FUSE Mount: what df reports, and -df.logical

FUSE-Mount.md

@@ -405,6 +405,8 @@ weed mount \
 
 Once usage reaches the limit, space-consuming operations — writes, file and directory creation, hard/symbolic links, and extended-attribute updates — fail with `ENOSPC`. Reads and deletes keep working, so an over-quota mount can be drained back under its limit. `df` inside the mount reports the quota as the filesystem size, with the collection's used size as "used".
 
+The quota is measured against the space the collection occupies — replicas and EC shards included — the same figure [`df` reports](#what-does-df-output-means). Add [`-df.logical`](#reporting-data-sizes-instead--dflogical) to measure the data instead, so a 5 GiB quota on a `-replication=001` mount admits 5 GiB of files rather than 2.5 GiB.
+
 Notes:
 * The quota is **collection-scoped**, not per-directory. Usage is summed across the whole collection, so multiple mounts (or other clients) writing to the same collection share a single budget.
 * Enforcement is **mount-side**, not an authoritative server-side limit. A client that writes to the collection without setting a quota is not restricted, and two writers on separate mounts can briefly exceed the limit before the next usage poll catches up.
@@ -587,11 +589,39 @@ The issue is with samba.conf. If you see NT_STATUS_ACCESS_DENIED error, try to a
 
 
 ### What does "df" output means?  ###
+
+`df` reports the space the cluster gives up to the data, not the size of the data itself.
+
 ```
-Size: total number of volumes * volume size limit
-Used: (Logical Total Size of files - Logical Deleted File Size) * replication = physical disk size taken
+Size: max volume count across all volume servers * volume size limit
+Used: regular volumes: (file bytes - deleted bytes) * number of replicas
+      ec volumes:      every shard on disk, parity included
 Available = Size - Used
 ```
+
+So a file written with `-replication=001` is counted twice, and a 10+4 EC volume is counted as its 14 shards — about 1.4x the data it holds. Encoding a replicated volume therefore lowers `Used`, because 14 shards take less room than 2 replicas.
+
+`Size` is always cluster-wide. `Used` covers only the mount's collection when it was started with `-collection`, so on such a mount `Available` also counts what other collections have taken.
+
+#### Reporting data sizes instead (`-df.logical`)
+
+To have `df` answer "how much of my data is stored" rather than "how much room does it take", mount with `-df.logical`:
+
+```bash
+weed mount -filer=localhost:8888 -dir=/mnt/seaweedfs -df.logical
+```
+
+```
+Size: Used + Available
+Used: regular volumes: file bytes - deleted bytes, counted once
+      ec volumes:      data shards only, counted once
+Available = free space / number of copies the mount's replication makes
+```
+
+`Available` is scaled because the free space holds that many copies fewer of whatever you write next: on a `-replication=001` mount, 100 GiB of free disk accepts 50 GiB of new data. The replication used is the mount's `-replication` if set, otherwise the filer's default, otherwise the master's. Unlike the default output, this `Available` is computed from cluster-wide usage, so a collection-scoped mount is not told that another collection's space is free.
+
+The flag also switches `-collectionQuotaMB` to the same accounting, so a mount measures its usage one way.
+
 ### Can't mount as non-root user  ###
 From https://github.com/seaweedfs/seaweedfs/issues/877
 ```