The object server can be configured to leave a certain amount of disk
space free; default is 1%. This is useful in avoiding 100%-full
filesystems, as those can get Swift in a state where the filesystem is
too full to write tombstones, so you can't delete objects to free up
space.
When a cluster has accounts/containers and objects on the same disks,
then you can wind up with a 100%-full disk since account and container
servers don't respect fallocate_reserve. This commit makes account and
container servers respect fallocate_reserve so that disks shared
between account/container and object rings won't get 100% full.
When a disk's free space falls below the configured reserve, account
and container PUT, POST, and REPLICATE requests will fail with a 507
status code. These are the operations that can significantly increase
the disk space used by a given database.
I called the parameter "fallocate_reserve" for consistency with the
object server. No actual fallocate() call happens under Swift's
control in the account or container servers (sqlite3 might make such a
call, but it's out of our hands).
Change-Id: I083442eef14bf83c0ea717b1decb3e6b56dbf1d0
8e651a2d3d