From mboxrd@z Thu Jan 1 00:00:00 1970 From: "A. James Lewis" Subject: Re: layering question. Date: Fri, 07 Aug 2015 15:38:33 +0100 Message-ID: <55C4C2E9.7020501@fsck.co.uk> References: <55C0E63F.2030007@fsck.co.uk> <20150805090446.Horde.VMReCuJzW3PwyI8Gh_BZ_yJ@www3.nde.ag> <55C2B034.2090404@fsck.co.uk> <20150807144344.Horde.5xyJhwIOzk_C5q__CpHL4IV@www3.nde.ag> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from dozer.fsck.co.uk ([77.246.163.77]:58516 "EHLO dozer.fsck.co.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932072AbbHGOih (ORCPT ); Fri, 7 Aug 2015 10:38:37 -0400 In-Reply-To: <20150807144344.Horde.5xyJhwIOzk_C5q__CpHL4IV@www3.nde.ag> Sender: linux-bcache-owner@vger.kernel.org List-Id: linux-bcache@vger.kernel.org To: linux-bcache@vger.kernel.org, "Jens-U. Mozdzen" That's interesting, are you putting your MD on top of multiple bcache devices... rather than bcache on top of an MD device... I wonder what the rationale behind this is? Also, can anyone give me a summary of how bcache compares with dm-cache? James On 07/08/15 13:43, Jens-U. Mozdzen wrote: > Hi *, > > Zitat von Kai Krakow : >> Hi! >> >> A. James Lewis schrieb: >> >>> The problem is tho... with a very large backing store, I'm not really >>> happy with a single point of failure in the cache... is there another >>> way to mirror the cache device? >> >> Well, AFAIR there are plans to add such capabilities into bcache >> itself - >> read: make it possible to add more than one caching device to a cache >> set. >> It will use some sort of hybrid mirror / striping to get the best >> combination of speed and safety - at least that's what the idea is >> about. I >> just don't remember where I've read about it, neither do I know the >> status >> of it. >> >> If you want to eliminate the single point of failure, you may want to >> try >> mdadm with its write-mostly option instead of using bcache. It's >> slower for >> writes obviously but gracefully falls back if the SSD fails. >> Obviously, you >> can also not benefit from having a huge storage because it's classic >> RAID-1 >> and thus the smallest member will limit your storage size. >> >> Bcache also has countermeasures for a failing caching device but I >> didn't >> really look into that yet. You should read the documentation about it in >> Documentation/bcache.txt (Error Handling). The safest mode to use >> here is >> writethrough. > > A work of caution here: At least in my layered (kernel 3.18.8) > situation, the upper layers from time to time run into some sort of > time-out situation when writing to (bcached) disk. Teh writes abort > (bad, but tolerable in my circumstances), but on top this makes MD > mark the current disk faulty, degrading your RAID. > > When using "writeback", the likeliness for this to happen is > relatively small (not more than once every few days), probably because > the writes to SSD are fairly quick. These hit then have always been on > the caching device (MD-RAID1 in my case). > > When using "writethrough", the likeliness was extremely higher (I've > seen 2 hits within 6 hours, not later than 28 hours after switching to > "writethrough") and the hit was on the data device (MD-RAID6 in my case). > > Had I only set up RAID5, my data array would have dropped dead then. > > After switching back to "writeback", I've had *one* further incident, > again on the caching device, within 6 days. > > I would definitely not call "writethrough" "the safest mode" when > using MD-RAID for the bcache devices, on kernel 3.18.8. > > Regards, > Jens > > -- > To unsubscribe from this list: send the line "unsubscribe > linux-bcache" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html