Re: btrfs-image and btrfs send related queries

[sri <toyours_sridhar@yahoo.co.in>]

Henk Slager <eye1tm <at> gmail.com> writes:

> 
> On Mon, Apr 18, 2016 at 4:26 PM, Roman Mamedov <rm <at> romanrm.net> 
wrote:
> > On Mon, 18 Apr 2016 16:13:28 +0200
> > Henk Slager <eye1tm <at> gmail.com> wrote:
> >
> >> (your email keeps ending up in gmail spam folder)
> >>
> >> On Mon, Apr 18, 2016 at 9:24 AM, sri <toyours_sridhar <at> 
yahoo.co.in> wrote:
> >> > I tried btrfs-image and created image file and ran btrfs-image -r 
to a
> >> > different disk. Once recovered and mounted, I can able to see 
data is
> >> > not zeroed out as mentioned in btrfs-image man page.
> >>
> >> "different disk"  you mention, that is important info. If you doe 
the
> >> restore to a image file, that image file is sparse and all data 
blocks
> >> are read as zeros.
> >>
> >> However, if you restore to a block device, then you can assume it 
just
> >> writes the device blocks for metadata and leaves the rest 
untouched.
> >> So trim whole device first or brute-force overwrite completely with
> >> zeros.
> >>
> >> So maybe the man pages needs some correction / extra notes.
> >>
> >> > I tried on same machine.
> >
> > Does btrfs-image store/restore the FS UUID? If it does, then 
potentially both
> > the source FS and the restored one were visible at the same time to 
the kernel
> > with identical UUIDs, and maybe it was actually accessing/mounting 
the source
> > one.
> 
> Very good point! The UUID's are the same. I remember I used a VM to
> separate the source FS from the restored FS
> 
> Also, the assumption I made about restoring to a block device is not
> correct: If you restore to a loopdev that has a file with all
> non-zeros as backing-store, the files in the mounted restored FS are
> read as zeros.
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> 
thank you for inputs. Actually I have tried in the following way.
I have /dev/sdb , /dev/sdc. Using wipefs -fa I cleared both devices and 
cre=
ated btrfs on /dev/sdb. Mounted and written some files and unmounted it.
Then I ran btrfs-image /dev/sdc /img1.img and got the dump.
Once image created I again ran wipefs -fa /dev/sdb.
Then I ran btrfs-image -r /img1.img /dev/sdc and mounted /dev/sdc.
ls to dumped filesystem shows the file size as original and no file 
content=
. I guess btrfs-image doesn't modify files stat so i feel it is showing 
siz=
e as original.
However running cat on some of files give i/o error
qd67:/btrfs1 # cat shadow.hcat: shadow.h: Input/output error
These errors are not on all files on other files, since dump doesn't 
contai=
ns any data it just prompts for cat as below.
qd67:/btrfs1 # cat stab.hqd67:/btrfs1 # cat stdio_ext.h
Not sure why i/o errors are coming.