Re: test_fsync file overrun - Mailing list pgsql-hackers

Jeff Janes wrote:
> test_fsync in tools/fsync pre-creates a 16MB file.  If it is given a number
> of iterations greater than 1024 (like one might use if trying to see what
> happens when NVRAM gets filled, or on a journaling file system), than one of
> the writes being timed will have to extend the size of the pre-created test
> file, which can greatly skew the results.
>
> This patch uses lseek to periodically restart at the beginning of the file,
> rather than writing past the end of it.

Oh, I never noticed that the later tests kept appending to the file
rather then overwriting it.  I have applied the attached fix for CVS
HEAD that just uses lseek() before each write group, as you suggested.
I have backpatched it to 8.4.X because the original code created 16GB
files in tests (yikes).

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +
Index: src/tools/fsync/test_fsync.c
===================================================================
RCS file: /cvsroot/pgsql/src/tools/fsync/test_fsync.c,v
retrieving revision 1.24
diff -c -c -r1.24 test_fsync.c
*** src/tools/fsync/test_fsync.c    10 Aug 2009 18:19:06 -0000    1.24
--- src/tools/fsync/test_fsync.c    21 Sep 2009 16:52:00 -0000
***************
*** 149,156 ****
--- 149,160 ----
          die("Cannot open output file.");
      gettimeofday(&start_t, NULL);
      for (i = 0; i < loops; i++)
+     {
          if (write(tmpfile, buf, WRITE_SIZE) != WRITE_SIZE)
              die("write failed");
+         if (lseek(tmpfile, 0, SEEK_SET) == -1)
+             die("seek failed");
+     }
      gettimeofday(&elapse_t, NULL);
      close(tmpfile);
      printf("\tone 16k o_sync write   ");
***************
*** 167,172 ****
--- 171,178 ----
              die("write failed");
          if (write(tmpfile, buf, WRITE_SIZE / 2) != WRITE_SIZE / 2)
              die("write failed");
+         if (lseek(tmpfile, 0, SEEK_SET) == -1)
+             die("seek failed");
      }
      gettimeofday(&elapse_t, NULL);
      close(tmpfile);
***************
*** 188,195 ****
--- 194,205 ----
          die("Cannot open output file.");
      gettimeofday(&start_t, NULL);
      for (i = 0; i < loops; i++)
+     {
          if (write(tmpfile, buf, WRITE_SIZE / 2) != WRITE_SIZE / 2)
              die("write failed");
+         if (lseek(tmpfile, 0, SEEK_SET) == -1)
+             die("seek failed");
+     }
      gettimeofday(&elapse_t, NULL);
      close(tmpfile);
      printf("\topen o_dsync, write    ");
***************
*** 205,212 ****
--- 215,226 ----
          die("Cannot open output file.");
      gettimeofday(&start_t, NULL);
      for (i = 0; i < loops; i++)
+     {
          if (write(tmpfile, buf, WRITE_SIZE / 2) != WRITE_SIZE / 2)
              die("write failed");
+         if (lseek(tmpfile, 0, SEEK_SET) == -1)
+             die("seek failed");
+     }
      gettimeofday(&elapse_t, NULL);
      close(tmpfile);
      printf("\topen o_sync, write     ");
***************
*** 226,231 ****
--- 240,247 ----
          if (write(tmpfile, buf, WRITE_SIZE / 2) != WRITE_SIZE / 2)
              die("write failed");
          fdatasync(tmpfile);
+         if (lseek(tmpfile, 0, SEEK_SET) == -1)
+             die("seek failed");
      }
      gettimeofday(&elapse_t, NULL);
      close(tmpfile);
***************
*** 246,251 ****
--- 262,269 ----
              die("write failed");
          if (fsync(tmpfile) != 0)
              die("fsync failed");
+         if (lseek(tmpfile, 0, SEEK_SET) == -1)
+             die("seek failed");
      }
      gettimeofday(&elapse_t, NULL);
      close(tmpfile);
***************
*** 269,274 ****
--- 287,294 ----
              die("write failed");
          if (write(tmpfile, buf, WRITE_SIZE / 2) != WRITE_SIZE / 2)
              die("write failed");
+         if (lseek(tmpfile, 0, SEEK_SET) == -1)
+             die("seek failed");
      }
      gettimeofday(&elapse_t, NULL);
      close(tmpfile);
***************
*** 290,295 ****
--- 310,317 ----
              die("write failed");
          if (write(tmpfile, buf, WRITE_SIZE / 2) != WRITE_SIZE / 2)
              die("write failed");
+         if (lseek(tmpfile, 0, SEEK_SET) == -1)
+             die("seek failed");
      }
      gettimeofday(&elapse_t, NULL);
      close(tmpfile);
***************
*** 310,315 ****
--- 332,339 ----
          if (write(tmpfile, buf, WRITE_SIZE / 2) != WRITE_SIZE / 2)
              die("write failed");
          fdatasync(tmpfile);
+         if (lseek(tmpfile, 0, SEEK_SET) == -1)
+             die("seek failed");
      }
      gettimeofday(&elapse_t, NULL);
      close(tmpfile);
***************
*** 332,337 ****
--- 356,363 ----
              die("write failed");
          if (fsync(tmpfile) != 0)
              die("fsync failed");
+         if (lseek(tmpfile, 0, SEEK_SET) == -1)
+             die("seek failed");
      }
      gettimeofday(&elapse_t, NULL);
      close(tmpfile);