Re: mdadm Write-intent bitmap
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-
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