What is "Enable Bitmap" on my Qnap NAS?

"Bitmap" is used to allow faster RAID rebuild times, but you will be able to recover without it if you have a proper RAID level enabled.

When an array has a write-intent bitmap, a spindle (a device, often a hard drive) can be removed and re-added, then only blocks changes since the removal (as recorded in the bitmap) will be resynced.

Therefore a write-intent bitmap reduces rebuild/recovery (md sync) time if:

• the machine crashes (unclean shutdown)
• one spindle is disconnected, then reconnected

If one spindle fails and has to be replaced, a bitmap makes no difference.

A write-intent bitmap:

• does not improve performance
• may cause a degradation in write performance, it varies upon:
  • the size of the chunk data (on the RAID device) mapped to each bit in the bitmap, as expressed by cat /proc/mdstat. The ratio (bitmap size / RAID device size )
  • workload profile (long sequences of writes are more impacted, as spindle heads go back and forth between the data zone and the bitmap zone)
• can be removed/added at any time

If I remember correctly, it allows you to recover data faster. (Only in certain RAID scenarios though.) Here's a link -ignore the bickering- that should help.