On Mon, Aug 15, 2016 at 8:30 PM, Hugo Mills <h...@carfax.org.uk> wrote: > On Mon, Aug 15, 2016 at 10:32:25PM +0800, Anand Jain wrote: >> >> >> On 08/15/2016 10:10 PM, Austin S. Hemmelgarn wrote: >> >On 2016-08-15 10:08, Anand Jain wrote: >> >> >> >> >> >>>>IMHO it's better to warn user about 2 devices RAID5 or 3 devices RAID6. >> >>>> >> >>>>Any comment is welcomed. >> >>>> >> >>>Based on looking at the code, we do in fact support 2/3 devices for >> >>>raid5/6 respectively. >> >>> >> >>>Personally, I agree that we should warn when trying to do this, but I >> >>>absolutely don't think we should stop it from happening.
About a year ago I had a raid5 array in an disk upgrade situation from 5x 2TB to 4x 4TB. As intermediate I had 2x 2TB + 2x 4TB situation for several weeks. The 2x 2TB were getting really full and the fs was slow. just wondering if an enospc would happen, I started an filewrite task doing several 100 GB's and it simply did work to my surprise. At some point, chunks only occupying the 4TB disks must have been created. I also saw the expected write rate on the 4TB disks. CPU load was not especially high as far as I remember, like a raid1 fs as far as I remember. So it is good that in such a situation, one can still use the fs. I don't remember how the allocated/free space accounting was, probably not correct, but I did not fill up the whole fs to see/experience that. I have no strong opinion whether we should warn about amount of devices at mkfs time for raid56. It's just that the other known issues with raid56 draw more attention. >> >> How does 2 disks RAID5 work ? >> >One disk is your data, the other is your parity. >> >> >> >In essence, it works >> >like a really computationally expensive version of RAID1 with 2 disks, >> >which is why it's considered a degenerate configuration. >> >> How do you generate parity with only one data ? > > For plain parity calculations, parity is the value p which solves > the expression: > > x_1 XOR x_2 XOR ... XOR x_n XOR p = 0 > > for corresponding bits in the n data volumes. With one data volume, > n=1, and hence p = x_1. > > What's the problem? :) > > Hugo. > >> -Anand >> >> >> > Three disks in >> >RAID6 is similar, but has a slight advantage at the moment in BTRFS >> >because it's the only way to configure three disks so you can lose two >> >and not lose any data as we have no support for higher order replication >> >than 2 copies yet. > > -- > Hugo Mills | I always felt that as a C programmer, I was becoming > hugo@... carfax.org.uk | typecast. > http://carfax.org.uk/ | > PGP: E2AB1DE4 | -- 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
