Gather writes to larger chunks (MAXPHYS) instead of issuing them in sectors.

[delphij]

Summary

Gather writes to larger chunks (MAXPHYS) instead of issuing them in
sectors.

On my SanDisk Cruzer Blade 16GB USB stick this made formatting much faster:

x before
+ after
+--------------------------------------------------------------------------+
|+                                                                         |
|+                                                                      x  |
|+                                                                      x x|
|A                                                                      MA||
+--------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   3         15.89         16.38            16         16.09     0.2570992
+   3          0.32          0.37          0.35    0.34666667   0.025166115
Difference at 95.0% confidence
        -15.7433 +/- 0.414029
        -97.8455% +/- 0.25668%
        (Student's t, pooled s = 0.182665)

[delphij]

Tested on a hard drive; this reduced the time for format by 2/3:

x before
+ after
+------------------------------------------------------------+
|+++                                                       xx|
||A|                                                       AM|
+------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   3          7.24          7.33          7.29     7.2866667   0.045092498
+   3          2.36          2.49           2.4     2.4166667   0.066583281
Difference at 95.0% confidence
	-4.87 +/- 0.128884
	-66.8344% +/- 1.50101%
	(Student's t, pooled s = 0.0568624)

[emaste]

sbin/newfs_msdos/mkfs_msdos.c

776–777

This requires that MAXPHYS % bpb.bpbBytesPerSec == 0 right? Do we check that anywhere?

[delphij]

Address reviewer comment.

[delphij]

sbin/newfs_msdos/mkfs_msdos.c

776–777

No, but bpbBytesPerSec has to be greater than the buffer size (I don't think that would ever happen, though).

[delphij]

Round down chunksize if it's not fully aligned.

[delphij]

Instead of padding the buffer, assert that they always fit as the specification requires
sector size to be one of 512, 1024, 2048 or 4096 bytes.

[delphij]

ministat graph for time savings.

x before
+ after
+--------------------------------------------------------------------------+
|+                                                                         |
|+                                                                      x  |
|+                                                                      x x|
|A                                                                      MA||
+--------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   3         15.89         16.38            16         16.09     0.2570992
+   3          0.32          0.37          0.35    0.34666667   0.025166115
Difference at 95.0% confidence
	-15.7433 +/- 0.414029
	-97.8455% +/- 0.25668%
	(Student's t, pooled s = 0.182665)

[delphij]

Minor style fix.