From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rich Subject: Re: LINEAR RAID, little help Date: Sat, 07 Apr 2007 19:41:38 +0100 Message-ID: <4617E5E2.2090609@pcfusion.co.uk> References: <46177DD8.9080707@pcfusion.co.uk> <20070407173352.GA23645@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20070407173352.GA23645@gmail.com> Sender: linux-raid-owner@vger.kernel.org To: Gavin McCullagh Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Gavin McCullagh wrote: > Hi, > > The main reason I'm posting (given others can answer these questions > better) is to ask a further question:=20 > > Why would anyone use RAID-linear? If RAID-0 gives better performance= for > the same (reduced) reliability, what's the point of using Linear? Do= you > get slightly more space out of it? I presume it's for stuff where yo= u > don't place much value no your data. > > On Sat, 07 Apr 2007, Rich wrote: > > =20 >> I currently have a linear RAID setup via mdadm with is made up of 3=20 >> drives. I just have a few questions that I can't seem to find search= ing=20 >> around on Google, etc. >> =20 > > I must admit I've never used linear raid. May I ask what made you ch= oose > it over say raid-0? > =20 Er, I went with Linear as reading around people seemed to recommend thi= s=20 for odd sized drives (my old drives are 80's, 120 and 320's) also a rea= d=20 somewhere that data on the other drives is more recoverable that most o= f=20 the other RAID's. > =20 >> First question, what happens if one drive fails (I know I will loose= the=20 >> data on that drive) but how, if at all can I recover the data on the= =20 >> other drives can I plug them in (on their own) as though they were i= n=20 >> fact an individual drives? Do I need to execute a rebuild command in= =20 >> mdadm at all to rebuild the array? >> =20 > > http://tldp.org/LDP/EVMSUG/html/characraidlvls.html > > I'm sure someone else will give you a better answer but it would appe= ar > from these docs that your situation is similar to that of a person us= ing > RAID-0 who has a drive failure. Basically, you end up with a big por= tion > of your filesystem data missing so you may lose everything. I guess = you > might be able to recover some data using some filesystem recovery too= ls but > it's likely to be a pretty unreliable process and likely to vary from= one > filesystem to the next. It's probably a little more reliable than RA= ID-0 > in that what's left is probably contiguous but I would certainly not = want > to rely on such a process for my data. > > The faq says similar "RAID-linear is a simple concatenation of partit= ions > to create a larger virtual partition. It is handy if you have a numbe= r > small drives, and wish to create a single, large partition. This > concatenation offers no redundancy, and in fact decreases the overall > reliability: if any one disk fails, the combined partition will fail.= " > > http://tldp.org/HOWTO/Software-RAID-0.4x-HOWTO-2.html > =20 >> Second question, how can I go about adding a drive to my linear RAID= , I >> wish to add two new 500GB drives but I'm unsure how. I have found ho= wto's >> for RAID 5 but I just wanted to check it was a similar process for >> Linear? Also this won't effect any data currently on the drive will = it? >> =20 > > You don't say what you're doing on this array, but before modifying i= t, I'd > be seriously inclined to question whether RAID-linear is really the r= ight > thing to be using at all. Anyway, the mdadm manpage says=20 > =20 Well it contains stuff like TV and films a lot of which my friends have= ,=20 so loosing it is not that big of a deal just inconvenient. As I said=20 before I read linear was a better choice for odd sized drives and had=20 better data recovery that the other RAID's > "Currently the only support available is to > =B7 change the "size" attribute for RAID1, RAID5 and RAID6. > =B7 increase the "raid-disks" attribute of RAID1 and RAID5. > =B7 add a write-intent bitmap to any array which support these bitma= ps, or > remove a write-intent bitmap from such an array." > > which suggests you can't. That might be wrong though as it sounds (t= o me > anyway) like linear would be one of the easier ones to implement grow= for. > I'm not sure. > > Gavin > > =20 I'm sure I read somewhere on the mdadm page in one of the posts about=20 adding support for linear growing, and that was dated 2004 so it might=20 be in there? Just wasn't to sure how to do it... Thanks, Rich :) - 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