I tried thin pools and got the same snapshot creation time, about .45s. I also tried creating the entire vg in memory with losetup and that didn't change the creation time either. On 2020-02-25 04:38, Zdenek Kabelac wrote: > Dne 22. 02. 20 v 12:58 Eric Toombs napsal(a): >> Snapshot creation is already pretty fast: >> >>> $ time sudo lvcreate --size 512M --snapshot --name snap >>> /dev/testdbs/template >>> Logical volume "snap" created. >>> 0.03user 0.05system 0:00.46elapsed 18%CPU (0avgtext+0avgdata >>> 28916maxresident)k >>> 768inputs+9828outputs (0major+6315minor)pagefaults 0swaps >> >> That's about half a second in real time. But I have a scenario that >> would benefit from it being even faster. I'm doing many small unit tests >> starting from a template filesystem. I do the snapshot, run the unit >> test on the snapshot, then delete the snapshot afterwards using >> lvremove. Each unit test, though, takes much less than a second to run >> (often on the order of 10ms), so most of the time is being spent making >> these snapshots. >> >> So, is there a sort of "dumber" way of making these snapshots, maybe by >> changing the allocation algorithm or something? > > Hi > > IMHO - what takes most of the time are these couple things: > > Each command has 'non-trivial' time overhead on its startup (scanning > you system with devices and validating everything) > > For old snapshots - COW are needs to be 'created' & 'zeroed' as > separate LV. > > Then you need to 'flush' all existing IO on origin device (so it's in > the consistent states - i.e. the filesystem synchronizes all it's > content in its metadata) - this all takes some measurable amount of time. > > You can 'prepare' empty zeroed LV ahead of time and then use > 'lvconvert' to attach snapshot (with -Zn) - this should speed-up > attachment > of snapshot. For the 'second' point you could likely issue 'sync' ahead > of time so most buffers will be flushed (if there is no big IO traffic). > > Saying all this - why are you using old snapshot when you are targeting > for performance ?? > > You really should consider usage of thin-pool - where you could chain a > long series of snapshot without having a dramatic performance > degradation of the whole IO throughput - old snapshot are really meant > to be used only if you want to take i.e. backup of a filesystem and you > need some 'consistent' point in time - for everything else you should be > using thin-pools nowdays... > > Regards > > Zdenek > _______________________________________________ linux-lvm mailing list linux-lvm@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
