Date: Sun, 1 Jul 2018 14:17:33 -0700 From: bob prohaska <fbsd@www.zefox.net> To: "Rodney W. Grimes" <freebsd-rwg@pdx.rh.CN85.dnsmgr.net> Cc: Mark Millard <marklmi@yahoo.com>, freebsd-arm@freebsd.org, bob prohaska <fbsd@www.zefox.net> Subject: Re: RPI3 swap experiments Message-ID: <20180701211733.GC52656@www.zefox.net> In-Reply-To: <201807012011.w61KBZka029690@pdx.rh.CN85.dnsmgr.net> References: <20180701191741.GA52656@www.zefox.net> <201807012011.w61KBZka029690@pdx.rh.CN85.dnsmgr.net>
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On Sun, Jul 01, 2018 at 01:11:35PM -0700, Rodney W. Grimes wrote: > > IIRC the "delay" through a usb hub, as long as your not doing transactional > translation (TT, ie 480 mb/s down to a 12mb/s device) is on the order of > microseconds, this is the same weither it is a powered or un powered hub. > > I doubt that any flash drive can respond faster than a spinning rust > drive with a cache can, as the cache is DRAM with tens of nano seconds > access time. > > Flash drives DO have an access delay, the FTL still has to do the mapping > funcions, and a page of flash has to be pulled into a read buffer. I > actually believe that even the fastest USB flash drives are still > slower than any modern spinning rust at this operation. > There is a sorted list of gstat output from one of my tests at http://www.zefox.net/~fbsd/rpi3/swaptests/3gbusbflash/readdelay.sort The first few lines suggest that microSD responds in less than a millisecond, direct-connected USB flash requires 1.2 milliseconds and hub-connected USB flash takes 1.6 milliseconds. I'm not sure how representative these numbers are, the -u option was used in sort. There's a similar list for a mechanical drive at http://www.zefox.net/~fbsd/rpi3/swaptests/newtests/1.3gbusbmechanical_swapinfo/readdelay.sort where the minimum delay is 6.4 milliseconds. It's tempting to think that at least occasionally flash storage beats mechanical storage. The mechanical drive I used is an old 2.5" salvaged from a Dell compact desktop computer, stuffed into a USB enclosure, so it surely isn't the fastest in the world. Thanks for reading! bob prohaska
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