MGM: implement the concept of alternative spaces - fixes EOS-6286

DOC: document the concept of alternative spaces

doc/diopside/manual/interfaces.rst

@@ -1118,6 +1118,7 @@ The following policies can be configured
     key                 values                                          
     =================== =================================================
     space               default,...
+    altspaces           [space1[,space2[,space3]]]
     layout              plain,replica,raid5,raid6,raiddp,archive,qrain
     nstripes            1..255
     checksum            adler,md5,sha1,crc32,crc32c
@@ -1229,9 +1230,31 @@ Examples:
    # files uploaded selecting the rep4 space will be stored with 4 replicas, if non space is selected they will get the default for the target directory or the default space
 
    # define a space with 4 replica policy
-   eos space config rep4 space.policys.nstripes=4
+   eos space config rep4 space.policy.nstripes=4
    eos space config rep4 space.policy.layout=replica
     
+
+   ##############
+   # Example 3  #
+   ##############
+   # When the policy is consulted for a write operation, alternative storage spaces can be defined as fallbacks.
+   # These alternatives are used when the available space in the primary storage, as determined by the policy, is exceeded.
+   # For instance, if files are initially placed in an NVME storage space and it becomes full, the system can fall back
+   # to an alternative space, such as an HDD. If the HDD space is also exhausted, another fallback, like OLDHDD, can be tried.
+   # If all alternative spaces are exhausted, the policy will revert to the initially selected space, and the placement
+   # operation will fail due to insufficient space.
+
+
+   # define as default the NVME space
+   eos space config default space.policy.space=NVME
+
+   # define the HDD space as alternative to the NVME space
+   eos space config NVME space.policy.altspaces=HDD,OLDHDD
+
+   # define several alternative spaces HDD,OLDHDD if the NVME space has no nominal bytes left
+   # HDD is tried first and if there are nominal bytes left that one is taken
+   eos space config NVME space.policy.altspaces=HDD,OLDHDD
+
 Storage Tiering
 ^^^^^^^^^^^^^^^
 Currently we support two tiering scenarios:

mgm/Policy.cc

@@ -40,7 +40,8 @@ EOSMGMNAMESPACE_BEGIN
 const std::vector<std::string> Policy::gBasePolicyKeys = {
     "policy.space",         "policy.layout",          "policy.nstripes",
     "policy.checksum",      "policy.blocksize",       "policy.blockchecksum",
-    "policy.localredirect", "policy.updateconversion","policy.readconversion"};
+    "policy.localredirect", "policy.updateconversion","policy.readconversion",
+    "policy.altspaces"};
 
 const std::vector<std::string> Policy::gBasePolicyRWKeys = {
     "policy.bandwidth", "policy.iopriority", "policy.iotype",
@@ -170,6 +171,26 @@ Policy::GetLayoutAndSpace(const char* path,
     }
   }
 
+  // Check if the given space is under the nominal value, otherwise loop through altspaces and take the first having capacity
+  if (rw) {
+    if (FsView::gFsView.UnderNominalQuota(space, (vid.uid==0))) {
+      std::string altspaces_key = "policy.altspaces";
+      if (auto kv = spacepolicies.find(altspaces_key);
+          kv != spacepolicies.end() && (!kv->second.empty())) {
+	std::vector<std::string> alt_spaces;
+	eos::common::StringConversion::Tokenize(kv->second,
+						alt_spaces,
+						":");
+	for (auto aspace:alt_spaces) {
+	  if (!FsView::gFsView.UnderNominalQuota(aspace, (vid.uid==0))) {
+	    eos_static_info("msg=\"space '%s' is under nominal quota - selected alternative space '%s'", space.c_str(), aspace.c_str());
+	    space = aspace; // select this one and continue
+	    break;
+	  }
+	}
+      }
+    }
+  }
   // Replace the non empty settings from the default space have been already
   // defined before
   if (!conversion && space != "default") {