A master decides nothing from it. Every caller that read it was asking whether
a volume is remote, which the backend name answers, and the value itself is
reported on demand by the server holding the volume, through the volume info in
ReadVolumeFileStatus.
It is also the one string here that cannot be shared: unique per volume, so
unlike the collection and backend names it carries its own characters for every
volume a master tracks.
VolumeInfo goes from 136 bytes to 120. 800k volumes registered from a heartbeat
that has been over the wire go from 214 to 163 B/volume when tiered.
The volume server's own status page keeps showing the key, now read from the
volume it holds rather than relayed through a master, which is also where the
other volume server implementation reads it.
The heartbeat digest drops it on the same grounds: a change to something the
master does not hold cannot make its copy stale. Both implementations and their
shared vectors move together, and the field-coverage test now names what is
deliberately not retained rather than being loosened.
* storage: share the volume strings a cluster repeats
Decoding a heartbeat allocates a fresh string for the collection, disk type and
remote backend of every volume, and a master holding a million volumes then
holds a million copies of the same handful of names.
Not the remote storage key, which is unique per volume: interning that would
fill the table rather than share anything.
800k volumes registered from a heartbeat that has actually been over the wire:
227 -> 211 B/volume, and 238 -> 214 when the volumes are tiered, since the
backend name shares too.
* storage: hold the interned strings rather than let them be collected
unique.Make clears its entries by weak reference, and its canonical value does
not survive a collection even while a caller still holds the string it handed
back -- so a volume reported later would get a second copy of a name the rest
of the cluster already shares. With only changed volumes reported, most are
interned once and never again, so that is the common case rather than a corner.
The table therefore only grows, which is why it stays restricted to values
drawn from a small set. Ten thousand collections keep a few hundred kilobytes.
Chris Lu