On Fri, Sep 25, 2020 at 8:22 AM Damien Le Moal <Damien.LeMoal@xxxxxxx> wrote: > > On 2020/09/25 2:20, Kanchan Joshi wrote: > > On Tue, Sep 8, 2020 at 8:48 PM hch@xxxxxxxxxxxxx <hch@xxxxxxxxxxxxx> wrote: > >> > >> On Mon, Sep 07, 2020 at 12:31:42PM +0530, Kanchan Joshi wrote: > >>> But there are use-cases which benefit from supporting zone-append on > >>> raw block-dev path. > >>> Certain user-space log-structured/cow FS/DB will use the device that > >>> way. Aerospike is one example. > >>> Pass-through is synchronous, and we lose the ability to use io-uring. > >> > >> So use zonefs, which is designed exactly for that use case. > > > > Not specific to zone-append, but in general it may not be good to lock > > new features/interfaces to ZoneFS alone, given that direct-block > > interface has its own merits. > > Mapping one file to a one zone is good for some use-cases, but > > limiting for others. > > Some user-space FS/DBs would be more efficient (less meta, indirection) > > with the freedom to decide file-to-zone mapping/placement. > > There is no metadata in zonefs. One file == one zone and the mapping between > zonefs files and zones is static, determined at mount time simply using report > zones. Zonefs files cannot be renamed nor deleted in anyway. Choosing a zonefs > file *is* the same as choosing a zone. Zonfes is *not* a POSIX file system doing > dynamic block allocation to files. The backing storage of files in zonefs is > static and fixed to the zone they represent. The difference between zonefs vs > raw zoned block device is the API that has to be used by the application, that > is, file descriptor representing the entire disk for raw disk vs file descriptor > representing one zone in zonefs. Note that the later has *a lot* of advantages > over the former: enables O_APPEND use, protects against bugs with user write > offsets mistakes, adds consistency of cached data against zone resets, and more. > > > - Rocksdb and those LSM style DBs would map SSTable to zone, but > > SSTable file may be two small (initially) and may become too large > > (after compaction) for a zone. > > You are contradicting yourself here. If a SSTable is mapped to a zone, then its > size cannot exceed the zone capacity, regardless of the interface used to access > the zones. And except for L0 tables which can be smaller (and are in memory > anyway), all levels tables have the same maximum size, which for zoned drives > must be the zone capacity. In any case, solving any problem in this area does > not depend in any way on zonefs vs raw disk interface. The implementation will > differ depending on the chosen interface, but what needs to be done to map > SSTables to zones is the same in both cases. > > > - The internal parallelism of a single zone is a design-choice, and > > depends on the drive. Writing multiple zones parallely (striped/raid > > way) can give better performance than writing on one. In that case one > > would want to file that seamlessly combines multiple-zones in a > > striped fashion. > > Then write a FS for that... Or have a library do it in user space. For the > library case, the implementation will differ for zonefs vs raw disk due to the > different API (regular file vs block devicer file), but the principles to follow > for stripping zones into a single storage object remain the same. ZoneFS is better when it is about dealing at single-zone granularity, and direct-block seems better when it is about grouping zones (in various ways including striping). The latter case (i.e. grouping zones) requires more involved mapping, and I agree that it can be left to application (for both ZoneFS and raw-block backends). But when an application tries that on ZoneFS, apart from mapping there would be additional cost of indirection/fd-management (due to file-on-files). And if new features (zone-append for now) are available only on ZoneFS, it forces application to use something that maynot be most optimal for its need. Coming to the original problem of plumbing append - I think divergence started because RWF_APPEND did not have any meaning for block device. Did I miss any other reason? How about write-anywhere semantics (RWF_RELAXED_WRITE or RWF_ANONYMOUS_WRITE flag) on block-dev. Zone-append works a lot like write-anywhere on block-dev (or on any other file that combines multiple-zones, in non-sequential fashion). > > Also it seems difficult (compared to block dev) to fit simple-copy TP > > in ZoneFS. The new > > command needs: one NVMe drive, list of source LBAs and one destination > > LBA. In ZoneFS, we would deal with N+1 file-descriptors (N source zone > > file, and one destination zone file) for that. While with block > > interface, we do not need more than one file-descriptor representing > > the entire device. With more zone-files, we face open/close overhead too. > > Are you expecting simple-copy to allow requests that are not zone aligned ? I do > not think that will ever happen. Otherwise, the gotcha cases for it would be far > too numerous. Simple-copy is essentially an optimized regular write command. > Similarly to that command, it will not allow copies over zone boundaries and > will need the destination LBA to be aligned to the destination zone WP. I have > not checked the TP though and given the NVMe NDA, I will stop the discussion here. TP is ratified, if that is the problem you are referring to. > filesend() could be used as the interface for simple-copy. Implementing that in > zonefs would not be that hard. What is your plan for simple-copy interface for > raw block device ? An ioctl ? filesend() too ? As as with any other user level > API, we should not be restricted to a particular device type if we can avoid it, > so in-kernel emulation of the feature is needed for devices that do not have > simple-copy or scsi extended copy. filesend() seems to me like the best choice > since all of that is already implemented there. At this moment, ioctl as sync and io-uring for async. sendfile() and copy_file_range() takes two fds....with that we can represent copy from one source zone to another zone. But it does not fit to represent larger copy (from N source zones to one destination zone). Not sure if I am clear, perhaps sending RFC would be better for discussion on simple-copy. > As for the open()/close() overhead for zonefs, may be some use cases may suffer > from it, but my tests with LevelDB+zonefs did not show any significant > difference. zonefs open()/close() operations are way faster than for a regular > file system since there is no metadata and all inodes always exist in-memory. > And zonefs() now supports MAR/MOR limits for O_WRONLY open(). That can simplify > things for the user. > > > -- > Damien Le Moal > Western Digital Research -- Joshi
