On 8 November 2012 16:31, Alan Altmark <[email protected]> wrote:
> On Thursday, 11/08/2012 at 06:47 EST, Rob van der Heij <[email protected]> > wrote: >> On a real round brown disk, doing a "format write" is a delicate >> process that requires dedication and a steady hand. It's done one >> track at a time. This takes a full round trip per track, so roughly 1 >> million of those in your case. Given that, 20 would be explained. >> >> If there's more FICON things in between, there may be more hops to >> take and your I/O response might be worse. The amount of data does not >> look like it would saturate your NVS, but who knows what else is going >> on. If you upload an hour of data while this was running, I'd be most >> happy to investigate what is going on and whether there is room for >> improvement. > > DS8000s return Device End as soon as the data is in NVS (non-volatile > storage). Data is written to disk asynchronously, so the physical > organization of a track is transparent to the I/O operation. Managing the > cache to avoid I/O delays due to NVS destaging operations is one of the > things a smart controller has to handle.) > > I will make the rash assumption that all modern storage controllers with > NVS cache operate the same way. You're correct. However, formatting is a bit unfair competition because you can write short records and have the control unit provide the omitted zeroes. So the channel speed is not slowing you down. Depending on how the smarts are done in the subsystem, you might fill up NVS quicker than the collective back-end can absorb. At that point your I/O will be slowed down to the rate of the back-end (with all the write penalties). Among the smart things is also how to avoid a single I/O stream monopolize the cache before hitting the wall anyway. And knowing the OP, it's very well possible she was doing this on a device that is under synchronous PPRC and the Device End is waiting for the other side. Rob ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
