doc/dev/osd_internals: add pgpool.rst

Signed-off-by: Brad Hubbard <bhubbard@redhat.com>

diff --git a/doc/dev/osd_internals/pgpool.rst b/doc/dev/osd_internals/pgpool.rst
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+==================
+PGPool
+==================
+
+PGPool is a structure used to manage and update the status of removed
+snapshots.  It does this by maintaining two fields, cached_removed_snaps - the
+current removed snap set and newly_removed_snaps - newly removed snaps in the
+last epoch. In OSD::load_pgs the osd map is recovered from the pg's file store
+and passed down to OSD::_get_pool where a PGPool object is initialised with the
+map.
+
+With each new map we receive we call PGPool::update with the new map. In that
+function we build a list of newly removed snaps
+(pg_pool_t::build_removed_snaps) and merge that with our cached_removed_snaps.
+This function included checks to make sure we only do this update when things
+have changed or there has been a map gap.
+
+When we activate the pg we initialise the snap trim queue from
+cached_removed_snaps and subtract the purged_snaps we have already purged
+leaving us with the list of snaps that need to be trimmed. Trimming is later
+performed asynchronously by the snap_trim_wq.
+