[linux-lvm] How do you mount a snapshot?

[linux-lvm] How do you mount a snapshot?

[Tracy R Reed]

From what I've read it looks like the oopses on mounting snapshots have
been fixed since I last tried playing with them so I am at it again. I
have a freshly installed RH7.0 system with the 2.4.0-test10 kernel patched
for LVM 0.9 and reiserfs 3.6.22. I can successfully create a snapshot
with:

lvcreate -s -L16 -nsnapshot /dev/vg-sdb1/lv-sdb1

but when I try to mount it:

[root@computer /usr2]# mount -t reiserfs /dev/vg-sdb1/snapshot /snap/
mount: block device /dev/vg-sdb1/snapshot is write-protected, mounting read-only
mount: wrong fs type, bad option, bad superblock on /dev/vg-sdb1/snapshot,
       or too many mounted file systems

Without specifying the filesystem type I get:

[root@computer /usr2]# mount /dev/vg-sdb1/snapshot /snap/
mount: /dev/vg-sdb1/snapshot has wrong major or minor number

I was using mount mount-2.10m but I downloaded the fileutils and compiled
and tried mount-2.10r and I get the same results.

What else do I need to do in order to get a snapshot mounted?

-- 
Tracy Reed      http://www.ultraviolet.org

Re: [linux-lvm] How do you mount a snapshot?

[David Gould]

On Thu, Dec 07, 2000 at 03:39:11AM -0800, Tracy R Reed wrote:
> 
> [root@computer /usr2]# mount -t reiserfs /dev/vg-sdb1/snapshot /snap/
> mount: block device /dev/vg-sdb1/snapshot is write-protected, mounting read-only
> mount: wrong fs type, bad option, bad superblock on /dev/vg-sdb1/snapshot,
>        or too many mounted file systems

Unless they have changed it (possible), reiserfs does not support ro
hardware for mounting unless the filesystem was unmounted cleanly. The
reason is that to make the FS consistant the journal has to be replayed
which necessarily _writes_ to the device. This cannot work on an ro device.

-dg

-- 
David Gould                                                 dg@suse.com
SuSE, Inc.,  580 2cd St. #210,  Oakland, CA 94607          510.628.3380
"As I've gained more experience with Perl it strikes me that it
 resembles Lisp in many ways, albeit Lisp as channeled by an awk
 script on acid."  -- Tim Moore <moore@herschel.bricoworks.com>,
                      on comp.lang.lisp

Re: [linux-lvm] How do you mount a snapshot?

[Chris Mason]

On Thursday, December 07, 2000 11:55:38 -0800 David Gould <dg@suse.com>
wrote:

> On Thu, Dec 07, 2000 at 03:39:11AM -0800, Tracy R Reed wrote:
>> 
>> [root@computer /usr2]# mount -t reiserfs /dev/vg-sdb1/snapshot /snap/
>> mount: block device /dev/vg-sdb1/snapshot is write-protected, mounting
>> read-only mount: wrong fs type, bad option, bad superblock on
>> /dev/vg-sdb1/snapshot, or too many mounted file systems
> 
> Unless they have changed it (possible), reiserfs does not support ro
> hardware for mounting unless the filesystem was unmounted cleanly. The
> reason is that to make the FS consistant the journal has to be replayed
> which necessarily _writes_ to the device. This cannot work on an ro
> device.
> 

We didn't change that ;-)  But, the message would look different.  In this
case, it did not find a reiserfs filesystem at all, or perhaps the reiserfs
module wasn't loaded...

-chris

Re: [linux-lvm] How do you mount a snapshot?

[Tracy R Reed]

On Thu, Dec 07, 2000 at 03:05:08PM -0500, Chris Mason wrote:
> > Unless they have changed it (possible), reiserfs does not support ro
> > hardware for mounting unless the filesystem was unmounted cleanly. The
> > reason is that to make the FS consistant the journal has to be replayed
> > which necessarily _writes_ to the device. This cannot work on an ro
> > device.

Hmm....

> We didn't change that ;-)  But, the message would look different.  In this
> case, it did not find a reiserfs filesystem at all, or perhaps the reiserfs
> module wasn't loaded...

