9.2  Querying Available Space

   The following extension provides a way to discover the available
   space for an arbitrary path.

       byte   SSH_FXP_EXTENDED
       uint32 request-id
       string "space-available"
       string path     [UTF-8]

   path
      'path' for which the available space should be reported.  This
      'path' is not required to be the mount point path, but MAY be a
      directory or file contained within the mount.

   The reply to the request is as follows:

       byte   SSH_FXP_EXTENDED_REPLY
       uint32 request-id
       uint64 bytes-on-device
       uint64 unused-bytes-on-device
       uint64 bytes-available-to-user
       uint64 unused-bytes-available-to-user
       uint32 bytes-per-allocation-unit

   bytes-on-device
      The total number of bytes on the device which stores 'path', both
      used and unused, or 0 if unknown.

   unused-bytes-on-device
      The total number of unused bytes available on the device which
      stores 'path', or 0 if unknown.

   bytes-available-to-user
      The total number of bytes, both used and unused, available to the
      authenticated user on the device which stores 'path', or 0 if
      unknown.

   unused-bytes-available-to-user
      The total number of unused bytes available to the authenticated
      user on the device which stores 'path', or 0 if unknown.

   bytes-per-allocation-unit
      The number of bytes in each allocation unit on the device, or in
      other words, the minimum number of bytes that a file allocation
      size can grow or shrink by.  If the server does not know this
      information, or the file-system in use does not use allocation
      blocks, this value MUST be 0.