On Sat, Jan 18, 2020 at 11:06:50PM +1100, Andrew Greig wrote: > Yes, the problem was my Motherboard would not handle enough disks, and we > did Format sdc with btrfs and left the sdb alone so that btrfs could arrange > things between them. > > I was hoping to get an understanding of how the RAID drives remembered the > "Balance" command when the the whole of the root filesystem was replaced on > a new SSD.
Your rootfs and your /data filesystem(*) are entirely separate. Don't confuse them. The /data filesystem needed to be re-balanced when you added the second drive (making it into a raid-1 array). 'btrfs balance' reads and rewrites all the existing data on a btrfs filesystem so that it is distributed equally over all drives in the array. For RAID-1, that means mirroring all the data on the first drive onto the second, so that there's a redundant copy of everything. Your rootfs is only a single partition, it doesn't have a raid-1 mirror, so re-balancing isn't necessary (and would do nothing). BTW, there's nothing being "remembered". 'btrfs balance' just re-balances the existing data over all drives in the array. It's a once-off operation that runs to completion and then exits. All **NEW** data will be automatically distributed across the array. If you ever add another drive to the array, or convert it to raid-0 (definitely NOT recommended), you'll need to re-balance it again. until and unless that happens you don't need to even think about re-balancing, it's no longer relevant. (*) I think you had your btrfs raid array mounted at /data, but I may be mis-remembering that. To the best of my knowledge, you have two entirely separate btrfs filesystems - one is the root filesystem, mounted as / (it also has /home on it, which IIRC you have made a separate btrfs sub-volume for). Anyway, it's a single-partition btrfs fs with no raid. The other is a 2 drive btrfs fs using raid-1, which I think is mounted as /data. > I thought that control would have rested with /etc/fstab. How do the > drives know to balance themselves, is there a command resident in sdc1? /etc/fstab tells the system which filesystems to mount. It gets read at boot time by the system start up scripts. > My plan is to have auto backups, and given that my activity has seen an SSD > go down in 12 months, maybe at 10 months I should build a new box, something > which will handle 64Gb RAM and have a decent Open Source Graphics driver. > And put the / on a pair of 1Tb SSDs. That would be a very good idea. Most modern motherboards will have more than enough NVME and SATA slots for that (e.g. most Ryzen x570 motherboards have 2 or 3 NVME slots for extremely fast SSDs, plus 6 or 8 SATA ports for SATA HDDs and SSDs. They also have enough RAM slots for 64GB DDR-4 RAM, and have at least 2 or 3 PCI-e v4 slots - you'll use one for your graphics card). 2 SSDs for the rootfs including your home dir, and 2 HDDs for your /data bulk storage filesystem. And more than enough drive ports for future expansion if you ever need it. ----------------------- some info on nvme vs sata: NVME SSDs are **much** faster then SATA SSDs. SATA 3 is 6 Gbps (600 MBps), so taking protocol overhead into account SATA drives max out at around 550 MBps. NVME drives run at **up to** PCI-e bus speeds - with 4 lanes, that's a little under 40 Gbps for PCIe v3 (approx 4000 MBps minus protocol overhead), double that for PCIe v4. That's the theoretical maximum speed, anyway. In practice, most NVME SSDs run quite a bit slower than that, about 2 GBps - that's still almost 4 times as fast as a SATA SSD. Some brands and models (e.g. those from samsung and crucial) run at around 3200 to 3500 MBps, but they cost more (e.g. a 1TB Samsung 970 EVO PLUS (MZ-V7S1T0BW) costs around $300, while the 1TB Kingston A2000 (SA2000M8/1000G) costs around $160 but is only around 1800 MBps). AFAIK there are no NVME drives that run at full PCI-e v4 speed (~8 GBps with 4 lanes) yet, it's still too new. That's not a problem, PCI-e is designed to be backwards-compatible with earlier versions, so any current NVME drive will work in pcie v4 slots. NVME SSDs cost about the same as SATA SSDs of the same capacity so there's no reason not to get them if your motherboard has NVME slots (which are pretty much standard these days). BTW, the socket that NVME drives plug into is called "M.2". M.2 supports both SATA & NVME protocols. SATA M.2 runs at 6 Gbps. NVME runs at PCI-e bus speed. So you have to be careful when you buy to make sure you get an NVME M.2 drive and not a SATA drive in M.2 form-factor...some retailers will try to exploit the confusion over this. craig -- craig sanders <c...@taz.net.au> _______________________________________________ luv-main mailing list luv-main@luv.asn.au https://lists.luv.asn.au/cgi-bin/mailman/listinfo/luv-main
