On Thu, 25 Oct 2018, Paul Lawrence wrote: > Thank you for the suggestion. I spent part of yesterday experimenting with > this idea, and it is certainly very promising. However, it does have some > disadvantages as compared to dm-bow, if I am understanding the setup > correctly: > > 1) Since dm-snap has no concept of the free space on the underlying file > system any write into the free space will trigger a backup, so using twice the > space of dm-bow. Changing existing data will create a backup with both > drivers, but since we have to reserve the space for the backups up-front with > dm-snap, we would likely only have half the space for that. Either way, it > seems that dm-bow is likely to double the amount of changes we could make. > > (Might it be possible to dynamically resize the backup file if it is mostly > used up? This would fix the problem of only having half the space for changing Yes - the snapshot store can be extended while the snapshot is active. > existing data. The documentation seems to indicate that you can increase the > size of the snapshot partition, and it seems like it should be possible to > grow the underlying file without triggering a lot of writes. OTOH this would > have to happen in userspace which creates other issues.) > > 2) Similarly, since writes into free space do not trigger a backup in dm-bow, > dm-bow is likely to have a lower performance overhead in many circumstances. > On the flip side, dm-bow's backup is in free space and will collide with other > writes, so this advantage will reduce as free space fills up. But by choosing > a suitable algorithm for how we use free space we might be able to retain most > of this advantage. > > I intend to put together a fully working prototype of your suggestion next to > better compare with dm-bow. But I do believe there is value in tracking free > space and utilizing it in any such solution. The snapshot target could be hacked so that it remembers space trimmed with REQ_OP_DISCARD and won't reallocate these blocks. But I suspect that running discard over the whole device would degrade performance more than copying some unneeded data. How much data do you intend to backup with this solution? Mikulas
