add nosparse volflag + update s3 readme:

may improve upload performance in some particular uncommon scenarios,
for example if hdd-writes are uncached, and/or the hdd is drastically
slower than the network throughput

one particular usecase where nosparse *might* improve performance
is when the upload destination is cloud-storage provided by FUSE
(for example an s3 bucket) but this is educated guesswork

README.md

@@ -1545,12 +1545,16 @@ connecting to an aws s3 bucket and similar
 
 there is no built-in support for this, but you can use FUSE-software such as [rclone](https://rclone.org/) / [geesefs](https://github.com/yandex-cloud/geesefs) / [JuiceFS](https://juicefs.com/en/) to first mount your cloud storage as a local disk, and then let copyparty use (a folder in) that disk as a volume
 
-you may experience poor upload performance this way, but that can sometimes be fixed by specifying the volflag `sparse` to force the use of sparse files; this has improved the upload speeds from `1.5 MiB/s` to over `80 MiB/s` in one case, but note that you are also more likely to discover funny bugs in your FUSE software this way, so buckle up
+you will probably get decent speeds with the default config, however most likely restricted to using one TCP connection per file, so the upload-client won't be able to send multiple chunks in parallel
+
+> before [v1.13.5](https://github.com/9001/copyparty/releases/tag/v1.13.5) it was recommended to use the volflag `sparse` to force-allow multiple chunks in parallel; this would improve the upload-speed from `1.5 MiB/s` to over `80 MiB/s` at the risk of provoking latent bugs in S3 or JuiceFS. But v1.13.5 added chunk-stitching, so this is now probably much less important. On the contrary, `nosparse` *may* now increase performance in some cases. Please try all three options (default, `sparse`, `nosparse`) as the optimal choice depends on your network conditions and software stack (both the FUSE-driver and cloud-server)
 
 someone has also tested geesefs in combination with [gocryptfs](https://nuetzlich.net/gocryptfs/) with surprisingly good results, getting 60 MiB/s upload speeds on a gbit line, but JuiceFS won with 80 MiB/s using its built-in encryption
 
 you may improve performance by specifying larger values for `--iobuf` / `--s-rd-sz` / `--s-wr-sz`
 
+> if you've experimented with this and made interesting observations, please share your findings so we can add a section with specific recommendations :-)
+
 
 ## hiding from google
 

copyparty/cfg.py

@@ -144,6 +144,7 @@ flagcats = {
         "noclone": "take dupe data from clients, even if available on HDD",
         "nodupe": "rejects existing files (instead of linking/cloning them)",
         "sparse": "force use of sparse files, mainly for s3-backed storage",
+        "nosparse": "deny use of sparse files, mainly for slow storage",
         "daw": "enable full WebDAV write support (dangerous);\nPUT-operations will now \033[1;31mOVERWRITE\033[0;35m existing files",
         "nosub": "forces all uploads into the top folder of the vfs",
         "magic": "enables filetype detection for nameless uploads",

copyparty/up2k.py

@@ -4905,7 +4905,8 @@ class Up2k(object):
             except:
                 pass
 
-        xbu = self.flags[job["ptop"]].get("xbu")
+        vf = self.flags[job["ptop"]]
+        xbu = vf.get("xbu")
         ap_chk = djoin(pdir, job["name"])
         vp_chk = djoin(job["vtop"], job["prel"], job["name"])
         if xbu:
@@ -4935,7 +4936,7 @@ class Up2k(object):
                 if x:
                     zvfs = vfs
                     pdir, _, job["name"], (vfs, rem) = x
-                    job["vcfg"] = vfs.flags
+                    job["vcfg"] = vf = vfs.flags
                     job["ptop"] = vfs.realpath
                     job["vtop"] = vfs.vpath
                     job["prel"] = rem
@@ -4985,8 +4986,13 @@ class Up2k(object):
                 fs = self.fstab.get(pdir)
                 if fs == "ok":
                     pass
-                elif "sparse" in self.flags[job["ptop"]]:
-                    t = "volflag 'sparse' is forcing use of sparse files for uploads to [%s]"
+                elif "nosparse" in vf:
+                    t = "volflag 'nosparse' is preventing creation of sparse files for uploads to [%s]"
+                    self.log(t % (job["ptop"],))
+                    relabel = True
+                    sprs = False
+                elif "sparse" in vf:
+                    t = "volflag 'sparse' is forcing creation of sparse files for uploads to [%s]"
                     self.log(t % (job["ptop"],))
                     relabel = True
                 else: