On Mon, May 28, 2018 at 8:51 PM, frostyplanet via openzfs-developer < [email protected]> wrote:
> Greetings, I am currently learning ZFS code, having some questions about > spacemap writing and condensing. Is there some people can enlighten me on > the big picture (because I could not find articles about these elsewhere ): > You might find the following talks informative on this subject: ZFS overview and read / write code walkthrough: https://www.youtube.com/watch?v=ptY6-K78McY Part of this is about spacemap on-disk format: https://www.youtube.com/watch?v=uJGkyMxdNFE Log spacemap: https://www.youtube.com/watch?v=jj2IxRkl5bQ > > 1) Space map store on disk as dnode, writing dnode blocks again needs > allocation and free, is this a feed back loop and how to break this > cycling dependence ? > Yes, we call this "sync to convergence". The cycle is broken by overwriting the block in place, thus not requiring any change of the allocation information. This is safe since we are only overwriting blocks that were allocated in the current txg, so if we crash it's as if nothing happened. > > 2) During disk space near full situation, is it possible some syncing > will fail becauses of cannot allocate to space map appending writes ? （I > have seen some posts about failure to delete files on a full disk） > No, we reserve some free space so that this can't happen. It isn't really related to file deletion, which may fail due to a (since fixed) bug in the space checking code, or if we are extremely low on space (in which case you can still "zfs destroy" to free up space). See also zfs_space_check_t in dsl_synctask.h. > > 3) Is it possible to condense a spacemap even if disk space not enough to > allocate a new space? > If there's zero allocatable space, then you've hit a bug in ZFS, and your pool is now permanently read-only. Normally there is always enough free space to condense a spacemap. --matt > *openzfs <https://openzfs.topicbox.com/latest>* / openzfs-developer > <https://openzfs.topicbox.com/groups/developer/members> / Permalink > <https://openzfs.topicbox.com/groups/developer/discussions/Td20524f29bfb36ea-M43f6fd1390923df8c3a777cd> > Delivery > options <https://openzfs.topicbox.com/groups> > ------------------------------------------ openzfs: openzfs-developer Permalink: https://openzfs.topicbox.com/groups/developer/discussions/Td20524f29bfb36ea-Mc09706bb1d1dc7999b00fba7 Delivery options: https://openzfs.topicbox.com/groups
