Date: Wed, 3 Jul 2013 13:17:35 -0700 From: Jeremy Chadwick <jdc@koitsu.org> To: Zoltan Arnold NAGY <zoltan.arnold.nagy@gmail.com> Cc: freebsd-fs@freebsd.org Subject: Re: O_DIRECT|O_SYNC semantics? Message-ID: <20130703201735.GB70533@icarus.home.lan> In-Reply-To: <CAGFYgbMPCcr7HmOyStLPUGkzPBHH1Xy5=pyVi1wFFwBAiehN9g@mail.gmail.com> References: <CAGFYgbMPCcr7HmOyStLPUGkzPBHH1Xy5=pyVi1wFFwBAiehN9g@mail.gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Wed, Jul 03, 2013 at 09:05:40PM +0200, Zoltan Arnold NAGY wrote: > Could someone have a look here: > http://serverfault.com/questions/520141/please-explain-my-fio-results-is-o-synco-direct-misbehaving-on-linux > > Basically, I'm seeing wastly different results on Linux and on FreeBSD 9.1. > Either FreeBSD's not honoring O_SYNC properly, or Linux does something > wicked. > > I've been at it for a few days, without any real progress. > > I do realize that since I'm operating at a block device level not with any > filesystem it's strange to ask on -fs, but I came to this results while > experimenting with the SSD as a ZIL device, and was surprised at the low > numbers. Block devices on FreeBSD are ***always*** O_DIRECT. There is no "caching mechanism" with such. Block devices on Linux result in caching, unless O_DIRECT is used. Because you're asking about some underlying kernel behaviour, I might recommend this be discussed on the -hackers list, where many of the low-level folks hang out. I can assure you that you're going to be asked to provide "dmesg" (on FreeBSD) from the system you're testing with, so you'd best have that ready. -- | Jeremy Chadwick jdc@koitsu.org | | UNIX Systems Administrator http://jdc.koitsu.org/ | | Making life hard for others since 1977. PGP 4BD6C0CB |
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20130703201735.GB70533>