Re: [RFC PATCH v1 0/5] BTRFS hot relocation support

[Kent Overstreet <koverstreet@google.com>]

On Tue, May 21, 2013 at 02:22:34AM +0000, Duncan wrote:
> zwu.kernel posted on Mon, 20 May 2013 23:11:22 +0800 as excerpted:
> 
> > The patchset is trying to introduce hot relocation support
> > for BTRFS. In hybrid storage environment, when the data in rotating disk
> > get hot, it can be relocated to nonrotating disk by BTRFS hot relocation
> > support automatically; also, if nonrotating disk ratio exceed its upper
> > threshold, the data which get cold can be looked up and relocated to
> > rotating disk to make more space in nonrotating disk at first, and then
> > the data which get hot will be relocated to nonrotating disk
> > automatically.
> 
> One advantage of a filesystem implementation, as opposed to bcache or 
> dmcache, is arguably a corner-case, but it's /my/ corner-case, so...
> 
> I run an intr*-less (I guess technically, empty initramfs) monolithic-
> kernel boot, using the kernel commandline root= and (formerly) md= and 
> related logic to choose/assemble/mount root directly from the kernel 
> command line via bootloader (grub2).  Thus, any user-space-required-to-
> mount-root is out, since I don't have an initr* and thus no early 
> userspace.  That means both lvm2 and dmcache (AFAIK) are out.  I'm not 
> sure about bcache, but it has other negatives, particularly against btrfs-
> raid-1 and I'd guess md/raid-1 as well.
> 
> Much like md before it, btrfs, while normally requiring the user-space-
> required device-scan to properly handle multiple devices, has kernel-
> command-line options that allow direct kernel multi-device assembly 
> without the help of early-userspace/initr*.

I wouldn't be averse to adding such functionality to bcache, provided it
could be done reasonably cleanly/sensibly. It's not high on my list but
I'd accept patches :)