So...can I or can I not use snapshots with reiserfs? The snapshot was
taken of a currently mounted and functioning (reiserfs module
loaded...compiled into the kernel, actually) so if I did the snapshot
correctly there should be a filesystem there. I'm going to wipe it out
(nothing on it but some test files) and try again with ext2 and see what
happens. 

This should work, shouldn't it?

-- 
Tracy Reed      http://www.ultraviolet.org

Re: [linux-lvm] How do you mount a snapshot?

[Chris Mason]

On Thursday, December 07, 2000 12:37:43 -0800 Tracy R Reed
<treed@ultraviolet.org> wrote:

> So...can I or can I not use snapshots with reiserfs? The snapshot was
> taken of a currently mounted and functioning (reiserfs module
> loaded...compiled into the kernel, actually) so if I did the snapshot
> correctly there should be a filesystem there. I'm going to wipe it out
> (nothing on it but some test files) and try again with ext2 and see what
> happens. 
> 
> This should work, shouldn't it?
> 

It should work with ext2.

You can make reiserfs snapshots, but you also need to apply the
linux-2.4.0-test10-VFS-lock.patch that comes with the LVM 0.9 code.  This
adds the hooks LVM needs to ask reiserfs to make the FS consistent before
creating the snapshot.

And, you need to edit linux/drivers/block/lvm.c, and uncomment the
LVM_VFS_ENHANCEMENT #define.  This enables the LVM code that uses the hooks.

-chris

Re: [linux-lvm] How do you mount a snapshot?

[Tracy R Reed]

On Thu, Dec 07, 2000 at 03:56:11PM -0500, Chris Mason wrote:
> You can make reiserfs snapshots, but you also need to apply the

Ah-ha! It works. Thank you!

-- 
Tracy Reed      http://www.ultraviolet.org

Re: [linux-lvm] How do you mount a snapshot?

[David Gould]

On Thu, Dec 07, 2000 at 03:56:11PM -0500, Chris Mason wrote:
> 
> On Thursday, December 07, 2000 12:37:43 -0800 Tracy R Reed
> <treed@ultraviolet.org> wrote:
> 
> > So...can I or can I not use snapshots with reiserfs? The snapshot was
> > taken of a currently mounted and functioning (reiserfs module
> > loaded...compiled into the kernel, actually) so if I did the snapshot
> > correctly there should be a filesystem there. I'm going to wipe it out
> > (nothing on it but some test files) and try again with ext2 and see what
> > happens. 
> > 
> > This should work, shouldn't it?
> 
> It should work with ext2.

Really? I wonder. Maybe right after a sync before anything gets dirtied,
but otherwise, how could the disk image be consistantly, er, consistant?
And would not e2fsck have the same issues about writing?

Or did I just wake up pessimistic today?

-dg
 
-- 
David Gould                                                 dg@suse.com
SuSE, Inc.,  580 2cd St. #210,  Oakland, CA 94607          510.628.3380
"As I've gained more experience with Perl it strikes me that it
 resembles Lisp in many ways, albeit Lisp as channeled by an awk
 script on acid."  -- Tim Moore <moore@herschel.bricoworks.com>,
                      on comp.lang.lisp

Re: [linux-lvm] How do you mount a snapshot?

[Chris Mason]

On Thursday, December 07, 2000 22:33:00 -0800 David Gould <dg@suse.com>
wrote:

> On Thu, Dec 07, 2000 at 03:56:11PM -0500, Chris Mason wrote:

[ snapshots ]

>> It should work with ext2.
> 
> Really? I wonder. Maybe right after a sync before anything gets dirtied,
> but otherwise, how could the disk image be consistantly, er, consistant?
> And would not e2fsck have the same issues about writing?
> 
> Or did I just wake up pessimistic today?
> 

The reason it will not work on reiserfs without the extra API is that on
reiserfs, sync == commit to log, not commit to disk.  So, there is no way
over the command line (or in the kernel) to ask the FS to sync to disk,
even when there is no traffic at all on the FS.

With ext2, if nothing is going on, you can type sync and have all the
buffers on disk.  It is at least possible ;-)  It would be better if ext2
could provide the write suspension hooks as well, but I'm guessing that
will be done in ext3 first (where the journal already does most of the
work).

