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

David myth at dgreaves.com
Thu Sep 15 21:57:42 UTC 2005


Brandon Beattie wrote:

>  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.
>
That's true - and a valid observation
I suppose it depends whether you are a fiddler or not.
My big filesystem is dedicated to Myth. If a disk dies then it needs
replacing.
As always it depends what you require in order to decide upon a solution.

>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.
>  
>
Not if you read the Howto or any other LVM docs that mention shrinking...
  http://www.tldp.org/HOWTO/LVM-HOWTO/reducelv.html

>For me, I see a disk is starting to reallocate blocks so I know it will
>begin failing in weeks to months
>
<insert>or seconds.</insert>

> and if (for example) I had used
>ReiserFS instead of XFS I could have 
>
<insert>tried to..</insert>

>shrunk the filesystem and LVM and
>taken the disk out right then.  Having chosen XFS I can not do this.
>  
>
correct.

>>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
>

OK. But in your original message (see "My terrible options, 1,2 & 3")
seemed to be saying:
"If I'd used LVM with a filesystem that can shrink then I would have had
_some_ protection against disk failure"

And that's, IMHO, a bad message.

I know you understand what you mean in the case of a SMART alert - but
frankly, for most people, most of the time, using non-redundant LVM
without raid *increases* your chance of data loss by a factor roughly
equal to the number of disks in the volume(1). So any suggestion that
the filesystem choice assists in the case of disk failure is probably a
disservice to most people who are of an experience level where they are
reading this mailing list for advice.

David
(1) For other readers, yes, if you have a vanilla 5 disk LVM you are
about 5 times _more_ likely to suffer a massive loss of your data.
Brandon, I know you understand this :)



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