What's a bitmap:

A mdadm bitmap, also called a "write intent bitmap", is a mechanism to speed up RAID rebuilds after an unclean shutdown or after removing and re-adding a disk.

With a bitmap, writing data to the RAID goes like this:

• Update bitmap: Mark the RAID chunks you are about to write to as dirty.


• Write the data to the RAID.


• Update bitmap: Mark the RAID chunks that were just written as clean.

The advantage of a bitmap is that if the system goes down in the middle of a write, the rebuild needs to check only the chunks marked as dirty, rather than the whole multi-TB RAID. This can speed up the rebuild process from taking several hours to completing in just a few seconds.

The drawback is lower write performance under normal use (outside rebuilds), since mdadm does additional disk access to update the bitmap.

External vs internal:

• external: Stored as a file on a disk outside the RAID. The advantage over an internal bitmap is better write performance during normal use (outside rebuilds).


• internal: Stored as RAID metadata. The advantage over an external bitmap is that you don't need a non-RAID disk and you save a bit on configuration (the path to the bitmap).

According to the mdadm man page:

Note:  external bitmaps are only known to work on ext2 and ext3.

       Storing bitmap files on other filesystems may result in  serious

       problems.

According to a post by Neil Brown, the mdadm author, external bitmaps should work on ext4 too:

I haven't looked inside ext4 but I am fairly confident that external bitmaps 

will work properly.

HOWTO:

Bitmaps are added and removed using mdadm --grow --bitmap=XXX ..., where the XXX is one of:

• --bitmap=internal: Create an internal bitmap.


• --bitmap=/var/my_bitmap.bin: Create an external bitmap at the specified path. The path must reside outside the RAID. A bitmap=... parameter must be added to the ARRAY entry in /etc/mdadm/mdadm.conf, and the --bitmap=... parameter must be passed if you are assembling the RAID from the command line.


• --bitmap=none: Remove/disable any bitmaps.

References:

• HowTo: Speed Up Linux Software Raid Building And Re-syncing


• mdadm man page - see flag -b, --bitmap=


• mdadm.conf man page - see option bitmap

Comments:

IMO, bitmaps are perhaps primarily of interest for RAID levels 5 and 6, since these have the slowest rebuilds.

I switched from RAID 5 to RAID 10 myself; the rebuilds are so much faster that I don't feel the need for a bitmap, and RAID 10 seems to require far fewer rebuilds in the first place.

My RAID 5 setup used to drop a disk something like once a month, causing 12-14 hour rebuilds. The RAID 10 has only dropped a disk once in half a year, rebuilt in less than an hour.

I don't know if the frequent disk drops I experienced was caused by something other than the RAID level, but the RAID 10 has been far more stable and rebuild speed isn't much of a concern anymore.