A reader Alex asked if I could post a comparative set of tests from my previous 3000 series Infiniband testing but using the PCIe direct-attached method. I was actually very keen to test this myself as I wanted to see how close the Infiniband connectivity method could get to the PCIe latencies. Why? Well, PCIe offers the lowest overhead but also causes some HA problems.
When SSDs first came out they were just that, solid state disks – or at least they looked like them. They had the same form factor and plugged into existing disk controllers, but had no spinning magnetic parts. This offered performance benefits but those benefits were restricted to the performance of those very disk controllers, which were never designed for this sort of technology. We call this the first generation of flash.
To overcome this architectural limitation, flash vendors came out with a new solution – placing flash on PCIe cards which can then attached direct to the system board, reducing latency and providing extreme performance. This is what we call the second generation of flash. It is what vendors such as Fusion IO provide – and looking at FIO’s share price you would have to congratulate them on getting to market and making a success of this.
However, there are other architectural limitations to this PCIe approach. One is that you cannot physical share the storage provided by PCIe – sure you can run some sort of sharing software to make it available outside of the server it is plugged into, but that increases latency and defeats the object of having super-fast flash storage plugged right into the system board. Even worse, if the system goes down then that flash (and everything that was on it) is unavailable. This makes PCIe flash cards a non-starter for HA solutions. If you want HA then the best you can do with them is use them for caching data which is still available on shared storage elsewhere (the Oracle Database Smart Flash Cache being one possible solution).
At Violin we don’t like that though. We don’t believe in spending time and CPU resources (or even worse, human resources) managing a cache of data trying to improve the probability and predictability of cache hits. Not when flash is now available as a tier 1 storage medium, giving faster results whilst using less space, power and cooling.
Another problem with PCIe is that the number of slots on a system board will always be limited – for reasons of heat, power, space etc there will always be a limit beyond which you cannot expand.
And there’s another even more major problem with PCIe flash cards, which no PCIe flash vendor can overcome: you cannot replace a PCIe card without taking the server down. That’s hardly the sort of enterprise HA solution that most customers are looking for.
This is where we get to the third generation of flash storage, which is to place the flash memory into arrays which connect via storage fabrics such as fibre-channel or Infiniband. This allows for the flash storage to be shared, to be extended, to offer resilience (e.g. RAID) and to have high-availability features such as online patching and maintenance, hot-swappable components etc.
This is the approach that Violin Memory took when designing their flash memory arrays from the ground up. And it’s an approach which has resulted in both families of array having a host of connectivity features: PCIe (for those who don’t want HA), iSCSI, Fibre-Channel and now Infiniband.
But what does the addition of a fibre-channel gateway do to the latency? Well, it adds a few hundred microseconds to the latency… In the scheme of things, when legacy disk arrays deliver latencies of >5ms that’s nothing, but when we are talking about flash memory with latencies of <1ms that suddenly becomes a big deal. And that’s why the Infiniband connectivity is so important – because it ostensibly offers the latency of PCIe but with the HA and management features of FC.
So let’s have a look at the latencies of the 3000 series using PCIe direct attach to see how the latency measures up against the Infiniband testing in my previous post:
Filename Event Waits Time(s) Latency IOPS
------------- ------------------------ ------------ -------- ------- ----------
awr_0_1.txt db file sequential read 308,185 33 107 7,139.2
awr_0_4.txt db file sequential read 4,166,252 510 122 24,883.1
awr_0_8.txt db file sequential read 9,146,095 1,245 136 41,569.2
awr_0_16.txt db file sequential read 19,496,201 3,112 160 70,121.9
awr_0_32.txt db file sequential read 40,159,185 11,079 275 92,185.0
awr_0_64.txt db file sequential read 81,342,725 49,049 602 99,060.1
We can see that again the latency is pretty much scaling at a linear rate. And up to 16 readers (which is double the number of CPU cores I have available) the latency remains under 200us. This is very similar to the Infiniband results, where up to (and including) 16 readers I also had <200us latency.
A couple of points to note:
- Again the lack of CPU capability in my Supermicro servers is prohibiting me from really pushing the arrays – causing the tests above 16 readers to get skewed. I have requested a new set of lab servers with ten-core Westmere-EX CPUs so I just need to sit back and wait for Father Christmas to visit
- The database block size is 8k
- To make matters even more complicated, this was actually a RAC system (although I ran the SLOB tests from a single instance)
That last point is worth expanding. I said that PCie does not allow for HA. That’s not strictly true for Violin however. In this system I have a pair of Supermicro servers, each connected via PCie to my single 3205 SLC array and presenting a single LUN, which I have partitioned and presented to ASM as a series of ASM disks.
Because ASM does not require SCSI-3 persistent reservations or any other such nastiness, I am able to use this as shared storage and run a 184.108.40.206 RAC and Grid Infrastructure system on it. I’ve run all the usual cable-pulling tests and not managed to break it yet, although I’m not convinced it is a design I would choose over Infiniband if I had to choose… mainly because the PCIe method does not incorporate the Violin Memory HA Gateway, which gives me the management GUI and an additional layer of protection from partial / unaligned IO.
I now need to go and beg for that bigger server so I can get some serious testing done on the 6000 series array which is currently laughing at me every time I tickle it with SLOB…