Introduce six new properties: limit_{bw,op}_{read,write,total}.

The limit_bw_* properties limit the read, write, or combined bandwidth,
respectively, that a dataset and its descendants can consume.
Limits are applied to both file systems and ZFS volumes.

The configured limits are hierarchical, just like quotas; i.e., even if
a higher limit is configured on the child dataset, the parent's lower
limit will be enforced.

The limits are applied at the VFS level, not at the disk level.
The dataset is charged for each operation even if no disk access is
required (e.g., due to caching, compression, deduplication,
or NOP writes) or if the operation will cause more traffic (due to
the copies property, mirroring, or RAIDZ).

Read bandwidth consumption is based on:

- read-like syscalls, eg., aio_read(2), pread(2), preadv(2), read(2),
  readv(2), sendfile(2)

- syscalls like getdents(2) and getdirentries(2)

- reading via mmaped files

- zfs send

Write bandwidth consumption is based on:

- write-like syscalls, eg., aio_write(2), pwrite(2), pwritev(2),
  write(2), writev(2)

- writing via mmaped files

- zfs receive

The limit_op_* properties limit the read, write, or both metadata
operations, respectively, that dataset and its descendants can generate.

Read operations consumption is based on:

- read-like syscalls where the number of operations is equal to the
  number of blocks being read (never less than 1)

- reading via mmaped files, where the number of operations is equal
  to the number of pages being read (never less than 1)

- syscalls accessing metadata: readlink(2), stat(2)

Write operations consumption is based on:

- write-like syscalls where the number of operations is equal to the
  number of blocks being written (never less than 1)

- writing via mmaped files, where the number of operations is equal
  to the number of pages being written (never less than 1)

- syscalls modifing a directory's content: bind(2) (UNIX-domain
  sockets), link(2), mkdir(2), mkfifo(2), mknod(2), open(2) (file
  creation), rename(2), rmdir(2), symlink(2), unlink(2)

- syscalls modifing metadata: chflags(2), chmod(2), chown(2),
  utimes(2)

- updating the access time of a file when reading it

Just like limit_bw_* limits, the limit_op_* limits are also
hierarchical and applied at the VFS level.

Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>

Introduce six new properties: limit_{bw,op}_{read,write,total}.

The limit_bw_* properties limit the read, write, or combined bandwidth,
respectively, that a dataset and its descendants can consume.
Limits are applied to both file systems and ZFS volumes.

The configured limits are hierarchical, just like quotas; i.e., even if
a higher limit is configured on the child dataset, the parent's lower
limit will be enforced.

The limits are applied at the VFS level, not at the disk level.
The dataset is charged for each operation even if no disk access is
required (e.g., due to caching, compression, deduplication,
or NOP writes) or if the operation will cause more traffic (due to
the copies property, mirroring, or RAIDZ).

Read bandwidth consumption is based on:

- read-like syscalls, eg., aio_read(2), pread(2), preadv(2), read(2),
  readv(2), sendfile(2)

- syscalls like getdents(2) and getdirentries(2)

- reading via mmaped files

- zfs send

Write bandwidth consumption is based on:

- write-like syscalls, eg., aio_write(2), pwrite(2), pwritev(2),
  write(2), writev(2)

- writing via mmaped files

- zfs receive

The limit_op_* properties limit the read, write, or both metadata
operations, respectively, that dataset and its descendants can generate.

Read operations consumption is based on:

- read-like syscalls where the number of operations is equal to the
  number of blocks being read (never less than 1)

- reading via mmaped files, where the number of operations is equal
  to the number of pages being read (never less than 1)

- syscalls accessing metadata: readlink(2), stat(2)

Write operations consumption is based on:

- write-like syscalls where the number of operations is equal to the
  number of blocks being written (never less than 1)

- writing via mmaped files, where the number of operations is equal
  to the number of pages being written (never less than 1)

- syscalls modifing a directory's content: bind(2) (UNIX-domain
  sockets), link(2), mkdir(2), mkfifo(2), mknod(2), open(2) (file
  creation), rename(2), rmdir(2), symlink(2), unlink(2)

- syscalls modifing metadata: chflags(2), chmod(2), chown(2),
  utimes(2)

- updating the access time of a file when reading it

Just like limit_bw_* limits, the limit_op_* limits are also
hierarchical and applied at the VFS level.

Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>