In my mind, the people that need snapshots the most are the database
programs, where disk traffic can be carefully controlled. So the current
ext2 method is still useful.

-chris

[linux-lvm] LVM Upgrade

[Magnus Naeslund(f)]

Hello, this is for sure a newbie question, but i havent found any docs about
it.
I am running 2.2.16 + LVM 0.8 (a Suse precompiled kernel) and want to
upgrade to 0.9.
If i just patch 2.2.17 vanilla + LFS + LVM + RAWIO, compile, install and
reboot, using the 0.9 lvm tools, it doesnt work.
What commands should i use?
vgscan doesnt work...

Magnus

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
 Programmer/Networker [|] Magnus Naeslund
 PGP Key: http://www.genline.nu/mag_pgp.txt
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

Re: [linux-lvm] How do you mount a snapshot?

[David Gould]

On Fri, Dec 08, 2000 at 09:09:57AM -0500, Chris Mason wrote:
> 
> 
> On Thursday, December 07, 2000 22:33:00 -0800 David Gould <dg@suse.com>
> wrote:
> 
> > On Thu, Dec 07, 2000 at 03:56:11PM -0500, Chris Mason wrote:
> 
> [ snapshots ]
> 
> >> It should work with ext2.
> > 
> > Really? I wonder. Maybe right after a sync before anything gets dirtied,
> > but otherwise, how could the disk image be consistantly, er, consistant?
> > And would not e2fsck have the same issues about writing?
> > 
> > Or did I just wake up pessimistic today?
> > 
> 
> The reason it will not work on reiserfs without the extra API is that on
> reiserfs, sync == commit to log, not commit to disk.  So, there is no way
> over the command line (or in the kernel) to ask the FS to sync to disk,
> even when there is no traffic at all on the FS.
> 
> With ext2, if nothing is going on, you can type sync and have all the
> buffers on disk.  It is at least possible ;-)  It would be better if ext2
> could provide the write suspension hooks as well, but I'm guessing that
> will be done in ext3 first (where the journal already does most of the
> work).

Ok, so we are agreeing, it does not work on ext2 either unless you sync
and do not write between the sync and the snapshot. Doable, but not exactly
"safe".

-dg
 
-- 
David Gould                                                 dg@suse.com
SuSE, Inc.,  580 2cd St. #210,  Oakland, CA 94607          510.628.3380
"As I've gained more experience with Perl it strikes me that it
 resembles Lisp in many ways, albeit Lisp as channeled by an awk
 script on acid."  -- Tim Moore <moore@herschel.bricoworks.com>,
                      on comp.lang.lisp

Re: [linux-lvm] How do you mount a snapshot?

[Christian Limpach]

> Ok, so we are agreeing, it does not work on ext2 either unless you sync
> and do not write between the sync and the snapshot. Doable, but not
exactly
> "safe".

hmm, I think it works for ext2 because you can always mount an ext2
filesystem even if it's in an inconsistent state.  This is what happens
everytime when your machine panics and you need to fsck your disks during
the reboot, the root filesystem is mounted read-only...  It's my
understanding, that reiserfs will always "fsck" (replay it's log) on mount
and that's why it won't mount read-only when the filesystem is not in a
consistent state.  What I don't know is if it would be possible to make
reiserfs not care about the inconsistencies (i.e. not replay the log) when
it mounts read-only?

An atomic sync-snapshot is of course the (implemented) solution, but it
would be nice if it would also work without that...

     christian

[linux-lvm] OOPS while doing vgextend

[Magnus Naeslund(f)]

LVM 0.9 kernel 2.2.17.

What's up?
I'll disable "show lvm in proc" stuff, because the EIP:
lvm_do_create_proc_entry_of_pv
look a little suspect, man...

