Interesting, thanks for the details. Adding the dev(a)ceph.io list. On Thu, Mar 26, 2020 at 12:07 PM <akmd(a)metaswitch.com> wrote: The bucket index is generally only consulted when listing objects. For GET/HEAD requests on objects, we read the etag and other object attributes directly from the associated 'head object' in the buckets.data pool. For small objects (ie. non-multipart uploads), this head object also stores the first 4MB of object data. For GET requests, radosgw will prefetch this data in the same osd request that reads the object's attributes, even in the 304 case where no data is returned to the client. The 'head object' concept works differently for versioned buckets, because each object version has its own head object, and the 'current' version's head object (internally called the 'object logical head', or OLH) acts like a symlink to one of those. So a GET/HEAD request that doesn't specify a ?versionId will require (at least*) two rados reads - one on the OLH object to determine the current version, and another on that version's head object. * OLH performance isn't necessarily deterministic, because it may involve replaying a journal of racing writes of new versions. Given the above, I would expect the performance to be very similar with and without If-None-Match. I'd suggest running radosgw with '--debug-ms=1' and comparing the number of osd_op requests it sends between the 206 and 304 cases.