Things I learned today

[bigtomrodney]

I have just recovered from a rather large fright. I'll spare you all the details as it something most of you have or will experience at some point. Well after this time-stopping event passed I came out with new knowledge, which is the real benefit of these things. Just thought I'd throw them out there.

• Don't put off the scheduled fsck on your filesystems, even if you are busy
• Even with backups of the essentials, there's no easy way to lose 2TB of data
• LVM isn't as scary as it seems
• fsck is amazing, once you remember to run it against the LVM mapper instead of the partitions

As a side note, since I ran through the full 2TB filesystem and had a few inodes cleaned up my system is noticeably faster. Might be worth a routine run of fsck for any of you guys who like to skip through it

[jmadero]

how do you run fsck against LVM instead of partition? I just let fsck run the routine check after 23 boots or w/e it is, is this not adequate?

[elija]

how do you run fsck against LVM instead of partition? I just let fsck run the routine check after 23 boots or w/e it is, is this not adequate?

That's exactly what I do. Should I be doing more?

[sarlacii]

I'm suddenly feeling queasy about my terabyte externals. I just rely on the default number of boots check too. Have never bothered to play with the settings tool for my ext3 partitions. However, I have enabled Smart check on all drives, with reporting.

[bigtomrodney]

how do you run fsck against LVM instead of partition? I just let fsck run the routine check after 23 boots or w/e it is, is this not adequate?

I might have phrased that badly

What I was doing was forgetting myself and trying to run fsck against the physical partition instead of the filesystem within the volume, the volume itself spread across two partitions.

Disk /dev/sdb: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x0000be9a

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1      121601   976760001   83  Linux

Disk /dev/sdc: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x0002bffd

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1               1      121601   976760001   83  Linux

tom@btrprime:~/Desktop$ sudo lvmdiskscan 
  /dev/ram0      [       64.00 MB] 
  /dev/home/home [        1.82 TB] 
  /dev/ram1      [       64.00 MB] 
  /dev/sda1      [      956.97 MB] 
  /dev/ram2      [       64.00 MB] 
  /dev/ram3      [       64.00 MB] 
  /dev/sda3      [      200.00 MB] 
  /dev/ram4      [       64.00 MB] 
  /dev/sda4      [       42.97 GB] 
  /dev/ram5      [       64.00 MB] 
  /dev/sda5      [       11.18 GB] 
  /dev/ram6      [       64.00 MB] 
  /dev/sda6      [       13.97 GB] 
  /dev/ram7      [       64.00 MB] 
  /dev/ram8      [       64.00 MB] 
  /dev/ram9      [       64.00 MB] 
  /dev/ram10     [       64.00 MB] 
  /dev/ram11     [       64.00 MB] 
  /dev/ram12     [       64.00 MB] 
  /dev/ram13     [       64.00 MB] 
  /dev/ram14     [       64.00 MB] 
  /dev/ram15     [       64.00 MB] 
  /dev/sdb1      [      931.51 GB] LVM physical volume
  /dev/sdc1      [      931.51 GB] LVM physical volume
    1 disk
  22 partitions
  0 LVM physical volume whole disks
  2 LVM physical volumes

tom@btrprime:~/Desktop$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda5              12G  6.9G  3.6G  66% /
tmpfs                 994M     0  994M   0% /lib/init/rw
varrun                994M  328K  993M   1% /var/run
varlock               994M     0  994M   0% /var/lock
udev                  994M  220K  993M   1% /dev
tmpfs                 994M  228K  993M   1% /dev/shm
lrm                   994M  2.7M  991M   1% /lib/modules/2.6.28-11-generic/volatile
/dev/sda1             942M   46M  850M   6% /boot
/dev/sda6              14G  177M   13G   2% /tmp
/dev/mapper/home-home
                      1.8T  930G  812G  54% /home
/dev/sr0              7.4G  7.4G     0 100% /media/cdrom1

I made the initial mistake of running fdisk against /dev/sdb1 and /dev/sdc1, whereas I should have in fact run it against /dev/mapper/home-home

[jmadero]

what exactly is lvm? I don't even think I use it....

[MikeTbob]

http://en.wikipedia.org/wiki/Logical...Manager_(Linux)
That about says it all. I don't use it myself.......I guess because I don't know much about it or don't really need it since I only use one HD for Linux with my own partition layout.

[bigtomrodney]

It's the Linux Logical Volume Manager. Think of it as a way of abstracting filesystems away from physical harddisks. It works amazingly well in conjunction with RAID arrays and is based on VVM, Veritas Volume Manager which was used with HP-UX/Tru64 as far as I remember.

Here's a simple example: Say you have one hard disk. You want to create your home partition. You could create a logical partition and format it as ext3. Then you run out of space...so what do you do? Buy another disk and then mount it at an arbitrary subdirectory of home. Sure, it works but this can get messy and particularly in complicated multi-user scenarios.

But what if instead of that you created an LVM partition and then a group inside that called home. Space runs out, you pop in another disk and you add the new disk to the group. Magic, your filesystem just got bigger without any funny mounting schemes or workarounds.

Fedora has used it by default for a few releases now. Basically, if I bought another 1TB drive I could add it in transparently as if I was using one big harddisk. You can also do crazy things with multiple groups on one LVM volume so that each can be expanded at a later date.

Check out the wiki page. It may seem a little mind boggling at first but it's definitely worth a look even if just from an educational point of view.