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:
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.
Best Answer
No. But if I understand correctly, the update of mdadm has replaced your configuration. It should've asked you if you wanted to keep the old one or replace it. If that is the case, then you can either use "sudo dpkg-reconfigure mdadm" or edit /etc/initramfs-tools/conf.d/mdadm so that BOOT_DEGRADED=true.