Magnus

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
 Programmer/Networker [|] Magnus Naeslund
 PGP Key: http://www.genline.nu/mag_pgp.txt
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-


Dec 11 04:59:28 gprods1 kernel: Unable to handle kernel NULL pointer
dereference at virtual address 0000002c
Dec 11 04:59:28 gprods1 kernel: current->tss.cr3 = 1f7e1000, %cr3 = 1f7e1000
Dec 11 04:59:28 gprods1 kernel: *pde = 00000000
Dec 11 04:59:28 gprods1 kernel: Oops: 0000
Dec 11 04:59:28 gprods1 kernel: CPU:    0
Dec 11 04:59:28 gprods1 kernel: EIP:
0010:[lvm_do_create_proc_entry_of_pv+23/100]
Dec 11 04:59:28 gprods1 kernel: EFLAGS: 00010246
Dec 11 04:59:28 gprods1 kernel: eax: 00002f2f   ebx: 00000000   ecx:
de85d400   edx: 00000000
Dec 11 04:59:28 gprods1 kernel: esi: 0000002c   edi: 08071ff8   ebp:
0000002c   esp: ddd97d94
Dec 11 04:59:28 gprods1 kernel: ds: 0018   es: 0018   ss: 0018
Dec 11 04:59:28 gprods1 kernel: Process vgextend (pid: 595, process nr: 21,
stackpage=ddd97000)
Dec 11 04:59:28 gprods1 kernel: Stack: 08071ff8 dffdc000 c01b3fa2 dffdc000
00000000 08071ff8 dffdc000 00000002
Dec 11 04:59:28 gprods1 kernel:        4004fe03 08071ff8 c01b19bb dffdc000
08071ff8 df1d98e0 ffffffe7 08071ff8
Dec 11 04:59:28 gprods1 kernel:        4004fe03 00000000 08071ff8 c01ee5e9
00000001 00000000 00000000 00000000
Dec 11 04:59:28 gprods1 kernel: Call Trace: [lvm_do_vg_extend+78/140]
[lvm_chr_ioctl+671/1696] [number+377/704] [min_sb_len.516+27276/27788]
[lf+47/92] [vsprintf+892/952] [vsprintf+892/952]
Dec 11 04:59:28 gprods1 kernel:        [sprintf+20/5456]
[head_vals.693+682/15960] [sprintf+20/5456] [head_vals.693+683/15960]
[get_partition_list+114/152] [head_vals.693+666/15960]
[do_follow_link+120/132] [lookup_dentry+351/488]
Dec 11 04:59:28 gprods1 kernel:        [get_chrfops+30/36]
[tvecs+10300/13824] [chrdev_open+63/76] [filp_open+256/328]
[sys_ioctl+441/484] [free_pages+41/44] [sys_open+130/172]
[system_call+52/56]
Dec 11 04:59:28 gprods1 kernel: Code: ac 38 e0 75 03 8d 56 ff 84 c0 75 f4 85
d2 75 05 89 ea eb 02
Dec 11 05:02:45 gprods1 kernel: Unable to handle kernel NULL pointer
dereference at virtual address 0000002c
Dec 11 05:02:45 gprods1 kernel: current->tss.cr3 = 1dde8000, %cr3 = 1dde8000
Dec 11 05:02:45 gprods1 kernel: *pde = 00000000
Dec 11 05:02:45 gprods1 kernel: Oops: 0000
Dec 11 05:02:45 gprods1 kernel: CPU:    1
Dec 11 05:02:45 gprods1 kernel: EIP:
0010:[lvm_do_create_proc_entry_of_pv+23/100]
Dec 11 05:02:45 gprods1 kernel: EFLAGS: 00010246
Dec 11 05:02:45 gprods1 kernel: eax: 00002f2f   ebx: 00000000   ecx:
de85d400   edx: 00000000
Dec 11 05:02:45 gprods1 kernel: esi: 0000002c   edi: 08071ff8   ebp:
0000002c   esp: ddd97d94
Dec 11 05:02:45 gprods1 kernel: ds: 0018   es: 0018   ss: 0018
Dec 11 05:02:45 gprods1 kernel: Process vgextend (pid: 621, process nr: 21,
