Peter Becker posted on Sat, 10 Oct 2015 21:48:31 +0200 as excerpted: > btrfs balance start -m /media/RAID > > complete with out any error but the resulte of device usage is confusing > me. > Metadata on sdb and sdc are 2 GiB, but on sdd (the new added device) > is 4 GiB. And the 2. one that's confusing me, is that sdd has a "System" > entry but sdb and sdc dosn't > > floyd@nas ~ $ sudo btrfs dev us /media/RAID/ > /dev/sdb, ID: 1 > Device size: 2.73TiB > Data,RAID1: 2.11TiB > Metadata,RAID1: 2.00GiB > System,RAID1: 32.00MiB > Unallocated: 628.49GiB > > /dev/sdc, ID: 2 > Device size: 2.73TiB > Data,RAID1: 2.11TiB > Metadata,RAID1: 2.00GiB > Unallocated: 628.52GiB > > /dev/sdd, ID: 3 > Device size: 2.73TiB > Data,RAID1: 792.00GiB > Metadata,RAID1: 4.00GiB > System,RAID1: 32.00MiB > Unallocated: 1.95TiB
FWIW, there's also btrfs fi usage, which prints a somewhat different layout of pretty much the same statistics. It may be useful to compare output styles and choose the one you prefer. I prefer fi usage to dev usage in most cases, but YMMV. The key thing to remember about btrfs raid1 on more than two devices is that it's exactly two copies, not N copies, where N is the number of devices. In a three-device raid1, by definition, for each chunk that will mean one copy each on two devices, with the third device not getting a copy of that particular chunk, since btrfs raid1 is exactly two copies, no more, no less. So system is raid1, and sdb and sdd each have a copy of the (apparently just one) system chunk, one copy each for two copies total, leaving no system chunk to be placed on sdc, which is why it has none. And, given the stats, there are 4 GiB of raid1 metadata chunks comprising two copies of 2 GiB worth of metadata. Half that metadata has a copy each on sdb and sdd, while the other half has a copy each on sdc and sdd. IOW, sdd has a copy of all metadata, but sdb and sdc only have a copy of half the metadata each. Since the chunk allocator creates new chunks on the device with the most available space, subject to the restriction that for raid1, there's two copies and both copies cannot be on the same device, because sdd was recently added and thus the one most empty, when you ran the metadata balance, it created one copy of the raid1 two copies on the new device as it had the most free space, and then had to select one of the other two devices for the other copy. Since the other two devices were basically evenly filled, it alternated, selecting one and then the other, so each one got the second copy of half of the metadata, while the new device with the most free space got the first copy of all metadata as it was rewritten by the balance. -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
