[linux-lvm] Saving files from full failing volume

[linux-lvm] Saving files from full failing volume

[Chris Jensen]

Hi,
I've just had a disk die, and when I replace it I'm thinking of
setting up LVM, however I'm wondering how the same scenario would pan
out under LVM:

The disk was 160GB and completely full. On thursday the disk reported
a smart error, so I started transferring data off the disk, but seeing
as how I didn't have another 160GB handy, I had to simply copy
directory by directory to vairous bits of scattered space that I was
able to find, starting with the most important data. (the disk then
completely died after getting about half the data off :-( )

If this disk had been setup with LVM, the filesystem on it would've
spanned that disk and an 80G disk (also close to full). Under LVM
would it have been possible to determine which files were being stored
on the 160G so I could use filesystem tools to move them off before
shrinking the volume and moving it entirely to the 80G disk?

(I've read up about LVM and understand I can replace failed
blocks/disks with /dev/zero, so the shrinking and moving part doesn't
worry me to much, it's just the identifying of where files live that
I'm curious about.)

Thanks for any responses
Chris

Re: [linux-lvm] Saving files from full failing volume

[Eric M. Hopper]

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On Sat, 2005-08-20 at 17:22 +1000, Chris Jensen wrote:
> (I've read up about LVM and understand I can replace failed
> blocks/disks with /dev/zero, so the shrinking and moving part doesn't
> worry me to much, it's just the identifying of where files live that
> I'm curious about.)

The only way to really identify where files live in LVM is to identify
which filesystem they're on.  Then they might end up being on any random
block of that filesystem.  You can identify which physical extents a
particular filesystem is using easily, but not which physical extents a
particular file is using.

Perhaps someone else can correct me if I'm wrong about this, but as I
understand things this is how it works.

Have fun (if at all possible),
-- 
The best we can hope for concerning the people at large is that they
be properly armed.  -- Alexander Hamilton
-- Eric Hopper (hopper@omnifarious.org  http://www.omnifarious.org/~hopper) --

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Re: [linux-lvm] Saving files from full failing volume

[Chris Jensen]

> The only way to really identify where files live in LVM is to identify
> which filesystem they're on.  Then they might end up being on any random
> block of that filesystem.  You can identify which physical extents a
> particular filesystem is using easily, but not which physical extents a
> particular file is using.
> 
> Perhaps someone else can correct me if I'm wrong about this, but as I
> understand things this is how it works.

Thanks for the response Eric. Let me check that I understand this correctly:
 If I were to use LVM to create a file system that spans 2 hard disks,
of capacity 80G each (for simplicities sake), giving a total capacity
of 160G, if that filesystem grows to (say) 100G and then one of the
disks begins to fail, there is no way for me to backup the files that
would be endangered, short of backing up the entire 100G, to another
disk entirely.

Is there a better way I should go about organising my disks and LVM so
that surviving a hard disk failure is not so difficult?

Re: [linux-lvm] Saving files from full failing volume

[Fredrik Tolf]

On Sat, 2005-08-27 at 23:00 +1000, Chris Jensen wrote:
> > The only way to really identify where files live in LVM is to identify
> > which filesystem they're on.  Then they might end up being on any random
> > block of that filesystem.  You can identify which physical extents a
> > particular filesystem is using easily, but not which physical extents a
> > particular file is using.
> > 
> > Perhaps someone else can correct me if I'm wrong about this, but as I
> > understand things this is how it works.
> 
> Thanks for the response Eric. Let me check that I understand this correctly:
>  If I were to use LVM to create a file system that spans 2 hard disks,
> of capacity 80G each (for simplicities sake), giving a total capacity
> of 160G, if that filesystem grows to (say) 100G and then one of the
> disks begins to fail, there is no way for me to backup the files that
> would be endangered, short of backing up the entire 100G, to another
> disk entirely.

That's how I understand it as well. The thing is also that you can't
really say that "a file is endangered", because it may even only be
parts of the file that are endangered. The blocks that make up the
storage area of a single file may well be spread over several disks.

With Linux LVM, there is no way for the filesystem driver to determine
on what disk a certain block resides, so the filesystem cannot safely
partition files on only one disk. Even if Linux LVM provided an
operation to inquire about such things, all filesystem drivers would
have to implement that independently of each other.

> Is there a better way I should go about organising my disks and LVM so
> that surviving a hard disk failure is not so difficult?

With a larger budget, you could buy double disk space and mirror the
volume (similar to RAID10). Depending on your budget, though, I guess
that may well not be possible.

I'm no expert on it, but from what I've heard, the new ZFS filesystem in
Solaris 10 would "fix" many of these issues. Whereas Linux LVM operates
on the block device layer (and thus also allows any filesystem to reside
on a LV), the ZFS filesystem contains a volume manager in the filesystem
layer, so that individual files can be striped, mirrored, and kept on
distinct disks, individually.

Hope it helps,
Fredrik Tolf

Re: [linux-lvm] Saving files from full failing volume

[Dan Pritts]

On Sat, Aug 27, 2005 at 04:58:57PM +0200, Fredrik Tolf wrote:
> > Thanks for the response Eric. Let me check that I understand this correctly:
> >  If I were to use LVM to create a file system that spans 2 hard disks,
> > of capacity 80G each (for simplicities sake), giving a total capacity
> > of 160G, if that filesystem grows to (say) 100G and then one of the
> > disks begins to fail, there is no way for me to backup the files that
> > would be endangered, short of backing up the entire 100G, to another
> > disk entirely.

effectively, all the files in the filesystem are "endangered" - i'd hate
to try to recover the data from the half of the filesystem that was on
the good disk.

Re: [linux-lvm] Saving files from full failing volume

[Chris Jensen]

> With a larger budget, you could buy double disk space and mirror the
> volume (similar to RAID10). Depending on your budget, though, I guess
> that may well not be possible.

It's for personal use, so I don't have that budget - if I did, then I
wouldn't be scrounging for recovery space in the first place :-)

So it seems, unless the data on the partition is without value, or you
can afford enough disks to setup mirroring, I'd be better off just
paritioning the two disks sepeartely and using a sym link farm!

Damn, I was looking forward to playing with LVM.

Chris

Re: [linux-lvm] Saving files from full failing volume

[Fredrik Tolf]

On Sun, 2005-08-28 at 10:47 +1000, Chris Jensen wrote:
> > With a larger budget, you could buy double disk space and mirror the
> > volume (similar to RAID10). Depending on your budget, though, I guess
> > that may well not be possible.
> 
> It's for personal use, so I don't have that budget - if I did, then I
> wouldn't be scrounging for recovery space in the first place :-)

I guessed that. :)

> So it seems, unless the data on the partition is without value, or you
> can afford enough disks to setup mirroring, I'd be better off just
> paritioning the two disks sepeartely and using a sym link farm!

You may at least want to look into software RAID. The disadvantage is
that you cannot change the configuration of a S/W RAID array once you
have created it, and all the disks need to be the same size, but you can
get much cheap redundancy by using RAID 5. The end line is, of course,
that to get redundancy, you will always need at least one extra disk,
but RAID 5 at least only requires _one_ extra disk, and no more.

I have the same problem myself, though. To the LVM devs: Could there
possibly not be a way to create the like of RAID 5 in LVM, and also make
it extensible (i.e. that it has the ability to be extended by more
disks) as LVM already is?

Fredrik Tolf