tdb2: overallocate the recovery area.

I noticed a counter-intuitive phenomenon as I tweaked the coalescing
code: the more coalescing we did, the larger the tdb grew!  This was
measured using "growtdb-bench 250000 10".

The cause: more coalescing means larger transactions, and every time
we do a larger transaction, we need to allocate a larger recovery
area.  The only way to do this is to append to the file, so the file
keeps growing, even though it's mainly unused!

Overallocating by 25% seems reasonable, and gives better results in
such benchmarks.

The real fix is to reduce the transaction to a run-length based format
rather then the naive block system used now.

Before:
$ time ./growtdb-bench 250000 10 > /dev/null && ls -l /tmp/growtdb.tdb && time ./tdbtorture -s 0 && ls -l torture.tdb && ./speed --transaction 2000000
real	0m57.403s
user	0m11.361s
sys	0m4.056s
-rw------- 1 rusty rusty 689536976 2011-04-27 21:10 /tmp/growtdb.tdb
testing with 3 processes, 5000 loops, seed=0
OK

real	1m24.901s
user	0m0.380s
sys	0m0.512s
-rw------- 1 rusty rusty 655368 2011-04-27 21:12 torture.tdb
Adding 2000000 records:  941 ns (110551992 bytes)
Finding 2000000 records:  603 ns (110551992 bytes)
Missing 2000000 records:  428 ns (110551992 bytes)
Traversing 2000000 records:  416 ns (110551992 bytes)
Deleting 2000000 records:  741 ns (199517112 bytes)
Re-adding 2000000 records:  819 ns (199517112 bytes)
Appending 2000000 records:  1228 ns (376542552 bytes)
Churning 2000000 records:  2042 ns (553641304 bytes)

After:
$ time ./growtdb-bench 250000 10 > /dev/null && ls -l /tmp/growtdb.tdb && time ./tdbtorture -s 0 && ls -l torture.tdb && ./speed --transaction 2000000
real	0m59.687s
user	0m11.593s
sys	0m4.100s
-rw------- 1 rusty rusty 752004064 2011-04-27 21:14 /tmp/growtdb.tdb
testing with 3 processes, 5000 loops, seed=0
OK

real	1m17.738s
user	0m0.348s
sys	0m0.580s
-rw------- 1 rusty rusty 663360 2011-04-27 21:15 torture.tdb
Adding 2000000 records:  926 ns (110556088 bytes)
Finding 2000000 records:  592 ns (110556088 bytes)
Missing 2000000 records:  416 ns (110556088 bytes)
Traversing 2000000 records:  422 ns (110556088 bytes)
Deleting 2000000 records:  741 ns (244003768 bytes)
Re-adding 2000000 records:  799 ns (244003768 bytes)
Appending 2000000 records:  1147 ns (295244592 bytes)
Churning 2000000 records:  1827 ns (568411440 bytes)

ccan/tdb2/transaction.c

@@ -701,10 +701,11 @@ static enum TDB_ERROR tdb_recovery_allocate(struct tdb_context *tdb,
 	/* the tdb_free() call might have increased the recovery size */
 	*recovery_size = tdb_recovery_size(tdb);
 
-	/* round up to a multiple of page size */
+	/* round up to a multiple of page size. Overallocate, since each
+	 * such allocation forces us to expand the file. */
 	*recovery_max_size
-		= (((sizeof(rec) + *recovery_size) + PAGESIZE-1)
-		   & ~(PAGESIZE-1))
+		= (((sizeof(rec) + *recovery_size + *recovery_size / 2)
+		    + PAGESIZE-1) & ~(PAGESIZE-1))
 		- sizeof(rec);
 	*recovery_offset = tdb->file->map_size;
 	recovery_head = *recovery_offset;