stackpage=ddd97000)
Dec 11 05:02:45 gprods1 kernel: Stack: 08071ff8 dffdc000 c01b3fa2 dffdc000
00000000 08071ff8 dffdc000 00000002
Dec 11 05:02:45 gprods1 kernel:        4004fe03 08071ff8 c01b19bb dffdc000
08071ff8 ddf806c0 ffffffe7 08071ff8
Dec 11 05:02:45 gprods1 kernel:        4004fe03 00000000 08071ff8 c01ee5e9
00000001 00000000 00000000 00000000
Dec 11 05:02:45 gprods1 kernel: Call Trace: [lvm_do_vg_extend+78/140]
[lvm_chr_ioctl+671/1696] [number+377/704] [min_sb_len.516+27276/27788]
[lf+47/92] [vsprintf+892/952] [vsprintf+892/952]
Dec 11 05:02:45 gprods1 kernel:        [sprintf+20/5456]
[head_vals.693+682/15960] [sprintf+20/5456] [head_vals.693+683/15960]
[get_partition_list+114/152] [head_vals.693+666/15960]
[do_follow_link+120/132] [lookup_dentry+351/488]
Dec 11 05:02:45 gprods1 kernel:        [get_chrfops+30/36]
[tvecs+10300/13824] [chrdev_open+63/76] [filp_open+256/328]
[sys_ioctl+441/484] [free_pages+41/44] [sys_open+130/172]
[system_call+52/56]
Dec 11 05:02:45 gprods1 kernel: Code: ac 38 e0 75 03 8d 56 ff 84 c0 75 f4 85
d2 75 05 89 ea eb 02
Dec 11 05:02:48 gprods1 kernel: Unable to handle kernel NULL pointer
dereference at virtual address 0000002c
Dec 11 05:02:48 gprods1 kernel: current->tss.cr3 = 1dde8000, %cr3 = 1dde8000
Dec 11 05:02:48 gprods1 kernel: *pde = 00000000
Dec 11 05:02:48 gprods1 kernel: Oops: 0000
Dec 11 05:02:48 gprods1 kernel: CPU:    0
Dec 11 05:02:48 gprods1 kernel: EIP:
0010:[lvm_do_create_proc_entry_of_pv+23/100]
Dec 11 05:02:48 gprods1 kernel: EFLAGS: 00010246
Dec 11 05:02:48 gprods1 kernel: eax: 00002f2f   ebx: 00000000   ecx:
de85d200   edx: 00000000
Dec 11 05:02:48 gprods1 kernel: esi: 0000002c   edi: 08071ff8   ebp:
0000002c   esp: ddd97d94
Dec 11 05:02:48 gprods1 kernel: ds: 0018   es: 0018   ss: 0018
Dec 11 05:02:48 gprods1 kernel: Process vgextend (pid: 622, process nr: 21,
stackpage=ddd97000)
Dec 11 05:02:48 gprods1 kernel: Stack: 08071ff8 dffdc000 c01b3fa2 dffdc000
00000000 08071ff8 dffdc000 00000003
Dec 11 05:02:48 gprods1 kernel:        4004fe03 08071ff8 c01b19bb dffdc000
08071ff8 df1d9560 ffffffe7 08071ff8
Dec 11 05:02:48 gprods1 kernel:        4004fe03 00000000 08071ff8 c01ee5e9
00000001 00000000 00000000 00000000
Dec 11 05:02:48 gprods1 kernel: Call Trace: [lvm_do_vg_extend+78/140]
[lvm_chr_ioctl+671/1696] [number+377/704] [min_sb_len.516+27276/27788]
[lf+47/92] [vsprintf+892/952] [vsprintf+892/952]
Dec 11 05:02:48 gprods1 kernel:        [sprintf+20/5456]
[head_vals.693+682/15960] [sprintf+20/5456] [head_vals.693+683/15960]
[get_partition_list+114/152] [head_vals.693+666/15960]
[do_follow_link+120/132] [lookup_dentry+351/488]
Dec 11 05:02:48 gprods1 kernel:        [get_chrfops+30/36]
[tvecs+10300/13824] [chrdev_open+63/76] [filp_open+256/328]
[sys_ioctl+441/484] [free_pages+41/44] [sys_open+130/172]
[system_call+52/56]
