From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ric Wheeler Subject: Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md. Date: Thu, 12 Jul 2007 15:43:10 -0400 Message-ID: <4696844E.5060407@emc.com> References: <18006.38689.818186.221707@notabene.brown> <18010.12472.209452.148229@notabene.brown> <20070528094358.GM25091@agk.fab.redhat.com> <5201e28f0705290225v14fdac44hb0382a4137a84d01@mail.gmail.com> <20070529220500.GA6513@agk.fab.redhat.com> <5201e28f0705300212g3be16464u5ee1a4c80db27a11@mail.gmail.com> <465DAC72.1010201@cfl.rr.com> <5201e28f0705310414u1a9aebc4je135748274543946@mail.gmail.com> <465F9197.7060002@gmail.com> <465FC7B1.3060309@gmail.com> <4693D26D.2060004@emc.com> <4758.1184110800@turing-police.cc.vt.edu> <46955D45.8010606@emc.com> <7718.1184261676@turing-police.cc.vt.edu> Reply-To: ric@emc.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <7718.1184261676@turing-police.cc.vt.edu> Sender: linux-raid-owner@vger.kernel.org To: Valdis.Kletnieks@vt.edu Cc: Tejun Heo , david@lang.hm, Stefan Bader , Phillip Susi , device-mapper development , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org, Jens Axboe , David Chinner , Andreas Dilger List-Id: linux-raid.ids Valdis.Kletnieks@vt.edu wrote: > On Wed, 11 Jul 2007 18:44:21 EDT, Ric Wheeler said: >> Valdis.Kletnieks@vt.edu wrote: >>> On Tue, 10 Jul 2007 14:39:41 EDT, Ric Wheeler said: >>> >>>> All of the high end arrays have non-volatile cache (read, on power loss, it is a >>>> promise that it will get all of your data out to permanent storage). You don't >>>> need to ask this kind of array to drain the cache. In fact, it might just ignore >>>> you if you send it that kind of request ;-) >>> OK, I'll bite - how does the kernel know whether the other end of that >>> fiberchannel cable is attached to a DMX-3 or to some no-name product that >>> may not have the same assurances? Is there a "I'm a high-end array" bit >>> in the sense data that I'm unaware of? >>> >> There are ways to query devices (think of hdparm -I in S-ATA/P-ATA drives, SCSI >> has similar queries) to see what kind of device you are talking to. I am not >> sure it is worth the trouble to do any automatic detection/handling of this. >> >> In this specific case, it is more a case of when you attach a high end (or >> mid-tier) device to a server, you should configure it without barriers for its >> exported LUNs. > > I don't have a problem with the sysadmin *telling* the system "the other end of > that fiber cable has characteristics X, Y and Z". What worried me was that it > looked like conflating "device reported writeback cache" with "device actually > has enough battery/hamster/whatever backup to flush everything on a power loss". > (My back-of-envelope calculation shows for a worst-case of needing a 1ms seek > for each 4K block, a 1G cache can take up to 4 1/2 minutes to sync. That's > a lot of battery..) I think that we are on the same page here - just let the sys admin mount without barriers for big arrays. 1GB of cache, by the way, is really small for some of us ;-) ric