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Homework and Emergencies Emergency UNIX and Linux Support Is this how LVM snapshots work? Post 302510804 by majid.merkava on Tuesday 5th of April 2011 03:57:14 AM
Old 04-05-2011
Is this how LVM snapshots work?

Hi guys.

I'm confused how LVM snapshots work. Here is what i understood:

1. we have a Logical Volume holding our data.
2. we make a snap shot of it with this command:
Code:
lvcreate -L 1000M -s -n backup /dev/vg01/lv01

3. mount the snap shot
4. take your backup
5. remove the snapshot --> in this step LVM will write new changes back to our main volume.? am i right?

In second step we specify the size. is that 1000MB holds the changes from the point we had created the snapshot?
 

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

NAME
lvextend - extend the size of a logical volume SYNOPSIS
lvextend [--alloc AllocationPolicy] [-A|--autobackup y|n] [-d|--debug] [-h|-?|--help] [--noudevsync] [-i|--stripes Stripes [-I|--stripesize StripeSize]] {-l|--extents [+]LogicalExtentsNumber[%{VG|LV|PVS|FREE|ORIGIN}] | -L|--size [+]LogicalVolumeSize[bBsSkKmMgGtTpPeE]} [-f|--force] [-n|--nofsck] [-r|--resizefs] [-t|--test] [-v|--verbose] LogicalVolumePath [PhysicalVolumePath[:PE[-PE]]...] DESCRIPTION
lvextend allows you to extend the size of a logical volume. Extension of snapshot logical volumes (see lvcreate(8) for information to cre- ate snapshots) is supported as well. But to change the number of copies in a mirrored logical volume use lvconvert(8). OPTIONS
See lvm for common options. --noudevsync Disable udev synchronisation. The process will not wait for notification from udev. It will continue irrespective of any possible udev processing in the background. You should only use this if udev is not running or has rules that ignore the devices LVM2 cre- ates. -l, --extents [+]LogicalExtentsNumber[%{VG|LV|PVS|FREE|ORIGIN}] Extend or set the logical volume size in units of logical extents. With the + sign the value is added to the actual size of the logical volume and without it, the value is taken as an absolute one. The number can also be expressed as a percentage of the total space in the Volume Group with the suffix %VG, relative to the existing size of the Logical Volume with the suffix %LV, of the remaining free space for the specified PhysicalVolume(s) with the suffix %PVS, as a percentage of the remaining free space in the Volume Group with the suffix %FREE, or (for a snapshot) as a percentage of the total space in the Origin Logical Volume with the suffix %ORIGIN. The resulting value is rounded upward. -L, --size [+]LogicalVolumeSize[bBsSkKmMgGtTpPeE] Extend or set the logical volume size in units of megabytes. A size suffix of M for megabytes, G for gigabytes, T for terabytes, P for petabytes or E for exabytes is optional. With the + sign the value is added to the actual size of the logical volume and with- out it, the value is taken as an absolute one. -i, --stripes Stripes Gives the number of stripes for the extension. Not applicable to LVs using the original metadata LVM format, which must use a sin- gle value throughout. -I, --stripesize StripeSize Gives the number of kilobytes for the granularity of the stripes. Not applicable to LVs using the original metadata LVM format, which must use a single value throughout. StripeSize must be 2^n (n = 2 to 9) -f, --force Proceed with size extension without prompting. -n, --nofsck Do not perform fsck before extending filesystem when filesystem requires it. You may need to use --force to proceed with this option. -r, --resizefs Resize underlying filesystem together with the logical volume using fsadm(8). Examples "lvextend -L +54 /dev/vg01/lvol10 /dev/sdk3" tries to extend the size of that logical volume by 54MB on physical volume /dev/sdk3. This is only possible if /dev/sdk3 is a member of volume group vg01 and there are enough free physical extents in it. "lvextend /dev/vg01/lvol01 /dev/sdk3" tries to extend the size of that logical volume by the amount of free space on physical volume /dev/sdk3. This is equivalent to specifying "-l +100%PVS" on the command line. "lvextend -L+16M vg01/lvol01 /dev/sda:8-9 /dev/sdb:8-9" tries to extend a logical volume "vg01/lvol01" by 16MB using physical extents /dev/sda:8-9 and /dev/sdb:8-9 for allocation of extents. SEE ALSO
fsadm(8), lvm(8), lvcreate(8), lvconvert(8), lvreduce(8), lvresize(8), lvchange(8) Sistina Software UK LVM TOOLS 2.02.95(2) (2012-03-06) LVEXTEND(8)
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