Dec 11 05:02:48 gprods1 kernel: Code: ac 38 e0 75 03 8d 56 ff 84 c0 75 f4 85
d2 75 05 89 ea eb 02
Dec 11 05:02:53 gprods1 kernel: Unable to handle kernel NULL pointer
dereference at virtual address 0000002c
Dec 11 05:02:53 gprods1 kernel: current->tss.cr3 = 1dde8000, %cr3 = 1dde8000
Dec 11 05:02:53 gprods1 kernel: *pde = 00000000
Dec 11 05:02:53 gprods1 kernel: Oops: 0000
Dec 11 05:02:53 gprods1 kernel: CPU:    0
Dec 11 05:02:53 gprods1 kernel: EIP:
0010:[lvm_do_create_proc_entry_of_pv+23/100]
Dec 11 05:02:53 gprods1 kernel: EFLAGS: 00010246
Dec 11 05:02:53 gprods1 kernel: eax: 00002f2f   ebx: 00000000   ecx:
de85d600   edx: 00000000
Dec 11 05:02:53 gprods1 kernel: esi: 0000002c   edi: 08071ff8   ebp:
0000002c   esp: ddd97d94
Dec 11 05:02:53 gprods1 kernel: ds: 0018   es: 0018   ss: 0018
Dec 11 05:02:53 gprods1 kernel: Process vgextend (pid: 624, process nr: 21,
stackpage=ddd97000)
Dec 11 05:02:53 gprods1 kernel: Stack: 08071ff8 dffdc000 c01b3fa2 dffdc000
00000000 08071ff8 dffdc000 00000004
Dec 11 05:02:53 gprods1 kernel:        4004fe03 08071ff8 c01b19bb dffdc000
08071ff8 df1d96e0 ffffffe7 08071ff8
Dec 11 05:02:53 gprods1 kernel:        4004fe03 00000000 08071ff8 c01ee5e9
00000001 00000000 00000000 00000000
Dec 11 05:02:53 gprods1 kernel: Call Trace: [lvm_do_vg_extend+78/140]
[lvm_chr_ioctl+671/1696] [number+377/704] [min_sb_len.516+27276/27788]
[vsprintf+892/952] [vsprintf+892/952] [sprintf+20/5456]
Dec 11 05:02:53 gprods1 kernel:        [head_vals.693+682/15960]
[sprintf+20/5456] [head_vals.693+683/15960] [get_partition_list+114/152]
[head_vals.693+666/15960] [do_follow_link+120/132] [lookup_dentry+351/488]
[get_chrfops+30/36]
Dec 11 05:02:53 gprods1 kernel:        [tvecs+10300/13824]
[chrdev_open+63/76] [filp_open+256/328] [sys_ioctl+441/484]
[free_pages+41/44] [sys_open+130/172] [system_call+52/56]
Dec 11 05:02:53 gprods1 kernel: Code: ac 38 e0 75 03 8d 56 ff 84 c0 75 f4 85
d2 75 05 89 ea eb 02
Dec 11 05:03:21 gprods1 kernel: Unable to handle kernel NULL pointer
dereference at virtual address 0000002c
Dec 11 05:03:21 gprods1 kernel: current->tss.cr3 = 1dde8000, %cr3 = 1dde8000
Dec 11 05:03:21 gprods1 kernel: *pde = 00000000
Dec 11 05:03:21 gprods1 kernel: Oops: 0000
Dec 11 05:03:21 gprods1 kernel: CPU:    0
Dec 11 05:03:21 gprods1 kernel: EIP:
0010:[lvm_do_create_proc_entry_of_pv+23/100]
Dec 11 05:03:21 gprods1 kernel: EFLAGS: 00010246
Dec 11 05:03:21 gprods1 kernel: eax: 00002f2f   ebx: 00000000   ecx:
de85d000   edx: 00000000
Dec 11 05:03:21 gprods1 kernel: esi: 0000002c   edi: 08071d80   ebp:
0000002c   esp: ddd97d94
Dec 11 05:03:21 gprods1 kernel: ds: 0018   es: 0018   ss: 0018
Dec 11 05:03:21 gprods1 kernel: Process vgextend (pid: 628, process nr: 21,
stackpage=ddd97000)
Dec 11 05:03:21 gprods1 kernel: Stack: 08071d80 dffdc000 c01b3fa2 dffdc000
