7

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

6

"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
  • 1
    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
  • 1
3

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
3

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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