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Thread: Linux expertise required please!

  1. #17
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    Re: Linux expertise required please!

    Wait, so you want to be able to boot off /dev/sdb should /dev/sda fail right? Then why didn't you just make /boot an md raid1 volume?
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    Re: Linux expertise required please!

    Quote Originally Posted by oolon View Post
    Putty and Winscp are great tools. Putty will do keys and ssh-agents, more importantly it will also do port forwarding as well as using HTTPS proxies.
    ! Just hadn't seen the portable version - although I couldn't use that from my current location. on my own

    Quote Originally Posted by aidanjt View Post
    Wait, so you want to be able to boot off /dev/sdb should /dev/sda fail right? Then why didn't you just make /boot an md raid1 volume?
    Well at the time I was a little wary of booting off a software RAID device and I don't think Anaconda would let me create /boot on a RAID array - but I suppose I could convert sda1 into a raid partition and then add sb1 to it. Think I need to experiment with that before adding it to a production system though.

    So if I convert sda1 to a an autodetect partition - and alter /etc/fstab that should do it or would I need to recreate initrd? Think I need to build a test machine - I think I have a spare PC or two I can use!
    Last edited by peterb; 18-05-2010 at 02:12 PM.
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    Re: Linux expertise required please!

    Naw, you'll be fine, md keeps the blocks identical, the BIOS attempts to initialise sda first, if that isn't detected it'll move on to sdb, as long as you have grub installed on the mbrs of both, everything should behave as if it were one volume. The only cravet is if you use grub-0.97's 'savedefault' in a multiboot scenario, md might take one or the other as a failed partition. But since this is a server, I'd assume this isn't an issue for you.
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    Re: Linux expertise required please!

    Don't think there is anything unusual here - but this is fstab:

    Code:
    /dev/mapper/system-root /                       ext3    defaults        1 1
    UUID=3ae24813-2acc-4563-a29c-f8c70914ace3 /boot                   ext3    defaults        1 2
    /dev/mapper/system-home /home                   ext3    defaults        1 2
    /dev/mapper/system-photos1 /photos1                ext3    defaults        1 2
    /dev/mapper/system-photos2 /photos2                ext3    defaults        1 2
    /dev/mapper/system-photos3 /photos3                ext3    defaults        1 2
    /dev/mapper/system-photos4 /photos4                ext3    defaults        1 2
    /dev/mapper/system-var  /var                    ext3    defaults        1 2
    /dev/mapper/system-swap swap                    swap    defaults        0 0
    tmpfs                   /dev/shm                tmpfs   defaults        0 0
    devpts                  /dev/pts                devpts  gid=5,mode=620  0 0
    sysfs                   /sys                    sysfs   defaults        0 0
    proc                    /proc                   proc    defaults        0 0
    ~
    I think making /boot part of an array may be the answer, but I'm still curious as to why it will boot off one disk - but not the other which should be a logical copy - unless it is related to the RAID metadata.
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    Re: Linux expertise required please!

    Because UUID=3ae24813-2acc-4563-a29c-f8c70914ace3 is the filesystem assigned to /dev/sda1, so while grub is doing it's thing, the fedora init scripts governing mounting is failing when it can't find /dev/sda1 to do an fsck, it returns an error and bails out since it wouldn't know what to do in the case of mount failure. As a cheap work around you could add the 'noauto' option so it doesn't try to mount /boot at boot time, but making it raid1 really is the best way to save you pain in the long run.
    Quote Originally Posted by Agent View Post
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    Re: Linux expertise required please!

    Would that still be true though if the 'old' sda is removed and sdb reconnected as sda? (which is what I was doing). That does make sense though as one of the error messages was 'superblock missing or corrupt' which I thought referred to just/var - but in the light of that, prossibly referes to the entire FS... ?
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    Re: Linux expertise required please!

    As it's using a UUID instead of a the traditional /dev/sdXn, yes. Changing it still wont handle all failure cases, though.
    Quote Originally Posted by Agent View Post
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  10. #24
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    Re: Linux expertise required please!

    Scsi devices are enumated in order (sata disks work though this scsi interface) that means that the first disk on the first controller it finds is called sda, the second sdb and so on. This is different compared to PATA where the hda,b,c,d had physical signifance. The device name cannot be assumed and a different way is needed to identify the disk. One of the biggest moans about linux is its lack of persistent device names. This is particularly noticeable with fibre channel devices. As a result a number of ways have been devised to work out what disk is what, like using dev-mapper, the problem with these software solutions if you need a half active system before you can use them. So you get this kind of race condition, and other methods like looking at the UUID of the filesystem or label of it are used to identify the disk.

    If your case an easy solution would be to change the UUID= bit to /dev/sda1, so when the first disk is in place it uses that filesystem, if it fails the second one will float up from sdb to sda and will still work, however if the disk fails while the computer is active or the disk does bad reads this will not work, and mirroring is the way to go.

    Edit:

    Also of course if you put a blank disk in to replace "sda" or rather the disk with the lowest sata port number, then on boot this will be seen and have no filesystem on it.
    Last edited by oolon; 19-05-2010 at 10:20 AM.
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    Re: Linux expertise required please!

    Quote Originally Posted by oolon View Post
    <---snip

    If your case an easy solution would be to change the UUID= bit to /dev/sda1, so when the first disk is in place it uses that filesystem, if it fails the second one will float up from sdb to sda and will still work, however if the disk fails while the computer is active or the disk does bad reads this will not work, and mirroring is the way to go.
    That might be a good workaround - while I sort out mirroring the boot partition (although as it is static data (pretty much) mirring isn't immediately essential.

    Quote Originally Posted by oolon View Post
    Also of course if you put a blank disk in to replace "sda" or rather the disk with the lowest sata port number, then on boot this will be seen and have no filesystem on it.
    If sda failed, the plan would be to replace sda with sdb and put in a new sdb - and it was while testing that set up when the problem appeared. But I guess if the boot partition is mirrored AND Grub has been set up on sdb or the MBR copied over, that wouldn't be required as the mobo would boot off the first available disk with a valid MBR.
    Last edited by peterb; 19-05-2010 at 11:14 AM.
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