Ceph and NVME - not a good combination?

isotopp image Kristian Köhntopp -
July 16, 2019
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Saving an older twitter thread on Ceph .

I just have finished reading Part 4: RHCS 3.2 Bluestore Advanced Performance Investigation and now I do not know what to say.

Ceph set out with the idea to make storage a commodity by using regular PCs + a lot of local hard disks + software to make inexpensive storage. For that, you ran a number of control processes and an OSD process per disk (“object storage demon”).

Some people were very enthusiastic, envisioning OSD processes becoming an integrated part of any storage device as part of computational storage, even.

Something else happened: NVME. NVME is flash memory on a PCIe bus, giving you a very large number of IOPS with mediocre latency. You get around 1 million IO operations per second, but a read takes ~100 microseconds, and an unbuffered write takes about 420 micros. That means, you need to run 100s of parallel things.

Ceph is full of logs. Logs kill parallelism by design, or at least severely limit it. Also, Ceph is full of computational complexity. And that is ok, when a disk seek takes 5ms on a 3GHz CPU, and you got 15 million clock cycles think time per disk seek. Less so, when you need to keep 100s of ops in flight. Suddenly, the budget shrinks by /1000, and your serializing architecture gets in the way of things.

Now read this report. We are talking about NVME devices, 11T, maybe 16T per device, 4 PCIe lanes. These people are talking about 4 OSD processes (down to 2, and they consider this an improvement) per device. And they eat six (sic!) cores per NVME device. Apart from this never making it to computational storage for thermal reasons, this is an insane cost.

In modern storage, bandwidth is usually limited by the network, not the local devices: You can RAID your storage bandwidth until you saturated the 100 GBit/s network, and then you are done.

IOPS are unlimited in the same way: You get not quite 1 million IOPS per device, so if that is not sufficient, spread across many.

The only remaining challenge in this age of abundance in the data center is commit (fsync) latency: how long to push things to a persistent storage. Even that comes down a lot, with optane or NVRAM in the client or as early as possible in the server.

Relatively low powered all-flash iSCSI appliances do routinely 250 micros commit latency, and things involving NVMEoF and RDMA can come down to under 100 micros reliably, if you have the coin. Want faster? Put Optane or NVRAM into each client.

And it is telling that the paper here talks about latency improvements only in relative numbers (“30% less”), but never actually speaks about absolute timings.

Maybe it is time to stop and think, and re-evaluate if this Ceph thing is the smart thing to do with storage in the face of NVME, Optane and NVRAM. Because the numbers look all wrong to me. Even the units look wrong, actually.

Conversations deeper in the Thread :

There was were questions on what to use for low-latency disaggregated storage and if one needed redundancy at all.

Examples of low-latency storage are software solutions from Lightbitslabs , from Datera , and from Quobyte . There are many more.

And funnily enough, if you talk to your inhouse customers, most actually don’t want any of this, because they talk to a database (MySQL, Cassandra, Elastic, …) and not to “storage” directly.

Then you speak to the MySQL people and they say “We have replication and Orchestrator , even masters are throwaway”. You talk to the Cassandra team and they laugh in Paxos. And so on. So most of these teams actually prefer “RDMA to a NVME namespace” over storage software with redundancy, and do resiliency higher up on the stack, in application context.

As a storage team, you have to have some kind of low-latency non-replicated solution first, because that’s what is actually the primary demand. You also need a lowish-latency, OLTP-capable redundant solution (<500 microseconds is fine), but there is only unscaled, low level demand.

And then there is huge demand for volume at any latency, append-only preferred over rewriteable for many reasons. So S3 with heavy tiering. Ceph is actually pretty good at that, but that is likely not NVME, but many large disk drives.

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