From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ric Wheeler Subject: Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush Date: Mon, 23 Aug 2010 11:19:13 -0400 Message-ID: <4C729171.3030605@redhat.com> References: <1281616891-5691-1-git-send-email-tj@kernel.org> <20100820132214.GA6184@lst.de> <4C6E9CAF.5010202@redhat.com> <4C7269E9.9070304@kernel.org> <20100823124815.GA20095@lst.de> <4C727E96.5020801@redhat.com> <4C727F2B.6060501@fusionio.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Christoph Hellwig , Tejun Heo , "linux-fsdevel@vger.kernel.org" , "linux-scsi@vger.kernel.org" , "linux-ide@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-raid@vger.kernel.org" , "James.Bottomley@suse.de" , "tytso@mit.edu" , "chris.mason@oracle.com" , "swhiteho@redhat.com" , "konishi.ryusuke@lab.ntt.co.jp" , "dm-devel@redhat.com" , "vst@vlnb.net" , "jack@suse.cz" , "hare@suse.de" To: Jens Axboe Return-path: In-Reply-To: <4C727F2B.6060501@fusionio.com> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On 08/23/2010 10:01 AM, Jens Axboe wrote: > On 2010-08-23 15:58, Ric Wheeler wrote: >> On 08/23/2010 08:48 AM, Christoph Hellwig wrote: >>> On Mon, Aug 23, 2010 at 02:30:33PM +0200, Tejun Heo wrote: >>>> It might be useful to give several example configurations with >>>> different cache configurations. I don't have much experience with >>>> battery backed arrays but aren't they suppose to report write through >>>> cache automatically? >>> >>> They usually do. I have one that doesn't, but SYNCHRONIZE CACHE on >>> it is so fast that it effectively must be a no-op. >>> >> >> Arrays are not a problem in general - they normally have internally, redundant >> batteries to hold up the cache. >> >> The issue is when you have an internal hardware RAID card with a large cache. >> Those cards sit in your server and the batteries on the card protect its >> internal cache, but do not have the capacity to hold up the drives behind it. >> >> Normally, those drives should have their write cache disabled, but sometimes >> (especially with S-ATA disks) this is not done. > > The problem purely exists on arrays that report write back cache enabled > AND don't implement SYNC_CACHE as a noop. Do any of them exist, or are > they purely urban legend? > Hi Jens, There are actually two distinct problems: (1) arrays with a non-volatile write cache (battery backed, navram, whatever) that do not NOOP a SYNC_CACHE command. I know of one brand that seems to do this, but it is not a common brand. If we do not issue flushes for write through caches, I think that we will avoid this in any case. (2) hardware raid cards with internal buffer memory and on-card battery backup (they sit in your server, disks sit in jbod like expansion shelves). These are fine if the drives in those shelves have write cache disabled. ric