* master: cap the reported capacity at what the disks hold
Statistics reported max volume count times the volume size limit, which is
how many volumes the cluster is allowed to place, not how much space it has.
A cluster given far more slots than its disks can fill reported a capacity it
could never reach -- 65536 slots at 30GB read as 1.9PB on a 460GB disk -- and
the number never moved, since writing data changes neither the slot count nor
the size limit.
The volume servers already report each filesystem's total and free bytes in
their heartbeats, so bound the answer by what they say is left.
* mount: keep the last known sizes when filer statistics fails
A failed Statistics call returned before df's answer was filled in, so a
mount whose filer or master was briefly unreachable reported an empty
filesystem rather than the sizes it already had.
* master: drop the disk ceiling when a volume server does not report
A cluster part way through an upgrade has volume servers that predate the disk
bytes in the heartbeat. Summing only the ones that answered left the quiet
server's free space out of the total, and the server holding the room is
exactly the one that could make the cluster read as full.
Answer with the disks only when every one of them reported.
df on a mount shows the space the cluster gives up to the data: every
replica of a regular volume, every shard of an ec one. That is the honest
answer for capacity planning, but it is not the question a user asks when
they want to know how much of their data is stored.
Add -df.logical. The master reports the logical sizes alongside the raw
ones: one replica per regular volume, the data shards of each ec volume
counted once. Free space is divided by the copies the requested
replication makes, so used plus available stays the amount of data the
mount can still write, and it comes off the cluster-wide usage rather
than one collection's, since capacity is cluster-wide too.
Statistics through a filer resolves an unset replication to the filer's
default rather than the master's, matching where the writes it is sizing
for actually land.
The flag governs the quota check too, so a mount has one notion of how
much it is using. A filer that predates the new fields sends zeros, and
the mount keeps reporting the raw sizes.
Chris Lu