mount: improve read throughput with parallel chunk fetching (#7569)

* mount: improve read throughput with parallel chunk fetching

This addresses issue #7504 where a single weed mount FUSE instance
does not fully utilize node network bandwidth when reading large files.

Changes:
- Add -concurrentReaders mount option (default: 16) to control the
  maximum number of parallel chunk fetches during read operations
- Implement parallel section reading in ChunkGroup.ReadDataAt() using
  errgroup for better throughput when reading across multiple sections
- Enhance ReaderCache with MaybeCacheMany() to prefetch multiple chunks
  ahead in parallel during sequential reads (now prefetches 4 chunks)
- Increase ReaderCache limit dynamically based on concurrentReaders
  to support higher read parallelism

The bottleneck was that chunks were being read sequentially even when
they reside on different volume servers. By introducing parallel chunk
fetching, a single mount instance can now better saturate available
network bandwidth.

Fixes: #7504

* fmt

* Address review comments: make prefetch configurable, improve error handling

Changes:
1. Add DefaultPrefetchCount constant (4) to reader_at.go
2. Add GetPrefetchCount() method to ChunkGroup that derives prefetch count
   from concurrentReaders (1/4 ratio, min 1, max 8)
3. Pass prefetch count through NewChunkReaderAtFromClient
4. Fix error handling in readDataAtParallel to prioritize errgroup error
5. Update all callers to use DefaultPrefetchCount constant

For mount operations, prefetch scales with -concurrentReaders:
- concurrentReaders=16 (default) -> prefetch=4
- concurrentReaders=32 -> prefetch=8 (capped)
- concurrentReaders=4 -> prefetch=1

For non-mount paths (WebDAV, query engine, MQ), uses DefaultPrefetchCount.

* fmt

* Refactor: use variadic parameter instead of new function name

Use NewChunkGroup with optional concurrentReaders parameter instead of
creating a separate NewChunkGroupWithConcurrency function.

This maintains backward compatibility - existing callers without the
parameter get the default of 16 concurrent readers.

* Use explicit concurrentReaders parameter instead of variadic

* Refactor: use MaybeCache with count parameter instead of new MaybeCacheMany function

* Address nitpick review comments

- Add upper bound (128) on concurrentReaders to prevent excessive goroutine fan-out
- Cap readerCacheLimit at 256 accordingly
- Fix SetChunks: use Lock() instead of RLock() since we are writing to group.sections

1 files changed, 2 insertions, 0 deletions

diff --git a/weed/command/mount.go b/weed/command/mount.go
index 98f139c6f..618bbd3ae 100644
--- a/weed/command/mount.go
+++ b/weed/command/mount.go
@@ -17,6 +17,7 @@ type MountOptions struct {
 	ttlSec             *int
 	chunkSizeLimitMB   *int
 	concurrentWriters  *int
+	concurrentReaders  *int
 	cacheMetaTtlSec    *int
 	cacheDirForRead    *string
 	cacheDirForWrite   *string
@@ -65,6 +66,7 @@ func init() {
 	mountOptions.ttlSec = cmdMount.Flag.Int("ttl", 0, "file ttl in seconds")
 	mountOptions.chunkSizeLimitMB = cmdMount.Flag.Int("chunkSizeLimitMB", 2, "local write buffer size, also chunk large files")
 	mountOptions.concurrentWriters = cmdMount.Flag.Int("concurrentWriters", 32, "limit concurrent goroutine writers")
+	mountOptions.concurrentReaders = cmdMount.Flag.Int("concurrentReaders", 16, "limit concurrent chunk fetches for read operations")
 	mountOptions.cacheDirForRead = cmdMount.Flag.String("cacheDir", os.TempDir(), "local cache directory for file chunks and meta data")
 	mountOptions.cacheSizeMBForRead = cmdMount.Flag.Int64("cacheCapacityMB", 128, "file chunk read cache capacity in MB")
 	mountOptions.cacheDirForWrite = cmdMount.Flag.String("cacheDirWrite", "", "buffer writes mostly for large files")