[sheepdog-users] Object Cache Performance

Gerald Richter - ECOS richter at ecos.de
Thu Jan 9 09:44:36 CET 2014


I have done some performance tests with the object cache:

All tests were done with the same 20GB image. I have two nodes in the test cluster, so there is a full copy of the image on the local node.

1.       “dog vdi read vm > file” with object cache enabled first time: ~10min

2.       “dog vdi read vm > file” with object cache enabled second time: ~5min

3.       “dog vdi read vm > file” without object cache: ~5min

4.       “cp file file2” ~ 4min 

This shows read access via sheepdog without object cache (3.) is about 80% of the speed of a direct copy (4.) of the same amount of data. 

If the cache is already filled (2.), we have the same speed, but if the cache is used (1.) for the first time  we only get half of the speed. I guess this is because the cache needs to be filled, which requires twice the number of disk reads and writes. Since all data is already on the local node, the question is, is it really necessary to pass it through the cache or wouldn’t it be better to bypass the cache in this case?

I have made all this tests with dog, so no qemu(-img) is involved, to be sure to test the newest version (0.7.6)



P.S. I really need to use the object cache, because my network links are relatively slow and without object cache I get a poor write performance.

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