Sequential is faster than random on a disk, but we are not doing I/O to a disk, but a distributed storage cluster:
small random operations are striped over multiple objects and servers, and so can proceed in parallel and take advantage of more nodes and disks. This parallelism can overcome the added latencies of network I/O to yield very good throughput. small sequential read and write operations are serialized on a single server, NIC, and drive. This serialization eliminates parallelism, and the network and other queuing delays are no longer compensated for. This striping is a good idea for the small random I/O that is typical of the way Linux systems talk to their disks. But for other I/O patterns, it is not optimal. On 11/21/2012 01:47 PM, Sébastien Han wrote:
Hi Mark, Well the most concerning thing is that I have 2 Ceph clusters and both of them show better rand than seq... I don't have enough background to argue on your assomptions but I could try to skrink my test platform to a single OSD and how it performs. We keep in touch on that one. But it seems that Alexandre and I have the same results (more rand than seq), he has (at least) one cluster and I have 2. Thus I start to think that's not an isolated issue. Is it different for you? Do you usually get more seq IOPS from an RBD thant rand?
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