SANsymphony-V Sequential Storage Option

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Published: Wednesday, 19 November 2014 21:53

The Sequential Storage Option (aka Random Write Accelerator) is a new feature that comes with PSP1 for SANsymphony-V 10. The name itself already tells you what this feature could be used for but I will better explain a bit more about this main feature.

First to give you an overview, this feature will dramatically increase random write performance on every vdisk you enable this option assumed you really have random write I/O patterns. This feature will not speed up sequential writes or reads in any way, it is only designed to speed up random I/Os.

Why is this feature needed? During the last years storage capacity dramatically increased. On the other hand, speed didn't raise at the same level so today we have multi-terabyte SATA disks that offer the same speed as their pendants 8 years ago. Things get worse with the introduction of magnetic-shielded recording that allows higher capacities (currently up to 10TB on a single drive) but also increases write latency up to 6x. Even in the SAS area capacity raised but performance didn't. Only the change from 3,5" to 2,5" speeded up track-to-track seek performance a bit.

Why is seek performance so important? Seek performance is responsible for latency and as seek performance gets faster, latency goes down. And storage is all about latency....


So, current problem is huge disks and slow performance thus high latencies. On the other side, capacity needs to be increased as the demand for more and more store is unstoppable. So your DataCore environment probably has high amounts of capacity but performance lacks due to the quite small number of high capacity disks. This will keep costs low but also performance.

So how can we fill the gap between the high capacity, slow performance disk drives and the high performance, high cost drives in an efficient way? To find an answer to this question you have to find out how you can make slow SATA disks deliver the same performance as high-speed disks. Sound irrealistic.... but DataCore is one step further.

If you look at the weaks and strength of a high capacity SATA disk then random performance is slow but sequential performance is quite good. Sometimes SATA disks can even outperform SAS disks in sequential streams. If you look at the way storage is addressed in DataCore then you see that most reads and writes are random. Not a good starting point for the use of SATA disks. For reads you can use the DataCore cache to speed up things but for writes, even with cache, the data has to be destaged to the disks and that's where things get really slow.

Sequential Storage is a feature to address exactly that issue. Sequential Storage Option (SSO) more or less, transforms random write patterns into sequential only streams that can be perfectly served by even SATA disks. This way you can speed up random writes up to 33x. DataCore internal testings with 100% random 4k writes showed in combination with SSY-Vs write cache a sustained performance on a SINGLE SATA disk up to 10.000 IOPS. This is the performance of an entry level SSD. Peak performance was even faster. And even SSDs can profit for a performance gain up to 3x.

One more note on the mentioned performance gains. DataCore distinguishes between three classes within SSO. First there is "Minimum performance". This is the absolut minimum of performance you get from a single disk by using all DataCore optimization features EXCEPT SSO. Second there is "Maximum or peak performance". Peak performance can be reached right after the enablement of SSO on a vDisk. During this phase writes will get put into the index in a very fast and efficient way. Garbage collection is not neccessary at this phase making it very fast and efficient. Peak performance phase can last for a few minutes up to several hours depending on the I/O pattern. Third is "Sustained performance". This is the performance you see after peak phase ended and garbage collection needs to be done in the index table. This reduces the write performance but still keeping it at a level up to 10-25x faster than without SSO.

How can SSO accomplish that? To understand this you have to dig a bit deeper in the way SSO works.

SSO puts an extra layer of virtualization into the data path. SSY-V already virtualises underlying storage but in the past (and also today if you disable SSO on a vdisk) the addressing scheme still uses LBA and offset. This makes random writes on the frontend result in random writes in the backend. Optimization was only made through cache and coalesce of unneeded writes but the I/O pattern in the backend matches the pattern on the frontend. As most users use server virtualization in front of a DataCore SAN random traffic on the frontend makes ~90-95% resulting in heavily random access on the storage backend. No good place for SATA disks.

SSO now also virtualises the addressing scheme making it possible to "transform" random I/O to sequential I/O. To accomplish that, all write I/O is not automatically destaged to disk (the so called data rest area, where data is unoptimized stored) but rather stored on a log partition in some kind of "eternal table". As this table allows the writes to be stored sequentially the underlying disk only does sequential writes. The exact techique behind SSO is a bit hard to describe but the important thing to remember is that SSO makes sequential I/O out of random I/O fully transparent to the application servers.

Some more facts about SSO you should be aware of:

The last words: SSO is a cool feature, it is free of charge and it has only very few side effects (okay, 6x the storage could be a problem for some customers but the performance increase should be worth the invest in a few TB of low-cost storage). It can only speed up things in a specific I/O pattern but it won't make things worse even if this pattern doesn't apply to your vDisk.

So give it a try but in a reasonable way. Have fun.