Re: Is it possible to have metadata-only device with no data?

[Roman Mamedov <rm@romanrm.net>]

On Sun, 5 Feb 2017 22:55:42 +0100
Hans van Kranenburg <hans.van.kranenburg@mendix.com> wrote:

> On 02/05/2017 10:42 PM, Alexander Tomokhov wrote:
> > Is it possible, having two drives to do raid1 for metadata but keep data on a single drive only?
> 
> Nope.
> 
> Would be a really nice feature though... Putting metadata on SSD and
> bulk data on HDD...
> 

You can play around with this hack just to see how that would perform, but it
comes with no warranty and untested even by me. I was going to try it, but put
it on hold since you'd also need to make sure the SSD is being preferred for
metadata reads (and not HDD), but so far have not figured out a simple way of
ensuring that.

--- linux-amd64-4.4/fs/btrfs/volumes.c.orig	2016-11-01 22:41:41.970978721 +0500
+++ linux-amd64-4.4/fs/btrfs/volumes.c	2016-11-01 22:58:45.958977731 +0500
@@ -4597,6 +4597,14 @@
 		if (total_avail == 0)
 			continue;
 
+		/* If we have two devices and one is less than 25% of the total FS size, then
+		 * presumably it's a small device just for metadata RAID1, don't use it
+		 * for new data chunks. */
+		if ((fs_devices->num_devices == 2) &&
+				(device->total_bytes * 4 < fs_devices->total_rw_bytes) &&
+				(type & BTRFS_BLOCK_GROUP_DATA))
+			continue;
+
 		ret = find_free_dev_extent(trans, device,
 					   max_stripe_size * dev_stripes,
 					   &dev_offset, &max_avail);


-- 
With respect,
Roman