Re: parallel recovery slaves waiting on undo reads

That particular detail depends on the architecture of the machine. With the advent of the multi-core processors, like the latest generations of Intel Xeon E5 family or AMD Ryzen family there is certainly enough CPU power to perform I/O requests in parallel. Unix and Linux kernel generally have ligh-weight kernel threads that perform that kind of work. I am not sure about Solaris machines, but Linux machines usually use 3 types of I/O interfaces: SATA, for JBOD, FC/AL and iSCSI, with the last one becoming increasingly popular. SATA is not really well suited for async I/O because it can only get up to 6GB/sec and gets saturated very easy. The latest versions of FC/AL adapters, running up to 128 Gb/sec speeds are usually used for attaching EMC VMAX3 or high end NetApp SAN equipment and can easily sustain parallel I/O. Truth be told, the most popular versions are 32 Gb, not 128Gb, which is on the bleeding edge. However, increasingly popular iSCSI over 10Gb NIC cards can not. First, iSCSI causes a multitude of interrupts which are executing in interrupt mode and are preventing clock interrupts from occurring, which means - no time sharing. In other words, no parallelism. Second, many clients are using normal, run off the mill 10Gb adapters, instead of the adapters with iSCSI protocol built into NIC.That makes the situation much worse because Linux native iSCSI implementation is not the best one and is rather inefficient. Furthermore, Ethernet latency grows with utilization, which makes the I/O requests slower when the network utilization reaches around 75%. That is not that hard to achieve with an OLTP database. As of Oracle 12.2, it is possible to throttle down your PDB (haven't tried with non-PDB architecture) using MAX_IOPS and MAX_MBPS, which is where Kevin Closson's SLOB tools comes very handy. I would advise against using asynchronous I/O on any database that utilizes iSCSI attached storage.

On 2/29/20 1:43 PM, Noveljic Nenad wrote:
>
> With regard to ‘db file parallel read’ with sync IO, there seems to be
> a difference between Solaris x64 and Linux. On Solaris I’m seeing a
> bunch of serial pread calls (as opposed to preadv on Linux). The sum
> of their durations seems to be contained in the wait event time. My
> observation refers to a non-multitenant 12.1 instance running on
> Solaris 11.4. Therefore, the async IO would bring a huge benefit,
> provided there’s enough bandwidth to handle parallel IO calls.
>
> Best regards,
>
> Nenad
>
> https://nenadnoveljic.com/blog
>

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Mladen Gogala
Database Consultant
Tel: (347) 321-1217


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