Just thought id post my results here as there's not much information regarding this feature. From the limited information I've read about bitmaps, mdadm records that it is going to write to the array, writes the data and then delete the record if successful. If something happens during the write (eg. power outage), it will need to resync which takes many hours but with the log it just checks if it was doing a write and only fixes that section (or just loose the whole array?). There's a total of one single post (IIRC) of performance tests (which were crude) claiming 30% loss if the bitmap was internal and 3% if it was on a disk external to the array. I've found that you can enable/disable bitmaps on the fly without rebuilding/resyncing using --grow which is very handy. Code: mdadm /dev/md0 -G -b internal Code: mdadm /dev/md0 -G -b /location/of/external/bitmap Code: mdadm /dev/md0 -G -b none Here's my test results on a 7 disk RAID 5 array: Code: bonnie++ -n 0 -u 0 -d /path/to/folder/on/device/to/test/ | bon_csv2html > /var/www/results.html No bitmap Internal External And heres the test of my POS IDE hdd that had the external bitmap Results aren't 100% but i don't have the time to do more. I'm not sure how it compares to an expensive RAID card with a battery as backup. If anyone has any information regarding this feature please post, unless my google skill sucks i found like 3 or 4 useful websites which only mention the basic concept and what commands to type in to enable it.