Hi Stéphane, On 12/23/19 2:44 PM, Stéphane Lesimple wrote: > > Has this ever been considered to implement a feature so that metadata > chunks would always be allocated on a given set of disks part of the btrfs > filesystem? Yes, many times. > As metadata use can be intensive and some operations are known to be slow > (such as backref walking), I'm under the (maybe wrong) impression that > having a set of small ssd's just for the metadata would give quite a boost > to a filesystem. Maybe even make qgroups more usable with volumes having 10 > snapshots? No, it's not wrong. For bigger filesystems this would certainly help. > This could just be a preference set on the allocator, Yes. Now, the big question is, how do we 'just' set this preference? Be sure to take into account that the filesystem has no way to find out itself which disks are those ssds. There's no easy way to discover this in a running system. > so that a 6 disks > raid1 FS with 4 spinning disks and 2 ssds prefer to allocate metadata on > the ssd than on the slow drives (and falling back to spinning disks if ssds > are full, with the possibility to rebalance later). > > Would such a feature make sense? Absolutely. Hans
