IBM zHyperLink - Getting to extremely low I/O latencies
Kimmel 1200006UJF Visits (8291)
It's a completely new and different paradigm of doing I/O. Especially with flash becoming a general standard for the storage backends, it's the overhead of dispatching and handling the I/O as such, that generates a considerable add-on.
With zHyperLink, we can dramatically reduce this overhead. We are connecting the Z Central Electronics Complexes (CECs) directly to the I/O bays of a DS8880. The idea is the notion of "Synchronous I/O": For certain I/Os, of which we already know that they won't take too long, we (assuming the Z host perspective) don't dispatch them from the running tasks. Rather we regard the DS8000 as if it would be an integral part of the Z host, and for the I/O, there will be no queuing delays when resuming it back in. The CPU simply waits for the data.
Compared to classical z High-Performance FICON (zHPF) I/Os, we can dramatically reduce the overall latency, by a factor between 5 and 10 for such I/Os. Response times around 16 µs have been achieved in first installations, and values below 20 µs are generally possible.
zHyperLink does not replace FICON, rather it complements it. Only those I/Os which qualify will go over the new zHyperLink connections, and the remaining ones will still be routed over FICON. With typical Db2 installations, it's expected that this can mean about halving overall response times. SInce I/O service time can represent up to 65% of the latency for an IBM Db2 transaction, having major parts of I/Os which are suddenly extremely accelerated can open up completely new fields of application. As other flash storage systems often at best advertise single-I/O latencies in the range of around 100 µs, and some are even proud when only achieving below 0.5 ms.
In the initial delivery phase which has been made available late 2017 with the DS8880 R8.3 release, zHyperLink works just with read I/Os and only when using Db2 (V12 and higher). But there is already a Statement of Direction to support also writes, as well as other database accesses, at a certain point in the future not too far out.
zHyperLink is a direct optical point-to-point connection, no switches. It requires a z14 or z14 ZR1 host, and special new adapters. An important point to remember is a limit to the length of the optical cables, so the maximum distance between host and DS8880 is 150 metres.
Earlier DS8880 models can be upgraded to allow zHyperLink adapters into their I/O bays. Meanwhile, not only the DS8886 models, but also DS8888 and even DS8884 models can be equipped with zHyperLink, when eventually upgrading the CPU cores to a minimum needed amount.
More on zHyperLink find in the following IBM Systems Magazine article
and the new Redpaper REDP-5493