From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx1.redhat.com (mx1.redhat.com [172.16.48.31]) by int-mx1.corp.redhat.com (8.11.6/8.11.6) with ESMTP id j21LLpK01951 for ; Tue, 1 Mar 2005 16:21:51 -0500 Received: from dermis.amis.com (dermis.amis.com [207.141.5.253]) by mx1.redhat.com (8.12.11/8.12.11) with SMTP id j21LLnqV007583 for ; Tue, 1 Mar 2005 16:21:50 -0500 Received: from [172.16.98.4] (ws4.design.amis.com [172.16.98.4]) by mxrelayus.pc.amis.com (Postfix) with ESMTP id B6FD058847 for ; Tue, 1 Mar 2005 14:21:43 -0700 (MST) Message-ID: <4224DCE5.1010202@theserrs.net> Date: Tue, 01 Mar 2005 14:21:41 -0700 From: Scott Serr MIME-Version: 1.0 Subject: Re: [linux-lvm] Building up a RAID5 LVM home server (long) References: <42249BB2.7010901@theserrs.net> <38256.64.233.217.29.1109708080.squirrel@www.echohome.org> In-Reply-To: <38256.64.233.217.29.1109708080.squirrel@www.echohome.org> Content-Transfer-Encoding: 7bit Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: LVM general discussion and development Erik Ohrnberger wrote: >On Tue, March 1, 2005 11:43, Scott Serr said: > > >>Erik Ohrnberger wrote: >> >> >> >>>Dear Peeps of the LVM discussion list, >>> >>> > >....snip.... > > > >>>The Questions: >>>============== >>> It seems to me that RAID5 with at least one hot spare hard disk is >>>one >>>of the safest ways to go for this type of storage. The only concern that >>>I >>>have is specific to the wide variety of hard disk sizes that I have >>>available (2 40GB, 1 60GB, 2 80GB, and I'll probably add the 200GB drive >>>once I've migrated that data off it to the array). My limited >>>understanding >>>of RAID5 is that it's best if all the hard drives are exactly the same. >>>Is >>>this true? What are the downsides of using such a mix of hard disk >>>sizes? >>> >>> >>> >>> >>The down side is the partitions that make up a RAID5 have to match in >>size, if they don't - the RAID5 just uses the minimum partition size of >>the set for EACH partition. So if you have 20GB, 30GB, 40GB. 10GB of >>the 30GB will be wasted. 20GB of the 40GB will be wasted. So you might >>as well use the wasted space for scratch etc. You can optimize your >>disk use but you never want to include TWO partitions from one disk in >>the same RAID set. Right? >> >> > >Not including two partitions from the same drive into the same raid set >would make sense. What would the redundancy of that be? It wouldn't be. > >However, I've been thinking on this. What if I took two small drives >and raid-0'd them into a single larger block device, and then included >that into the raid5 set. Is this possible? > >20GB + 10GB (of 40GB) = md0 (raid 0) then >md0 + 40GB (what's left) + 40 GB = md1 (raid 5) > >Umm, right. That would break the rules, 40GB is contributing twice to the >raid5 set. Hmmm. > >What if the I broke everything into 10 GB pieces, and created multiple raid5 >sets? Then I could LVM2 them together and have a large filesystem that way. > >a=20GB, b=30GB, c=40GB > >a-1 + b-1 + c-1 = md0 (approx 30 GB storage) >a-2 + b-2 + c-2 = md1 (approx 30 GB storage) > b-3 + c-3 = md2 (waiting for one more drive) > c-4 = md3 (waiting for two more drives) > > This is sorta what I do. But in my opinion the gain of having RAID5 (over RAID1) is when you get over 3 disks... at 3 disks you are burning 33% for redudnacy... 25% or 20% or 17% sounds better to me. I guess if you go too far it costs in calculating the parity. >>> Being able to resize the storage is a key, as is having a robust and >>>reliable storage pool. As storage demands rise and fall, it's great to >>>have >>>the flexibility to add and drop hard disks from the storage pool and use >>>them for other things, resizing the file system and the volume group as >>>you >>>go along, of course. If the storage pool is RAID5, and I add a larger >>>hard >>>disk to the pool as a hot spare, and then use the software tools to fault >>>out the drive that I want, forcing a reconstruction, couldn't I pull the >>>faulted drive out, and use it for something else? What sort of shape or >>>state will the RAID5 array be in at this point? Will it use all of the >>>space on the newly added hot spare? >>> >>> >>> >>> >>I haven't use hot spares on Linux, a little on Solaris. You could do >>what you say in theory. But normally on low budget stuff it's not "hot >>plug" so you would have to shutdown and pull out your main drive. In my >>situation this would be bad, because I don't do my / (root) on RAID5. I >>could boot my "backup" root that I make with rsync, but then I would >>have to fix the fstab and make sure GRUB is installed on there and have >>a BIOS that will point to hdb (not just hda) for booting. >> >> > >Well, I wasn't thinking hot swapping. I'd shut the machine down to add and >remove hard disks, but the idea is to make use of the reconstruction as a >means for migrating hard disks into and out of the raid array, and it sounds >like that would work OK. > > I've thought about it... But once you have close to a terrabyte of stuff that isn't backed up, well... atleast I whimped out on this. I'm sure it would work in theory and 99% in practice.