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Slow response to expiration time update

Hi, I am using Swift as a cache for files that get up into the 10s of MB, and after fetching one I update its X-Delete-After time (in order to do approximate LRU by having unused stuff expire). I've noticed that for big files, this operation takes longer, which doesn't make sense since I am not sending any data, just headers, and I'm only getting back a small message, and on the server side this is (supposedly) handled asynchronously anyway. I don't have many files on swift since I am basically testing it out right now, so I don't think that is the problem. For example, updating the expiration time for a ~50MB file took min 2.21s, avg 7.5s, and max 28s (!). Here is the command I called 100 times to test this: curl -silent -k -X POST -H 'X-Auth-Token: [my auth token]' -H 'X-Delete-After: 604800' [my storage url]/[container]/[filename]

Any help would be greatly appreciated, taking 7 seconds on average to do this is really not acceptable for our use case (also, for the record, ping times to the servers in question are sub-ms and they have gigabit connections, so I don't think its a network problem).