b2: reduce default chunk size to 16M and increase upload concurrency to 8

The 96M default chunk size dates from 2016 when 100,000,000 bytes was
the minimum part size B2 allowed. The minimum is now 5,000,000 bytes.

Measurements against B2 EU on a 1 Gbit/s link show that each part has
a fixed overhead of about a second regardless of size, so throughput
is governed by chunk size multiplied by upload concurrency rather than
by chunk size alone. 16M chunks with concurrency 8 upload a 1 GiB file
as fast as 96M chunks with concurrency 4 while using a third of the
memory per transfer, and retry much less data when a part fails.

This also means that files between --multi-thread-cutoff and the chunk
size are no longer uploaded as a single part large file, which B2
rejects with "large files must have at least 2 parts", unless the
cutoff is set below 16M.

backend/b2/b2.go

@@ -64,7 +64,7 @@ const (
 	maxParts            = 10000
 	maxVersions         = 100 // maximum number of versions we search in --b2-versions mode
 	minChunkSize        = 5 * fs.Mebi
-	defaultChunkSize    = 96 * fs.Mebi
+	defaultChunkSize    = 16 * fs.Mebi
 	defaultUploadCutoff = 200 * fs.Mebi
 	largeFileCopyCutoff = 4 * fs.Gibi // 5E9 is the max
 	defaultMaxAge       = 24 * time.Hour
@@ -153,7 +153,12 @@ The minimum is 0 and the maximum is 4.6 GiB.`,
 When uploading large files, chunk the file into this size.
 
 Must fit in memory. These chunks are buffered in memory and there
-might a maximum of "--transfers" chunks in progress at once.
+might be a maximum of "--transfers" * "--b2-upload-concurrency"
+chunks in progress at once.
+
+B2 has a fixed per part overhead of about a second, so upload speed
+is roughly proportional to the chunk size multiplied by
+"--b2-upload-concurrency".
 
 5,000,000 Bytes is the minimum size.`,
 			Default:  defaultChunkSize,
@@ -168,7 +173,7 @@ concurrently.
 Note that chunks are stored in memory and there may be up to
 "--transfers" * "--b2-upload-concurrency" chunks stored at once
 in memory.`,
-			Default:  4,
+			Default:  8,
 			Advanced: true,
 		}, {
 			Name: "disable_checksum",

docs/content/b2.md

@@ -169,8 +169,9 @@ to load your computer, etc.  The default of `--transfers 4` is
 definitely too low for Backblaze B2 though.
 
 Note that uploading big files (bigger than 200 MiB by default) will use
-a 96 MiB RAM buffer by default.  There can be at most `--transfers` of
-these in use at any moment, so this sets the upper limit on the memory
+`--b2-upload-concurrency` (8 by default) buffers of `--b2-chunk-size`
+(16 MiB by default) each.  There can be at most `--transfers` files
+uploading at any moment, so this sets the upper limit on the memory
 used.
 
 ### Versions