On Sun, 30 Apr 2017 13:04:36 +0100 Nix <nix@xxxxxxxxxxxxx> wrote: > Aside: the storage server I've just set up has a different rationale for > having multiple mds. There's one in the 'fast part' of the rotating > rust, and one in the 'slow part' (for big archival stuff that is rarely > written to); the slow one has an LVM PV directly atop it, but the fast > one has a bcache and then an LVM PV built atop that. The fast disk also > has an md journal on SSD. Both are joined into one LVM VG. (The > filesystem journals on the fast part are also on the SSD.) It's not like the difference between the so called "fast" and "slow" parts is 100- or even 10-fold. Just SSD-cache the entire thing (I prefer lvmcache not bcache) and go. > So I have a chunk of 'slow space' for things like ISOs and video files > that are rarely written to (so a RAID journal is needless) and never > want to be SSD-cached, and another (bigger) chunk of space for > everything else, SSD-cached for speed and RAID-journalled for powerfail > integrity. > > (... actually it's more complex than that: there is *also* a RAID-0 > containing an ext4 sans filesystem journal at the start of the disk for > transient stuff like build trees that are easily regenerated, rarely > needed more than once, and where journalling the writes or caching the > reads on SSD is a total waste of SSD lifespan. If *that* gets corrupted, > the boot machinery simply re-mkfses it.) You have too much time on your hands if you have nothing better to do than to babysit all that b/s. -- With respect, Roman -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
