Software Raid

[Uwe Geercken]

hi gavin,

thanks for your feedback which is great as always !!!

I have currently two sata disks as indicated one with the system    
(250GB Samsung) and one with home (500GB Samsung).

on the home drive there is about 2GB of data, so I can put that on a dvd.

the system drive has edubuntu 7.10 and I will upgrade to 8.04 anyway  
by reinstalling from scratch. we haven't really installed a lot of  
programs. basically just java, mysql some codecs and some games. I can  
install all this quite quickly from the repository.

thanks for your words on the installation of raid. they will be very  
useful. as I am doing the LPI right now (6 of 9 month done) I have  
already setup raid during the course and I also have some  
documentation, which will get me going.

is there anything that I should not put on the raid or is not worth  
it, like eg. swap?

I will probably put system and home on one drive and have another disk  
for the raid because 500 GB are really enough for our purposes. and I  
can use the thrid disk for downloads or setting up a mirror of a  
repository (which I did in the past).

I think about it over the weekend.

thanks again,

uwe



Zitat von Gavin McCullagh <gmccullagh at gmail.com>:

> Hi,
>
> On Thu, 31 Jul 2008, Uwe Geercken wrote:
>
>> at the time I setup the system, I have used one harddisk for the
>> system and one for the home directories of the students. so these are
>> on seperate drives.
>
> What sizes are the respective drives?  Are they IDE, SATA, SCSI?
>
>> I am now thinking putting in a second drive and use software raid so
>> that I will have faster reads. my first thought was to use raid for
>> the system and still have the home folders on a seperate drive. or
>> maybe I should put home and the system on the same drive and use raid
>> for both?
>
> I presume you're talking about MD (linux kernel software raid).  Presumably
> it can read off the disks independently so you should probably get improved
> read speeds.  To my mind though, reliability is the best feature of RAID1.
>
> http://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_1_performance
>
> If you can get both the system and the home directories onto RAID1, you're
> system will keep running in the event of a single disk failure.  Disk
> failure is remarkably common these days.
>
> Migrating the running system to software RAID is not easy, so you may find
> it easiest to reinstall which is a bit of a downside.  Presumably you can
> just back up and restore the home directories with tar.  For the system, if
> you have sufficient space and don't wish to reinstall, you could possibly
> do something like
>
> 1. Shrink your / partition by 50% using gnu parted
> 2. Create a new equal sized partition in the resulting space.  Make it of
>    type linux-raid
> 3. Create a larger raid partition on the new disk.
> 4. Use mdadm to create a raid1 array using the two raid partitions.
> 5. Format the raid partition ext3 (or whatever you're using)
> 6. Reboot and make sure the raid partition comes back up.
> 7. Drop to single user mode and use something like cpio or rsync to sync
>    the new raid partition with the old one.
> 8. Configure grub with extra boot items to boot onto the new partition
> 9. Reboot using the new grub entry and say a prayer (if it doesn't work,
>    you should be able to boot onto the old partition anyway).
> 10.Once you're up on the new partition, you can re-run grub-install,
>    delete the old non-raid partition, expand the existing one back into the
>    freed space, expand the raid array into the space and finally expand the
>    root filesystem on the bigger partition.
>
> These instructions are untested and I've probably missed something.
> Frankly, I'd highly recommend the reinstall, backing up your config first
> of course!
>
> Gavin
>
>
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