[linux-lvm] Cannot create snapshots with kernel 2.4.24, lvm 1.0.4: Cannor allocate memory

[linux-lvm] Cannot create snapshots with kernel 2.4.24, lvm 1.0.4: Cannor allocate memory

[Steve Kann]

Hi,

    I've seen some others on this (and other lists) who are having this 
problem, but I haven't been able to find a solution.

    I have a new box that I'd like to use with LVM.  This will be a 
production fileserver.  I've installed debian woody on it, and upgraded 
to the latest debian kernel, 2.4.24.  I'm using the lvm 1.0.4 tools.

    Naturally, I'm spending this time before this box is in use to play 
with LVM so I know how to do what I need to quickly before the fires 
hit.  So, I've been able to happily create, grow, shrink, and remove 
lv's, as well as create and remove VGs and PVs.

    However, I cannot create a snapshot volume.

    This is pretty much why I wanted to use LVM in the first place 
(backing up a 400G active filesystem would be pretty impossible otherwise).

    The basic error I get is:

  520  vgcreate -s 128M vg0 /dev/sda7
  521  vgdisplay
  522  lvcreate -L 256M  -nhome vg0

u2:~# lvcreate -s -L128M -c 4k -nhomesnap /dev/vg0/home
lvcreate -- WARNING: the snapshot will be automatically disabled once it 
gets full
lvcreate -- ERROR "Cannot allocate memory" creating VGDA for 
"/dev/vg0/homesnap" in kernel

Other snapshot creation attempts, all of which fail the same way:
  525  lvcreate -s -L1M -nhomesnap /dev/vg0/home
  526  lvcreate -s -L128M -c 1M -nhomesnap /dev/vg0/home


    I've tried making a bunch of different volumes on which to base the 
snapshot, as well as a number of different parameters for the snapshot 
itself.  I thought that perhaps it couldn't handle snapshots that were 
larger than a certain size, or snapshots of volumes that are beyond a 
certain size, etc.

    I should have plenty of memory:
u2:~# free
             total       used       free     shared    buffers     cached
Mem:       4010744     111160    3899584          0      18444      58084
-/+ buffers/cache:      34632    3976112
Swap:      2097136          0    2097136

If 3.8GB of Real, or 5.8 GB of virtual memory aren't actually enough to 
create a 128MB snapshot of a 256Mb volume (I tried making it very 
small!), then something must be very wrong!

Any ideas?

[linux-lvm] Re: Cannot create snapshots with kernel 2.4.24, lvm 1.0.4: Cannor allocate memory

[Steve Kann]

Hi List,

    Thanks to some helpful off-list help from someone on the list, I've 
found out that the trouble is related to using LVM with a HIMEM kernel.  
I have rebuilt a new kernel without himem support, and snapshots are now 
working.

    On the other hand, if I run this way, I'll have 3GB of memory 
basically sitting there doing not much else other than contributing to 
chassis heat.

    I also have to get the VFS-Lock patch in there (i'm building a 
patched kernel now to test it) before snapshots are really useful.

    So, does anyone either (a) know what the problem is, or (b) at least 
know where I should look to try and fix it?  I'm guessing that the 
problem has been solved at least by some of the commerical distros..

Thanks.

-SteveK


Steve Kann wrote:

>
>
> Hi,
>
>    I've seen some others on this (and other lists) who are having this 
> problem, but I haven't been able to find a solution.
>
>    I have a new box that I'd like to use with LVM.  This will be a 
> production fileserver.  I've installed debian woody on it, and 
> upgraded to the latest debian kernel, 2.4.24.  I'm using the lvm 1.0.4 
> tools.
>
>    Naturally, I'm spending this time before this box is in use to play 
> with LVM so I know how to do what I need to quickly before the fires 
> hit.  So, I've been able to happily create, grow, shrink, and remove 
> lv's, as well as create and remove VGs and PVs.
>
>    However, I cannot create a snapshot volume.
>
>    This is pretty much why I wanted to use LVM in the first place 
> (backing up a 400G active filesystem would be pretty impossible 
> otherwise).
>
>    The basic error I get is:
>
>  520  vgcreate -s 128M vg0 /dev/sda7
>  521  vgdisplay
>  522  lvcreate -L 256M  -nhome vg0
>
> u2:~# lvcreate -s -L128M -c 4k -nhomesnap /dev/vg0/home
> lvcreate -- WARNING: the snapshot will be automatically disabled once 
> it gets full
> lvcreate -- ERROR "Cannot allocate memory" creating VGDA for 
> "/dev/vg0/homesnap" in kernel
>
> Other snapshot creation attempts, all of which fail the same way:
>  525  lvcreate -s -L1M -nhomesnap /dev/vg0/home
>  526  lvcreate -s -L128M -c 1M -nhomesnap /dev/vg0/home
>
>
>    I've tried making a bunch of different volumes on which to base the 
> snapshot, as well as a number of different parameters for the snapshot 
> itself.  I thought that perhaps it couldn't handle snapshots that were 
> larger than a certain size, or snapshots of volumes that are beyond a 
> certain size, etc.
>
>    I should have plenty of memory:
> u2:~# free
>             total       used       free     shared    buffers     cached
> Mem:       4010744     111160    3899584          0      18444      58084
> -/+ buffers/cache:      34632    3976112
> Swap:      2097136          0    2097136
>
> If 3.8GB of Real, or 5.8 GB of virtual memory aren't actually enough 
> to create a 128MB snapshot of a 256Mb volume (I tried making it very 
> small!), then something must be very wrong!
>
> Any ideas?
>
>
>