From mboxrd@z Thu Jan 1 00:00:00 1970 From: Raz Subject: Re: Subject: [001/002 ] raid0 reshape Date: Sun, 24 May 2009 01:33:42 +0300 Message-ID: <5d96567b0905231533i60ff7984t29b98152a4be7b85@mail.gmail.com> References: <1241300764.5607.36.camel@raz> <18965.16276.485692.812516@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <18965.16276.485692.812516@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Thu, May 21, 2009 at 2:48 PM, Neil Brown wrote: > On Tuesday May 19, dan.j.williams@intel.com wrote: >> On Sat, May 2, 2009 at 2:46 PM, raz ben yehuda wro= te: >> > Neil Hello >> > The bellow is the raid0 grow code.I have decided to fix raid0 and = not >> > perform the transformation raid0-raid4-raid0 due to two reasons: >> > 1. raid0 zones. this patch support any zone transformations. >> > 2. Undesired dependency of raid0 over raid4 re-striping code. >> >> Hi Raz, >> >> Can you explain a bit more about why the raid4 approach is >> undesirable? =A0I think making reshape only available to raid0 array= s >> where all the members are the same size is a reasonable constraint. >> We then get the nice benefit of reusing the raid5 reshape >> infrastructure. =A0In other words I am not convinced that the benefi= ts >> of reimplementing reshape in raid0 outweigh the costs. > > I've been thinking about this too... Is it something we really want t= o > do? > > My thoughts include: > > =A0- I don't like special cases - it would be nice to support reshape= on > =A0 all arrays, even RAID0 with different sizes devices. > =A0- Anyone who does this with a raid0 made of simple drives is askin= g > =A0 for trouble. =A0But a RAID0 over a bunch of RAID5 or RAID1 might = make > =A0 sense. > =A0- Maybe we should support different sized drives in RAID4. =A0As l= ong > =A0 as the parity drive is as big as the largest data drive it could = be > =A0 made to work. =A0Similarly hot spares would need to be big, but y= ou > =A0 could have 2 hot spares and take the smallest one that is big > =A0 enough. > =A0 If a drive in the RAID4+ (or is it is the thing called NORAID?) > =A0 failed and was replaced with a bigger drive, it would be cool to = be > =A0 able to incorporate that extra space into the array. > > =A0 If we did all that, then the 0->4->0 conversion could make use of > =A0 the same code. > =A0- Surely RAID0 is (like LVM) just a legacy idea until we get sensi= ble > =A0 file systems that actually understand multiple devices and do all > =A0 this stuff for you are a more sensible level - so why are we > =A0 busting a gut(*) to make RAID0 work well?? =A0Answer is of course > =A0 that no-one has made a sensible file system yet. (well... maybe z= fs > =A0 or btrfs, not sure) there is pvfs2 that stripes in a file level. though without redundancy. but i do consider pvfs2 as a profesional file system. > =A0- If you read the DDF spec carefully, you find there is a secondar= y > =A0 raid level which stripes over heterogeneous arrays a different wa= y. > =A0 You divide every primary array up into N chunks, so the chunk siz= es are > =A0 different on different arrays. =A0Then you make a secondary array= by > =A0 striping over those chunks. > =A0 So e.g. you might have a 4Gig RAID5 and a 1GIG RAID1. =A0So the > =A0 stripes array on top of these could take 4Meg from the RAID5, the= n > =A0 1Meg from the RAID1, then another 4 from the RAID5 etc. > =A0 So we want to support that? =A0And would we want to reshape such = a > =A0 thing?? i want;but what i do wonder is how raid awared file systems are to be t= uned. chunk=3Dstripe ? > So: lots of thoughts, some pointing in different directions. > But I'm not against reshape code appearing in RAID0 providing it is > well designed, maintainable, reliable, and doesn't slow down normal > RAID0 processing. =A0I suspect we can get there. > > NeilBrown > > > > > * is that an Australian term??? not sure. =A0http://www.wordwebonline= =2Ecom/en/BUSTAGUT > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid"= in > the body of a message to majordomo@vger.kernel.org > More majordomo info at =A0http://vger.kernel.org/majordomo-info.html > -- 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