd;
 
-	if (!my_write(fd, buf, len, MYF(MY_WME | MY_NABP))) {
-		posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED);
-		return 0;
+	if (my_write(fd, buf, len, MYF(MY_WME | MY_NABP))) {
+		return 1;
 	}
 
-	return 1;
+	return 0;
 }
 
 static

extra/mariabackup/ds_tmpfile.cc

@@ -115,6 +115,7 @@ tmpfile_open(ds_ctxt_t *ctxt, const char *path,
 	tmp_file->orig_path = (char *) tmp_file + sizeof(ds_tmp_file_t);
 
 	tmp_file->fd = fd;
+	posix_fadvise(tmp_file->fd, 0, 0, POSIX_FADV_DONTNEED);
 	memcpy(tmp_file->orig_path, path, path_len);
 
 	/* Store the real temporary file name in file->path */
@@ -138,12 +139,11 @@ tmpfile_write(ds_file_t *file, const uchar *buf, size_t len)
 {
 	File fd = ((ds_tmp_file_t *) file->ptr)->fd;
 
-	if (!my_write(fd, buf, len, MYF(MY_WME | MY_NABP))) {
-		posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED);
-		return 0;
+	if (my_write(fd, buf, len, MYF(MY_WME | MY_NABP))) {
+		return 1;
 	}
 
-	return 1;
+	return 0;
 }
 
 static int