00000000 08071d80 dffdc000 00000005
Dec 11 05:03:21 gprods1 kernel:        4004fe03 08071d80 c01b19bb dffdc000
08071d80 df1d9760 ffffffe7 08071d80
Dec 11 05:03:21 gprods1 kernel:        4004fe03 00000000 08071d80 c01ee5e9
00000001 00000000 00000000 00000000
Dec 11 05:03:21 gprods1 kernel: Call Trace: [lvm_do_vg_extend+78/140]
[lvm_chr_ioctl+671/1696] [number+377/704] [min_sb_len.516+27276/27788]
[lf+47/92] [vsprintf+892/952] [vsprintf+892/952]
Dec 11 05:03:21 gprods1 kernel:        [sprintf+20/5456]
[head_vals.693+682/15960] [sprintf+20/5456] [head_vals.693+683/15960]
[get_partition_list+114/152] [head_vals.693+666/15960]
[do_follow_link+120/132] [lookup_dentry+351/488]
Dec 11 05:03:21 gprods1 kernel:        [get_chrfops+30/36]
[tvecs+10300/13824] [chrdev_open+63/76] [filp_open+256/328]
[sys_ioctl+441/484] [free_pages+41/44] [sys_open+130/172]
[system_call+52/56]
Dec 11 05:03:21 gprods1 kernel: Code: ac 38 e0 75 03 8d 56 ff 84 c0 75 f4 85
d2 75 05 89 ea eb 02
Dec 11 05:05:14 gprods1 kernel: Unable to handle kernel NULL pointer
dereference at virtual address 0000002c
Dec 11 05:05:14 gprods1 kernel: current->tss.cr3 = 1dde8000, %cr3 = 1dde8000
Dec 11 05:05:14 gprods1 kernel: *pde = 00000000
Dec 11 05:05:14 gprods1 kernel: Oops: 0000
Dec 11 05:05:14 gprods1 kernel: CPU:    0
Dec 11 05:05:14 gprods1 kernel: EIP:
0010:[lvm_do_create_proc_entry_of_pv+23/100]
Dec 11 05:05:14 gprods1 kernel: EFLAGS: 00010246
Dec 11 05:05:14 gprods1 kernel: eax: 00002f2f   ebx: 00000000   ecx:
ddbc7e00   edx: 00000000
Dec 11 05:05:14 gprods1 kernel: esi: 0000002c   edi: 08071ff8   ebp:
0000002c   esp: ddd97d94
Dec 11 05:05:14 gprods1 kernel: ds: 0018   es: 0018   ss: 0018
Dec 11 05:05:14 gprods1 kernel: Process vgextend (pid: 638, process nr: 21,
stackpage=ddd97000)
Dec 11 05:05:14 gprods1 kernel: Stack: 08071ff8 dffdc000 c01b3fa2 dffdc000
00000000 08071ff8 dffdc000 00000006
Dec 11 05:05:14 gprods1 kernel:        4004fe03 08071ff8 c01b19bb dffdc000
08071ff8 df1d97e0 ffffffe7 08071ff8
Dec 11 05:05:14 gprods1 kernel:        4004fe03 00000000 08071ff8 c01ee5e9
00000001 00000000 00000000 00000000
Dec 11 05:05:14 gprods1 kernel: Call Trace: [lvm_do_vg_extend+78/140]
[lvm_chr_ioctl+671/1696] [number+377/704] [min_sb_len.516+27276/27788]
[lf+47/92] [vsprintf+892/952] [vsprintf+892/952]
Dec 11 05:05:14 gprods1 kernel:        [sprintf+20/5456]
[head_vals.693+682/15960] [sprintf+20/5456] [head_vals.693+683/15960]
[get_partition_list+114/152] [head_vals.693+666/15960]
[do_follow_link+120/132] [lookup_dentry+351/488]
Dec 11 05:05:14 gprods1 kernel:        [get_chrfops+30/36]
[tvecs+10300/13824] [chrdev_open+63/76] [filp_open+256/328]
[sys_ioctl+441/484] [free_pages+41/44] [sys_open+130/172]
[system_call+52/56]
Dec 11 05:05:14 gprods1 kernel: Code: ac 38 e0 75 03 8d 56 ff 84 c0 75 f4 85
d2 75 05 89 ea eb 02

