On Thu, Mar 31, 2016 at 12:18:19PM +0200, basti wrote: > Hello, > > I have a mdadm software raid with 3 disks so far so good. > > One disk (No. 3) is my backup disk which is connected once a week. > I have also a 2'nd Backup disk so one week backup disk 1 is connected to > other week backup disk 2 then backup disk 1 again and so on) > > Now I want to use an internal "Write-intent bitmap" because I update my > array to 2 TB disks and there is only USB 2 which takes to long to sync > the disks. > > Where is the bitmap saved?
You can either specify a file, or the word "internal" which means it will be stored in the RAID metadata area. "internal" is almost always safer. > How does backup disk know that is must sync changes of last 14 days? > when backup disk one was connected one week ago? It doesn't. When you connect the "backup" disk as a mirror to the existing mirror, mdadm decides that it is a damaged disk and attempts to repair it. The bitmap might help speed this up. > Is it possible to use a bitmap in this situation? Yes, but you should experiment to see whether this actually helps. Personally, I would consider making the backup disks look like separate mdadm RAID-1 devices, and using rsync to copy changed data across to them. If speed were important, I would strongly consider buying a PCI or PCIe USB3 card and upgrading the external enclosure(s) to USB3. -dsr-
