why a bitmap file never changes?

I'm just synchronizing a local LVM volume to the remote one using md and a bitmap file, as suggested by Neil Brown ("how to synchronize two devices (RAID-1, but not really?)" thread from 15 May 2007):

mdadm --build /dev/md11 --level=1 --bitmap=/root/backup-bitmap \
--write-behind --raid-disks=2 /dev/san1/backup \
--write-mostly /dev/iscsi/....../part

Certainly, it takes a while to complete this operation.

I noticed that the bitmap file never changes as the devices are being synced - md5sum stays always the same, no matter if resync progress was 10, 30, or 90%:

# md5sum /root/backup-bitmap
e8e0c4a8424dfd4d487b8facd231ce76  /root/backup-bitmap

# ls -l /root/backup-bitmap
-rw-r--r-- 1 root root 80676 2007-08-27 18:35 /root/backup-bitmap

The date of the file is the date of creation of this array, and as I look inside, it's basically almost empty. When looking in a hex editor, zeroes (+bitm etc.)at the beginning, and than FF to the end.

Is it normal?

# mdadm -D /dev/md11
/dev/md11:
        Version : 00.90.03
  Creation Time : Mon Aug 27 18:35:32 2007
     Raid Level : raid1
     Array Size : 658800640 (628.28 GiB 674.61 GB)
    Device Size : 658800640 (628.28 GiB 674.61 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 11
    Persistence : Superblock is not persistent

Intent Bitmap : /root/backup-bitmap

Update Time : Mon Aug 27 18:39:22 2007
State : active, resyncing
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0

Rebuild Status : 91% complete

Number Major Minor RaidDevice State
0 254 47 0 active sync /dev/dm-47
1 8 48 1 active sync writemostly /dev/sdd

# cat /proc/mdstat
Personalities : [raid1]
md11 : active raid1 sdd[1](W) dm-47[0]
      658800640 blocks super non-persistent [2/2] [UU]
      [==================>..]  resync = 91.8% (605314368/658800640) 
finish=65.6min speed=13584K/sec
      bitmap: 315/315 pages [1260KB], 1024KB chunk, file: 
/root/backup-bitmap

Tomasz Chmielewski
http://wpkg.org

Here is some more info about the bitmap itself - it's 100% dirty, what does it mean?

# mdadm -X /root/backup-bitmap
        Filename : /root/backup-bitmap
           Magic : 6d746962
         Version : 4
            UUID : 00000000.00000000.00000000.00000000
          Events : 0
  Events Cleared : 0
           State : OK
       Chunksize : 1 MB
          Daemon : 5s flush period
      Write Mode : Allow write behind, max 256
       Sync Size : 658800640 (628.28 GiB 674.61 GB)
          Bitmap : 643360 bits (chunks), 643360 dirty (100.0%)

Tomasz Chmielewski
http://wpkg.org

After the drives synchronized, the bitmap file was no longer 100% dirty.

So, I guess it gets 100% dirty when there is a substantial activity going on? Like resync, for example.

Tomasz Chmielewski
http://wpkg.org