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[Hemanth Makkapati <hemanth.makkapati@xxxxxxxxxxxxx>]

Public bug reported:

Glance cache uses  LRU strategy for cache pruning. It does so by keeping
track of the last accessed time on cached images. While the xattr driver
uses the last accessed time reported by the underlying filesystem, the
sqlite driver keeps track of its own last accessed time. The sqlite
driver doesn't set [1] the last accessed time the very first time an
image is added to cache. It updates [2] the last accessed time for every
subsequent access of the image from cache. This behavior makes the newly
cached images susceptible to cache pruning over older images because
they don't have a last accessed time set[3]. So, whenever the pruner
runs, it'll prune the newly cached images first and then look for older
images.

This bug may reduce Glance image download performance under certain
traffic patterns as images will be served from Swift instead of image
cache.

 [1] https://github.com/openstack/glance/blob/bb59c33ffcc6e1cde23c93bb25d38846e84c2cb9/glance/image_cache/drivers/sqlite.py#L324
[2] https://github.com/openstack/glance/blob/bb59c33ffcc6e1cde23c93bb25d38846e84c2cb9/glance/image_cache/drivers/sqlite.py#L376
[3] https://github.com/openstack/glance/blob/bb59c33ffcc6e1cde23c93bb25d38846e84c2cb9/glance/image_cache/drivers/sqlite.py#L280-L281

** Affects: glance
     Importance: Undecided
     Assignee: Hemanth Makkapati (hemanth-makkapati)
         Status: New

** Changed in: glance
     Assignee: (unassigned) => Hemanth Makkapati (hemanth-makkapati)

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https://bugs.launchpad.net/bugs/1438564

Title:
  Glance cache sqlite driver prunes newly cached images first over older
  images

Status in OpenStack Image Registry and Delivery Service (Glance):
  New

Bug description:
  Glance cache uses  LRU strategy for cache pruning. It does so by
  keeping track of the last accessed time on cached images. While the
  xattr driver uses the last accessed time reported by the underlying
  filesystem, the sqlite driver keeps track of its own last accessed
  time. The sqlite driver doesn't set [1] the last accessed time the
  very first time an image is added to cache. It updates [2] the last
  accessed time for every subsequent access of the image from cache.
  This behavior makes the newly cached images susceptible to cache
  pruning over older images because they don't have a last accessed time
  set[3]. So, whenever the pruner runs, it'll prune the newly cached
  images first and then look for older images.

  This bug may reduce Glance image download performance under certain
  traffic patterns as images will be served from Swift instead of image
  cache.

   [1] https://github.com/openstack/glance/blob/bb59c33ffcc6e1cde23c93bb25d38846e84c2cb9/glance/image_cache/drivers/sqlite.py#L324
  [2] https://github.com/openstack/glance/blob/bb59c33ffcc6e1cde23c93bb25d38846e84c2cb9/glance/image_cache/drivers/sqlite.py#L376
  [3] https://github.com/openstack/glance/blob/bb59c33ffcc6e1cde23c93bb25d38846e84c2cb9/glance/image_cache/drivers/sqlite.py#L280-L281

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