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

Brandon Beattie brandon+myth at linuxis.us
Thu Sep 15 18:28:38 UTC 2005


I have to put an exception on what I've said for 2 years now.  

I have been a supporter of XFS and JFS because of their much better 
performance for handling large (Many GB) files.  This last week when I
had yet another disk go out I found out why XFS and JFS are not good
choice for me or other people running a media system.  

LVM is almost mandatory for HD since Myth requires you to store all TV
shows in a single directory and a single drive isn't enough for most
people recording HD.  Everything regarding LVM works perfectly for this,
except when you want to make the size of the LVM partition smaller.

XFS and JFS support growing a filesystem, such as adding another drive
or allocation more space in LVM for that partition.  They however do not
support shrinking the filesystem.  

Shrinking I've found is very critical, and someone shouldn't ignore this
fact or they may not like their choices when things go bad.

I run smartmontools to watch for errors and I have a drive begining to
fail.  I can not remove the drive out of the LVM though because that
would require shrinking the filesystem and that's not possible with XFS
or JFS.

My terrible options are as follows:

1 Turn the system off so the drive doesn't fully faul until I get a new 
  drive and not record all the TV pilots and season premiers I've waited 
  4 months for. :)
2 Leave the drive in the system and hope it doesn't fully go bad before
  a new drive arrives.
3 Backup as much data as I can (No way I can backup 1TB) and destroy the
  rest and create a new LVM partition with the 4 good drives and hope a
  drive never goes bad ever again. (Yeah right)

If I were to do option 1 or 2, I have to deal with getting a new drive.
This means not waiting for a good deal and paying more, or dealing with
RMA'ing which has it's own issues.  

RMA'ing a "failing but not failed" drive is risky.  They often don't
check it, but sometimes companies do.  Companies have given hassle of a
drive that has not failed but is, to them it still works, to the user it
means they could lose data at any time.  If someone doesn't want to lose
data, RMA'ing is tricky.

Now for the other possible issues with LVM and a file system that does
not shrink.  Luckily I have an extra IDE channel to connect an extra 
drive to, but if someone is using 4 hard drives or 3 hard drives and a 
CDROM they can't add in that new hard drive, so they can't easily move
data to the new disk, or even save from having to completely redo the
LVM config.

Too bad for me I found out about this problem too late, but hopefully
I'll save others time and frustration.

If I could have just shrunk the FS down to not include this drive and
removed the drive from LVM I could have taken it out and then add in
another drive if or when I got another drive.

The only filesystems that support shrinking are Ext2, Ext3, and
ReiserFS.  Reiser4, Xfs, and JFS do not (Yet, but support for Reiser4
and Xfs is being worked on, but many months from being stable (early 
2006?).

Since ReiserFS (Reiser version 3) is usually better than ext2 and ext3
then it's sadly the best FS if you're going to be using LVM.  ResierFS
outperforms other FS's only when dealing with small files, which is
great for a root fs, but not a large media filesystem.  

Lucklily Myth doesn't need "perfect/the best FS", even when I'm
recording 4 HD streams at once, so as I had never, ever
wanted to do, I'm now going use reiserFS as my FS for LVM and my TB
partition for media.

I should also note, the disk and other hardware have more of an effect
on FS performance than the type of FS.

Also, when XFS comes out with shrinking I will be able to shrink the
reiserFS and create a new XFS LVM setup and grow it and move data from
the current reiserfs that I'll shrink away to nothing.

.... Happy Recording.

--Brandon


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