[dm-devel] software raid & intent bitmap on other device

[Brassow Jonathan]

On May 30, 2013, at 5:25 AM, folkert wrote:

> Hi,
> 
> I'm going to setup a software raid-1 and would like to improve its
> performance by storing things like the write intent bitmap (and fs
> journals but that's not relevant here) on a seperate disk. In this case
> and SSD.

"mirror" segment type or "raid1" segment type?

You can do this via device-mapper (i.e. 'dmsetup') for both target types.  However, if you are using LVM, you can only have separate placement of the log device with the "mirror" segment type.

> I was wondering: as SSDs frequently seem to die, what happens with the
> raid? Things will probably oops, panic and crash but after the reboot:
> will it refuse to come up? Or will it come up and just to a complete
> resync?

If you are using "mirror" segment type via LVM, it will simply convert to using an in-memory log.  However, that means that the entire mirror will need to be sync'ed if you deactivate the LV or reboot the machine.  (You could always replace the log on a different device though.)

So, there are a few options, but I'd be interested to see if there is any real benefit to putting the log on an SSD.

 brassow