[mythtv-users] Better think twice before choosing an FS especially for LVM

Brandon Beattie brandon+myth at linuxis.us
Thu Sep 15 21:12:28 UTC 2005


On Thu, Sep 15, 2005 at 04:38:44PM -0400, Scot L. Harris wrote:
> If you are concerned about failed drives LVM is not a solution.  RAID is
> a solution to handling failed drives.

I'm only mildly concerned.  If a drive just goes I'll accept that no
loss.

If I see one that is going to fail, but in no way has yet, it bothers me 
that I can't do anything about it.  This was my example of the 
pitfall that people think they can use LVM and change space around, for 
whatever reason, whenever they want.  If you use XFS, JFS, or Reiser4 
then you can only swap disks around and add space -- you can never shrink 
the filesystem and there's many uses that this feature is important.  
If you decided to just pull the drive because you need it in another
system, you can not do that with a file system that does not support
shrinking.  I'm sure very, very few people know this and those people
probably had thought they could, because it's LVM, which can grow and
shrink as it wants, and people are given a false impression that the
file system does not effect this.

For me, I see a disk is starting to reallocate blocks so I know it will
begin failing in weeks to months and if (for example) I had used
ReiserFS instead of XFS I could have shrunk the filesystem and LVM and
taken the disk out right then.  Having chosen XFS I can not do this.

> You have an excellent point regarding that fact that you can not shrink
> certain file systems.  IMHO trying to use LVM to provide a solution for
> handling failed drives or as a method of replacing failed drives is not
> the right tool for that job.

My choice for LVM was never to avoid failed disks, it was chosen to
isolate failure to partial loss for things I only minorly care about but
almost completely for convenience and functionality.  The only way
to avoid complete loss is make it less likely, and even raid5 complete
loss is something I've seen a few times in companies I've worked for.
Raid5 hardware and software raid are about 97% safer but to me that's
unacceptable for family photo's and videos.  My personal
choice is to backup anything I care about to something that if hit by a
power surge, accidental deletion, flood, or fire, would not cause any
data loss of something I needed.  Losing TV shows are wants, and to me
do not require raid.  I did raid 0, 1, and 5 for a year in my media
system with 5 drives.  I came to the conclusion that being able to add
and remove disks, partitions, isolate failure, and convenience were
better served with LVM and leaving Raid out of the picture.  I'm on my 
third year of HD PVR usage and have tried just about everything in the
book -- I'm very happy with my setup as it is now. .. I just wanted to
point this shrinking pitfall out so others don't fall in the "Why can't 
I shrink my [best filesystem for Myth] XFS or JFS LVM partition, I 
thought that's what LVM was for" group, regardless why they want to 
shrink it.


--Brandon


P.S. for other insight.. <google>Linux HTPC Howto</google>


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