From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: In this partition scheme, grub does not find md information? Date: Tue, 29 Jan 2008 18:44:20 -0500 Message-ID: <479FBA54.6010009@tmr.com> References: <479EAF42.6010604@pobox.com> <18334.46306.611615.493031@notabene.brown> <479F07E1.7060408@pobox.com> <479F0AAB.3090702@rabbit.us> <479F331F.7080902@msgid.tls.msk.ru> <479F3C74.1050605@rabbit.us> <479F42A5.8040007@msgid.tls.msk.ru> <479F5177.6060206@pobox.com> <479F557D.20502@rabbit.us> <479F7FCD.7030106@pobox.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <479F7FCD.7030106@pobox.com> Sender: linux-raid-owner@vger.kernel.org To: Moshe Yudkowsky Cc: Peter Rabbitson , Michael Tokarev , linux-raid@vger.kernel.org List-Id: linux-raid.ids Moshe Yudkowsky wrote: > I'd like to thank everyone who wrote in with comments and > explanations. And in particular it's nice to see that I'm not the only > one who's confused. > > I'm going to convert back to the RAID 1 setup I had before for /boot, > 2 hot and 2 spare across four drives. No, that's wrong: 4 hot makes > the most sense. > > And given that RAID 10 doesn't seem to confer (for me, as far as I can > tell) advantages in speed or reliability -- or the ability to mount > just one surviving disk of a mirrored pair -- over RAID 5, I think > I'll convert back to RAID 5, put in a hot spare, and do regular > backups (as always). Oh, and use reiserfs with data=journal. > Depending on near/far choices, raid10 should be faster than raid5, with far read should be quite a bit faster. You can't boot off raid10, and if you put your swap on it many recovery CDs won't use it. But for general use and swap on a normally booted system it is quite fast. > Comments back: > > Peter Rabbitson wrote: > >> Maybe you are, depending on your settings, but this is beyond the >> point. No matter what 1+0 you have (linux, classic, or otherwise) you >> can not boot from it, as there is no way to see the underlying >> filesystem without the RAID layer. > > Sir, thank you for this unequivocal comment. This comment clears up > all my confusion. I had a wrong mental model of how file system maps > work. > >> With the current state of affairs (available mainstream bootloaders) >> the rule is: >> Block devices containing the kernel/initrd image _must_ be either: >> * a regular block device (/sda1, /hda, /fd0, etc.) >> * or a linux RAID 1 with the superblock at the end of the device >> (0.9 or 1.2) > > Thaks even more: 1.2 it is. > >> This is how you find the actual raid version: >> >> mdadm -D /dev/md[X] | grep Version >> >> This will return a string of the form XX.YY.ZZ. Your superblock >> version is XX.YY. > > Ah hah! > > Mr. Tokarev wrote: > >> By the way, on all our systems I use small (256Mb for small-software >> systems, >> sometimes 512M, but 1G should be sufficient) partition for a root >> filesystem >> (/etc, /bin, /sbin, /lib, and /boot), and put it on a raid1 on all... >> ... doing [it] >> this way, you always have all the tools necessary to repair a damaged >> system >> even in case your raid didn't start, or you forgot where your root >> disk is >> etc etc. > > An excellent idea. I was going to put just /boot on the RAID 1, but > there's no reason why I can't add a bit more room and put them all > there. (Because I was having so much fun on the install, I'm using 4GB > that I was going to use for swap space to mount base install and I'm > working from their to build the RAID. Same idea.) > > Hmmm... I wonder if this more expansive /bin, /sbin, and /lib causes > hits on the RAID1 drive which ultimately degrade overall performance? > /lib is hit only at boot time to load the kernel, I'll guess, but /bin > includes such common tools as bash and grep. > >> Also, placing /dev on a tmpfs helps alot to minimize number of writes >> necessary for root fs. > > Another interesting idea. I'm not familiar with using tmpfs (no need, > until now); but I wonder how you create the devices you need when > you're doing a rescue. > > Again, my thanks to everyone who responded and clarified. > -- Bill Davidsen "Woe unto the statesman who makes war without a reason that will still be valid when the war is over..." Otto von Bismark