From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Garzik Subject: Re: Converting system to raid Date: Sun, 12 Apr 2009 07:11:43 -0400 Message-ID: <49E1CC6F.7020409@garzik.org> References: <003801c9b96d$21a0f420$0a00a8c0@vorg> <49DF0D06.8030705@musmo.com> <20090410195303.GB21242@cthulhu.home.robinhill.me.uk> <00cc01c9ba1d$3aefecf0$0a00a8c0@vorg> <20090410205942.GC21242@cthulhu.home.robinhill.me.uk> <00f301c9ba23$032025f0$0a00a8c0@vorg> <20090410215129.GD21242@cthulhu.home.robinhill.me.uk> <019701c9ba66$aeed5630$0a00a8c0@vorg> <20090411143642.GA9915@cthulhu.home.robinhill.me.uk> <49E0B188.8050009@hp.com> <49E0BAF9.3090209@garzik.org> <49E0D60E.7040904@hp.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <49E0D60E.7040904@hp.com> Sender: linux-raid-owner@vger.kernel.org To: jim owens Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids jim owens wrote: > Jeff Garzik wrote: >> jim owens wrote: >>> nanosecond time stamps are a myth! >> >> Not true, in the age of NO_HZ... > > While nanosecond time is real on some hardware platforms, > I don't know of any hard disk filesystem that does metadata > updates at that level of granularity and guarantees distinct > timestamps between writes at extremely short intervals. > > Even SSD performance would suck if we did that. > > The filesystem mod-time might have nanoseconds in it, but > that does not mean 2 writes 100 nanoseconds apart will have > different mod-times. Modern Linux filesystems absolutely do do metadata updates at that level of granularity. Are you guaranteed district timestamps between writes? No, but then again, metadata updates were never guaranteed between two writes either. You might just be dirtying a mmap'd page, for example. Jeff