initial code repo

[stor4nfv.git] / src / ceph / doc / dev / osd_internals / pgpool.rst

diff --git a/src/ceph/doc/dev/osd_internals/pgpool.rst b/src/ceph/doc/dev/osd_internals/pgpool.rst
new file mode 100644 (file)
index 0000000..45a252b
--- /dev/null
+++ b/src/ceph/doc/dev/osd_internals/pgpool.rst
@@ -0,0 +1,22 @@
+==================
+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.
+