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  • David Howells's avatar
    afs: Overhaul volume and server record caching and fileserver rotation · d2ddc776
    David Howells authored
    
    
    The current code assumes that volumes and servers are per-cell and are
    never shared, but this is not enforced, and, indeed, public cells do exist
    that are aliases of each other.  Further, an organisation can, say, set up
    a public cell and a private cell with overlapping, but not identical, sets
    of servers.  The difference is purely in the database attached to the VL
    servers.
    
    The current code will malfunction if it sees a server in two cells as it
    assumes global address -> server record mappings and that each server is in
    just one cell.
    
    Further, each server may have multiple addresses - and may have addresses
    of different families (IPv4 and IPv6, say).
    
    To this end, the following structural changes are made:
    
     (1) Server record management is overhauled:
    
         (a) Server records are made independent of cell.  The namespace keeps
         	 track of them, volume records have lists of them and each vnode
         	 has a server on which its callback interest currently resides.
    
         (b) The cell record no longer keeps a list of servers known to be in
         	 that cell.
    
         (c) The server records are now kept in a flat list because there's no
         	 single address to sort on.
    
         (d) Server records are now keyed by their UUID within the namespace.
    
         (e) The addresses for a server are obtained with the VL.GetAddrsU
         	 rather than with VL.GetEntryByName, using the server's UUID as a
         	 parameter.
    
         (f) Cached server records are garbage collected after a period of
         	 non-use and are counted out of existence before purging is allowed
         	 to complete.  This protects the work functions against rmmod.
    
         (g) The servers list is now in /proc/fs/afs/servers.
    
     (2) Volume record management is overhauled:
    
         (a) An RCU-replaceable server list is introduced.  This tracks both
         	 servers and their coresponding callback interests.
    
         (b) The superblock is now keyed on cell record and numeric volume ID.
    
         (c) The volume record is now tied to the superblock which mounts it,
         	 and is activated when mounted and deactivated when unmounted.
         	 This makes it easier to handle the cache cookie without causing a
         	 double-use in fscache.
    
         (d) The volume record is loaded from the VLDB using VL.GetEntryByNameU
         	 to get the server UUID list.
    
         (e) The volume name is updated if it is seen to have changed when the
         	 volume is updated (the update is keyed on the volume ID).
    
     (3) The vlocation record is got rid of and VLDB records are no longer
         cached.  Sufficient information is stored in the volume record, though
         an update to a volume record is now no longer shared between related
         volumes (volumes come in bundles of three: R/W, R/O and backup).
    
    and the following procedural changes are made:
    
     (1) The fileserver cursor introduced previously is now fleshed out and
         used to iterate over fileservers and their addresses.
    
     (2) Volume status is checked during iteration, and the server list is
         replaced if a change is detected.
    
     (3) Server status is checked during iteration, and the address list is
         replaced if a change is detected.
    
     (4) The abort code is saved into the address list cursor and -ECONNABORTED
         returned in afs_make_call() if a remote abort happened rather than
         translating the abort into an error message.  This allows actions to
         be taken depending on the abort code more easily.
    
         (a) If a VMOVED abort is seen then this is handled by rechecking the
         	 volume and restarting the iteration.
    
         (b) If a VBUSY, VRESTARTING or VSALVAGING abort is seen then this is
             handled by sleeping for a short period and retrying and/or trying
             other servers that might serve that volume.  A message is also
             displayed once until the condition has cleared.
    
         (c) If a VOFFLINE abort is seen, then this is handled as VBUSY for the
         	 moment.
    
         (d) If a VNOVOL abort is seen, the volume is rechecked in the VLDB to
         	 see if it has been deleted; if not, the fileserver is probably
         	 indicating that the volume couldn't be attached and needs
         	 salvaging.
    
         (e) If statfs() sees one of these aborts, it does not sleep, but
         	 rather returns an error, so as not to block the umount program.
    
     (5) The fileserver iteration functions in vnode.c are now merged into
         their callers and more heavily macroised around the cursor.  vnode.c
         is removed.
    
     (6) Operations on a particular vnode are serialised on that vnode because
         the server will lock that vnode whilst it operates on it, so a second
         op sent will just have to wait.
    
     (7) Fileservers are probed with FS.GetCapabilities before being used.
         This is where service upgrade will be done.
    
     (8) A callback interest on a fileserver is set up before an FS operation
         is performed and passed through to afs_make_call() so that it can be
         set on the vnode if the operation returns a callback.  The callback
         interest is passed through to afs_iget() also so that it can be set
         there too.
    
    In general, record updating is done on an as-needed basis when we try to
    access servers, volumes or vnodes rather than offloading it to work items
    and special threads.
    
    Notes:
    
     (1) Pre AFS-3.4 servers are no longer supported, though this can be added
         back if necessary (AFS-3.4 was released in 1998).
    
     (2) VBUSY is retried forever for the moment at intervals of 1s.
    
     (3) /proc/fs/afs/<cell>/servers no longer exists.
    
    Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
    d2ddc776