Re: [linux-lvm] How do you mount a snapshot?

[David Gould]

On Fri, Dec 08, 2000 at 09:30:50PM +0100, Christian Limpach wrote:
> > Ok, so we are agreeing, it does not work on ext2 either unless you sync
> > and do not write between the sync and the snapshot. Doable, but not
> exactly
> > "safe".
> 
> hmm, I think it works for ext2 because you can always mount an ext2
> filesystem even if it's in an inconsistent state.  This is what happens
> everytime when your machine panics and you need to fsck your disks during
> the reboot, the root filesystem is mounted read-only...  It's my

Ok, but then you have an inconsistant filesystem. Which is not exactly
what I think of as "works" or "safe". My point was that snapshots are
great, but don't make the problem of consistancy magically disappear.

-dg

-- 
David Gould                                                 dg@suse.com
SuSE, Inc.,  580 2cd St. #210,  Oakland, CA 94607          510.628.3380
"As I've gained more experience with Perl it strikes me that it
 resembles Lisp in many ways, albeit Lisp as channeled by an awk
 script on acid."  -- Tim Moore <moore@herschel.bricoworks.com>,
                      on comp.lang.lisp

Re: [linux-lvm] How do you mount a snapshot?

[Chris Mason]

On Sunday, December 10, 2000 22:48:08 -0800 David Gould <dg@suse.com> wrote:

> On Fri, Dec 08, 2000 at 09:30:50PM +0100, Christian Limpach wrote:
>> > Ok, so we are agreeing, it does not work on ext2 either unless you sync
>> > and do not write between the sync and the snapshot. Doable, but not
>> exactly
>> > "safe".
>> 
>> hmm, I think it works for ext2 because you can always mount an ext2
>> filesystem even if it's in an inconsistent state.  This is what happens
>> everytime when your machine panics and you need to fsck your disks during
>> the reboot, the root filesystem is mounted read-only...  It's my
> 
> Ok, but then you have an inconsistant filesystem. Which is not exactly
> what I think of as "works" or "safe". My point was that snapshots are
> great, but don't make the problem of consistancy magically disappear.
> 

Yes.  With ext2, the admin can take steps to make his FS consistent during
the snapshot.  It would be a painful manual process, but it is at least
possible.

It would not be useful for snapshots to allow reiserfs disks to be mounted
without a journal replay though.  An inconsistent ext2 FS might have a few
directories screwed up, or a rename in progress, or some bad bitmaps  (or
much worse, but we are pretending things are normal here).  But, most of
the time it will be mostly valid.

reiserfs just has one btree for everything, so if the snapshot comes in the
middle of a given operation, it affects the btree for the whole FS.  This
means an inconsistent reiserfs disk has a much higher chance of having
serious problems, almost regardless of the type of operation that was going
on.  So, the snapshot API does two things.  It waits for all transaction
operations to finish, and then it blocks new ones until the snapshot is
setup.

Christian, just let me know if that doesn't answer your question.

-chris

Re: [linux-lvm] How do you mount a snapshot?

[Andreas Dilger]

David Gould writes:
> On Fri, Dec 08, 2000 at 09:30:50PM +0100, Christian Limpach wrote:
> > > Ok, so we are agreeing, it does not work on ext2 either unless you sync
> > > and do not write between the sync and the snapshot. Doable, but not
> > exactly
> > > "safe".
> > 
> > hmm, I think it works for ext2 because you can always mount an ext2
> > filesystem even if it's in an inconsistent state.  This is what happens
> > everytime when your machine panics and you need to fsck your disks during
> > the reboot, the root filesystem is mounted read-only...  It's my
> 
> Ok, but then you have an inconsistant filesystem. Which is not exactly
> what I think of as "works" or "safe". My point was that snapshots are
> great, but don't make the problem of consistancy magically disappear.

Actually, the LVM snapshot code does an fsync_dev() call right before
the snapshot is created, so the window in which the filesystem can
become inconsistent is very small.  For ext2 this is relatively safe,
because fsync_dev() will flush everything to disk.  For ext3 and reiserfs,
the fsync_dev() isn't enough because it may only flush the data to the
journal, and these filesystems will (currently) refuse to mount with a
dirty journal on a read-only device.

Cheers, Andreas
-- 
Andreas Dilger  \ "If a man ate a pound of pasta and a pound of antipasto,
                 \  would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/               -- Dogbert