Symlink Caching (#206)

* symlink caching

* better location

---------

Co-authored-by: Valentin Volkl <valentin.volkl@cern.ch>

Author: HereThereBeDragons

apx-parameters.rst

@@ -29,6 +29,7 @@ CVMFS_BACKOFF_MAX               Maximum backoff in seconds when retrying to down
 CVMFS_CATALOG_WATERMARK         | Try to release pinned catalogs when their number surpasses the given watermark.
                                 | Defaults to 1/4 CVMFS_NFILES; explicitly set by shrinkwrap.
 CVMFS_CACHE_BASE                Location (directory) of the CernVM-FS cache.
+CVMFS_CACHE_SYMLINKS            If set to *yes*, enables symlink caching in the kernel. 
 CVMFS_CHECK_PERMISSIONS         If set to *no*, disable checking of file ownership and permissions (open all files).
 CVMFS_CLAIM_OWNERSHIP           If set to *yes*, allows CernVM-FS to claim ownership of files and directories.
 CVMFS_CPU_AFFINITY              Comma-separated list to set CPU affinity for all ``cvmfs`` components. 

cpt-configure.rst

@@ -1223,6 +1223,34 @@ They can also be exported in regular intervals (see :ref:`cpt_telemetry`).
 starts the catalog update routine.
 When using ``remount sync`` the system waits for the new file system snapshot to be served (if there is a new one).
 
+Kernel Cache Tuning
+~~~~~~~~~~~~~~~~~~~
+
+Using efficiently the kernel cache can increase the overall performance.
+Requests that would normally be answered by ``cvmfs``, can - if cached -
+be directly answered by the kernel which shortens the overall request time.
+There are multiple client config parameters that influence the kernel cache behavior.
+
+=============================== ========================================================================================
+**Parameter**                   **Meaning**
+=============================== ========================================================================================
+CVMFS_KCACHE_TIMEOUT            Timeout in seconds for path names and file attributes in the kernel file system buffers.
+CVMFS_CACHE_SYMLINKS            If set to *yes*, enables symlink caching in the kernel.
+CVMFS_STATFS_CACHE_TIMEOUT      | Caching time of  ``statfs()`` in seconds (no caching by default).
+                                | Calling ``statfs()`` in high frequency can be expensive.
+=============================== ========================================================================================
+
+Caching of symlink in the kernel means that the mangled name is stored, so that
+there is no need to resolve it again when it is requested for another time.
+Activating this option makes only sense if symlinks are heavily accessed.
+First performance measurement showed a slightly slower performance on the very first
+access (*cold cache*) but a better performance for multiple accesses (*warm* and *hot cache*).
+
+.. warning::
+    Symlink caching works best with ``kernel >= 6.2rc1`` and ``libfuse >= 3.16``.
+    It already works from version ``libfuse 3.10.0`` on but has restriction,
+    e.g. *mounts on top of mounts* will be destroyed if they are a symlink.
+
 
 File System Information
 ~~~~~~~~~~~~~~~~~~~~~~~