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| -rw-r--r-- | doc/dev/osd_internals/pgpool.rst | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/doc/dev/osd_internals/pgpool.rst b/doc/dev/osd_internals/pgpool.rst new file mode 100644 index 00000000..45a252bd --- /dev/null +++ b/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. + |