Adding Features and planning features to glusterfs-specs repo

As per the discussion (http://www.gluster.org/pipermail/gluster-users/2015-July/022918.html)
the specs are part of this repo.

Signed-off-by: hchiramm <hchiramm@redhat.com>

1 files changed, 49 insertions, 0 deletions

diff --git a/Features/memory-usage.md b/Features/memory-usage.md
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index 0000000..4e1a8a0
--- /dev/null
+++ b/Features/memory-usage.md
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+object expiry tracking memroy usage
+====================================
+
+Bitrot daemon tracks objects for expiry in a data structure known
+as "timer-wheel" (after which the object is signed). It's a well
+known data structure for tracking million of objects of expiry.
+Let's see the memory usage involved when tracking 1 million
+objects (per brick).
+
+Bitrot daemon uses "br_object" structure to hold information
+needed for signing. An instance of this structure is allocated
+for each object that needs to be signed.
+
+                struct br_object {
+                        xlator_t *this;
+                
+                        br_child_t *child;
+                
+                        void *data;
+                        uuid_t gfid;
+                        unsigned long signedversion;
+                
+                        struct list_head list;
+                };
+
+Timer-wheel requires an instance of the structure below per
+object that needs to be tracked for expiry.
+
+                struct gf_tw_timer_list {
+                        void *data;
+                        unsigned long expires;
+                
+                        /** callback routine */
+                        void (*function)(struct gf_tw_timer_list *, void *, unsigned long);
+                
+                        struct list_head entry;
+                };
+
+Structure sizes:
+
+-  sizeof (struct br_object): 64 bytes
+-  sizeof (struct gf_tw_timer_list): 40 bytes
+
+Together, these structures take up 104 bytes. To track all 1 million objects
+at the same time, the amount of memory taken up would be:
+
+** 1,000,000 * 104 bytes: ~100MB**
+
+Not so bad, I think.