Home / server / Questions / 208155
Accepted
Luke has no name
Asked: 2010-12-02 18:28:44 +0800 CST

LVM spanning over multiple disks: What disk is a file on? Can I lose a drive without total loss?

  • 772

I have three 990GB partitions over three drives in my server. Using LVM, I can create one ~3TB partition for file storage.

1) How does the system determine what partition to use first?
2) Can I find what disk a file or folder is physically on?
3) If I lose a drive in the LVM, do I lose all data, or just data physically on that disk?

lvm storage

3 Answers

  1. Best Answer

    1. The system fills from the first disk in the volume group to the last, unless you configure striping with extents.

    2. I don't think this is possible, but where I'd start to look is in the lvs/vgs commands man pages.

    3. If you lose a drive in a volume group, you can force the volume group online with the missing physical volume, but you will be unable to open the LV's that were contained on the dead PV, whether they be in whole or in part. So, if you had for instance 10 LV's, 3 total on the first drive, #4 partially on first drive and second drive, then 5-7 on drive #2 wholly, then 8-10 on drive 3, you would be potentially able to force the VG online and recover LV's 1,2,3,8,9,10.. #4,5,6,7 would be completely lost.

    • 13

  2. 1) How does the system determine what partition to use first?

    LVM doesn't really have the concept of a partition it uses PVs (Physical Volumes), which can be a partition. These PVs are broken up into extents and then these are mapped to the LVs (Logical Volumes). When you create the LVs you can specify if the data is striped or mirrored but the default is linear allocation. So it would use the extents in the first PV then the 2nd then the 3rd.

    2) Can I find what disk a file or folder is physically on?

    You can determine what PVs a LV has allocation extents on. But I don't know of a way to get that information for an individual file.

    3) If I lose a drive in the LVM, do I lose all data, or just data physically on that disk?

    As Peter has said the blocks appear as 0's if a PV goes missing. So you can potentially do data recovery on files that are on the other PVs. But I wouldn't rely on it. You normally see LVM used in conjunction with RAIDs for this reason.

    • 5

  3. I don't know the answer to #2, so I'll leave that to someone else. I suspect "no", but I'm willing to be happily surprised.

    1 is: you tell it, when you combine the physical volumes into a volume group.

    3 is: it's effectively as if a huge chunk of your disk suddenly turned to badblocks. You can patch things back together with a new, empty drive to which you give the same UUID, and then run an fsck on any filesystems on logical volumes that went acoss the bad drive to hope you can salvage something.

    And to the overall, unasked question: yeah, you probably don't really want to do that.

    • 3