From mboxrd@z Thu Jan 1 00:00:00 1970 From: Shaohua Li Subject: Re: RAID journal, requirements? Date: Wed, 3 Feb 2016 17:25:05 -0800 Message-ID: <20160204012505.GB29132@kernel.org> References: <20160127001520.GH3039@otheros> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: <20160127001520.GH3039@otheros> Sender: linux-raid-owner@vger.kernel.org To: Linus =?iso-8859-1?Q?L=FCssing?= Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Wed, Jan 27, 2016 at 01:15:20AM +0100, Linus L=FCssing wrote: > Hi, >=20 > I am very excited about the new journaling feature for RAID in md > and want to try it out soon. >=20 > Neil's article on LWN already gave a great overview and some hints > about it, but I am still a little unsure about the technical > requirements for the journal device. E.g. speed, reliability and > size-wise. And what the implications of failing them means. >=20 > I want to setup a small home NAS. My personal priorities are: > Firstly, low power-per-GB as it will be solar powered and battery > backed. Second priority is reliability. Performance is not that > important to me. >=20 > My current plan: Tiny ARM-based computer (Odroid C1+: 4x USB (2.0, > unfortunately) ports), 4x 2TB SATA HDDs (@0.7W idle, 1.7W max. each) > + SATA-to-USB -> RAID5, microSD for OS + applications, an eMMC for > a RAID journal. (later, once they hopefully get available in April, > switch to the Turris Omnia which is ARM-based again but has > mini-PCIe, too, where I would attach mini-PCIe-to-SATA adapters) >=20 > Is it a stupid idea to use an eMMC for the journal in the first > place? I am unsure how quickly it would wear out. And seems like I > do not have SMART for it either... So it would probably fail badly > at some point. Is that an issue, can I simply replace the eMMC > then without any data loss? Will the current code at least try a > write-through directly to the HDDs once a journal disk throws > errors? >=20 > Does MD assume that an underlying device performs some sort of wear > leveling if necessary like SSDs do for instance - or does MD > itself distribute writes evenly over the journaling disk? > (Does anyone know whether eMMCs usually do some kind of wear leveling= ?) >=20 >=20 > Thanks for all your work on this awesome new feature! (and special > thanks to Neil for all the years of maintenance - it is a pitty you > have to step down :( ) The only requirement of journal is it has a block interface. But all wr= ite IO write to journal first, so there could be a lot of write to journal dep= ending on your workload. I don't know about eMMC much, but suppose it doesn't = have good wear leveling support. If journal is broken, your data isn't lost, as data is still stored in = raid disks. Our current policy is the array becomes read-only if journal is = borken, this might be changed if necessary though. Thanks, Shaohua -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html