What is "Enable Bitmap" on my Qnap NAS? Will it allow me to recover data in case of failures?


"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.

  • I have RAID 5... So the only thing about it is speed, non increased safety, right? – Pitto Oct 13 '11 at 15:21
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    Yes, this is correct. The bitmap is said to reduce the speed a litle during normal operation while speeding up recovery. – Sven Oct 13 '11 at 15:22
  • Do I have a chance to disable it later? – Pitto Oct 21 '11 at 14:38
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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.

  • Nice link, though in fairness I feel that the bickering is fairly applicable in this case, too. – MadHatter Oct 13 '11 at 15:34
  • agreed, but not very suitable to helping everyone else get to the answer! – Ben Campbell Oct 13 '11 at 15:38

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

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