/etc/dump makes bad tape use estimates when files have holes

[donn at sdchema.UUCP]

Subject: /etc/dump makes bad tape use estimates when files have holes
Index:	etc/dump/dumpitime.c 4.2BSD

Description:
	When you run dump, it typically prints out an estimate of the
	number of tapes it needs and the expected number of 1K blocks
	of data on those tapes.  If a file on the filesystem being
	dumped has holes in it, the estimate can be way off.  (A hole
	is a place in a file where no blocks have been allocated,
	typically found in database files.  See lseek(2) if you're
	mystified.)

Repeat-By:
	Make a file with holes in it and try to dump the file system.
	In our case dump came back with:

		DUMP: estimated 1794142 tape blocks on 50.53 tape(s).

	It should have said:

		DUMP: estimated 132608 tape blocks on 3.74 tape(s).

	This is unnerving to say the least.

Fix:
	The problem is in the routine est() in dumpitime.c.  According
	to the comment at the top:  'It assumes that there are no
	unallocated blocks; hence the estimate may be high[.]'  It's
	actually reasonably simple to get a good estimate of the number
	of blocks needed even when the file has holes; there is a field
	in the inode structure, di_blocks, which contains the number of
	sectors used by the file, not counting indirect blocks.  Here
	are the changes I made:

 ----------------------------------------------------------------
 RCS file: RCS/dumpitime.c,v
 retrieving revision 1.1
 diff -c -r1.1 dumpitime.c
 *** /tmp/,RCSt1002440	Tue Mar 27 00:52:19 1984
 --- dumpitime.c	Tue Mar 27 00:33:03 1984
 ***************
 *** 204,211

   /*
    * This is an estimation of the number of TP_BSIZE blocks in the file.
 !  * It assumes that there are no unallocated blocks; hence
 !  * the estimate may be high
    */
   est(ip)
 	struct dinode *ip;

 --- 204,211 -----

   /*
    * This is an estimation of the number of TP_BSIZE blocks in the file.
 !  * WARNING: It uses the di_blocks field of the inode, which is not
 !  * maintained in 4.1c BSD.
    */
   est(ip)
 	struct dinode *ip;
 ***************
 *** 210,216
   est(ip)
 	struct dinode *ip;
   {
 ! 	long s;

 	esize++;
 	/* calc number of TP_BSIZE blocks */

 --- 210,216 -----
   est(ip)
 	struct dinode *ip;
   {
 ! 	long s, t;

 	esize++;

 ***************
 *** 213,220
 	long s;

 	esize++;
 ! 	/* calc number of TP_BSIZE blocks */
 ! 	s = howmany(ip->di_size, TP_BSIZE);
 	if (ip->di_size > sblock->fs_bsize * NDADDR) {
 		/* calc number of indirect blocks on the dump tape */
 		s += howmany(s - NDADDR * sblock->fs_bsize / TP_BSIZE,

 --- 213,235 -----
 	long s, t;

 	esize++;
 !
 ! 	/*
 ! 	 * ip->di_size is the size of the file in bytes.
 ! 	 * ip->di_blocks stores the number of sectors actually in the file.
 ! 	 * If there are more sectors than the size would indicate, this just
 ! 	 *	means that there are unused sectors in the last file block;
 ! 	 *	we can safely ignore these (s = t below).
 ! 	 * If the file is bigger than the number of sectors would indicate,
 ! 	 *	then the file has holes in it.  In this case we must use the
 ! 	 *	block count for keeping track of actual blocks used, but we
 ! 	 *	use the actual size for estimating the number of indirect
 ! 	 *	blocks (t vs. s in the indirect block calculation).
 ! 	 */
 ! 	s = howmany(dbtob(ip->di_blocks), TP_BSIZE);
 ! 	t = howmany(ip->di_size, TP_BSIZE);
 ! 	if ( s > t )
 ! 		s = t;
 	if (ip->di_size > sblock->fs_bsize * NDADDR) {
 		/* calculate the number of indirect blocks on the dump tape */
 		s += howmany(t - NDADDR * sblock->fs_bsize / TP_BSIZE,
 ***************
 *** 216,223
 	/* calc number of TP_BSIZE blocks */
 	s = howmany(ip->di_size, TP_BSIZE);
 	if (ip->di_size > sblock->fs_bsize * NDADDR) {
 ! 		/* calc number of indirect blocks on the dump tape */
 ! 		s += howmany(s - NDADDR * sblock->fs_bsize / TP_BSIZE,
 			TP_NINDIR);
 	}
 	esize += s;

 --- 231,238 -----
 	if ( s > t )
 		s = t;
 	if (ip->di_size > sblock->fs_bsize * NDADDR) {
 ! 		/* calculate the number of indirect blocks on the dump tape */
 ! 		s += howmany(t - NDADDR * sblock->fs_bsize / TP_BSIZE,
 			TP_NINDIR);
 	}
 	esize += s;
 ----------------------------------------------------------------

Donn Seeley    UCSD Chemistry Dept.       ucbvax!sdcsvax!sdchema!donn
32 52' 30"N 117 14' 25"W  (619) 452-4016  sdcsvax!sdchema!donn at nosc.ARPA