Linux Storage system: looking for advices Post: 302385275

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Special Forums Hardware Filesystems, Disks and Memory Linux Storage system: looking for advices Post 302385275 by Loic Domaigne on Thursday 7th of January 2010 04:15:48 PM

01-07-2010

Registered User

Good Evening,

Quote:

Originally Posted by pludi

No, it's not possible with LVM alone. LVM is designed to simplify the management of multiple, different devices by grouping them together. Bonus is a slight speed improvement. If you loose one drive with LVM, the data on it is gone for good too, but it's easy to extend the size.

Since I am not very fluent in LVM, I set-up a virtual KVM/Qemu guest with Linux in order to play with LVM and investigate possible failures scenario. Enclosed the results of my experiments.

I have the following setup: a volume group containing the following physical volume /dev/vda6, /dev/vdb2 and /dev/vdb3. I have only one logical volume than spans the entire volume group. ext3 is used as filesystem.

Code:

+--------------------------------------------------+
|                    ext3                          |
+--------------------------------------------------+
+--------------------------------------------------+
|                    mylv                          | logical volume = 100%VG
+--------------------------------------------------+
+--------------------------------------------------+
|                    myvg                          | volume group = vda6, vdb2, vdb3 
+--------------------------------------------------+
+-------------+     +-------------+    +-----------+
| /dev/vda6   |     | /dev/vdb2   |    | /dev/vdb3 |
+-------------+     +------------+    +-----------+

It is possible to save the current volume group meta information using vgcfgbackup.
I failed /dev/vdb3, /dev/vdb2 and /dev/vda6 respectively (zeroed the partition using dd). For vdb2 and vdb3, I could restore the ext3 filesystem as follows:
- recreate the physical volume of the failed partition, giving the right uuid label.
- restore the volume group meta information using vgcfgrestore
- repairing the ext3 filesystem using fsck.
As expected, only the files from the failed partition were missing after the restore operation.

However, I failed to restore the file system if /dev/vda6 gets damaged. I used an alternate superblock for fsck (one located on vdb2 or vdb3), but no avail. I lost information about the data stored on vdb2 and vdb3 (they can be found in lost+found, but name is lost). I didn't managed so far to recover the file system if the first disk (i.e. where the primary superblock is) failed. I still need to investigate what's wrong.

What do you think about these recovery possibilities? No point however that RAID+LVM looks safer.

Quote:

Originally Posted by DukeNuke2

how about opensolaris with ZFS? there are many tutorials on ZFS and how to build a home NAS system...

That could be definitively worse a try. But I'll first investigate on the system where I am most knowledgeable about.

Loic Domaigne

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LEARN ABOUT LINUX

pvck

PVCK(8) 						      System Manager's Manual							   PVCK(8)

NAME

       pvck - check physical volume metadata

SYNOPSIS

       pvck [-d|--debug] [-h|--help] [-v|--verbose] [--labelsector] PhysicalVolume [PhysicalVolume...]

DESCRIPTION

       pvck checks physical volume LVM metadata for consistency.

OPTIONS

       See lvm for common options.

       --labelsector sector
	      By  default, 4 sectors of PhysicalVolume are scanned for an LVM label, starting at sector 0.  This parameter allows you to specify a
	      different starting sector for the scan and is useful for recovery situations.  For example, suppose the partition table is corrupted
	      or  lost	on  /dev/sda, but you suspect there was an LVM partition at approximately 100 MB.  This area of the disk may be scanned by
	      using the --labelsector parameter with a value of 204800 (100 * 1024 * 1024 / 512 = 204800):

	      pvck --labelsector 204800 /dev/sda

	      Note that a script can be used with --labelsector to automate the process of finding LVM labels.

SEE ALSO

       lvm(8), pvcreate(8), pvscan(8) vgck(8)

Sistina Software UK					 LVM TOOLS 2.02.95(2) (2012-03-06)						   PVCK(8)