[Discuss] How do you backup "/" with rsync?
John Blomfield
jabfield at shaw.ca
Tue Dec 11 23:25:16 PST 2007
I have been following this discussion with interest and I have one
question, if you substitute the /dev/disk/by-id/ value in place of the
sda# in your fstab file, what how does this affect the grub designations
hd(#, #) type id's ??? I always thought that there was a one to one
correspondence between Linux id's and grub id's, that is, sdb1 ==
hd(1,0) etc. Without persistence if sdb1 becomes say sda1 if the
original sda1 is missing then grub id's change also and the remaining
drive is now hd(0,0) as far as grub is concerned. I think what I'm
asking is, if persistent Linux id's are used, are grub id's also persistent?
grub generates a file device.map which maps sdb1 to hd(1,0) etc and I
think it is possible to edit this file but I don't know if it stays
edited or whether it gets re-written each time you boot? If not you
could hard wire this to your persistent id's.
John Blomfield
Alan W. Irwin wrote:
> On 2007-12-11 11:31-0800 Steven Kurylo wrote:
>
>>> irwin at raven> ls -l /dev/disk/by-id/
>>> total 0
>>> lrwxrwxrwx 1 root root 9 2007-12-09 21:21
>>> ata-ST3500630AS-9QG3XJPX-part1 ->
>>> ../../sdb1
>>
>> You can just use /dev/disk/by-id/ata-ST3500630AS-9QG3XJPX-part1 in
>> your fstab, instead of /dev/sdb1. udev guaranties this id to be
>> unique. http://www.reactivated.net/writing_udev_rules.html#builtin
> [...]
>> You use udevinfo to find out what information is available to udev
>> when identifying drives:
>> http://www.reactivated.net/writing_udev_rules.html#udevinfo
>
> Thanks for the links to various parts of
> http://www.reactivated.net/writing_udev_rules.html. That's a useful
> resource.
>
> My previous results were somewhat obfuscated by non-persistent id's. For
> example, the above ls result pointed to ../../sdb1, and "vgdisplay
> --verbose" indicated only the raven volume group was defined and
> associated
> with the /dev/sdb1 physical volume. In contrast, the "mount" command
> output
> said the /boot partition was actually on sda1. I believe this
> inconsistency
> resulted from booting with the external drive on. I think for that
> situation /dev/sda was associated with the external drive and /dev/sdb
> associated with the internal drive. (IOW, the jmicron eSATA
> controller was
> being loaded and used before the ICH9 SATA controller.) Currently the
> external drive has no MBR so I have no idea how it found the correct grub
> menu. However, that boot with external drive on did work.
>
> Currently, I am not using a non-persistent drive identity for /boot in
> /etc/fstab which reads
>
> /dev/sda1 /boot ext3 defaults 0 2
>
> Thus, with the external disk turne on for booting, it must have
> mounted that
> filesystem for the external drive. After I turned off the external
> drive I
> never attempted to look at /boot, but if I had I presume I would have
> gotten
> i/o errors.
>
> I wanted to reboot in any case because of the /dev/sd[ab] confusion so
> this
> time I made sure the external drive was off, and only turned it on
> once the
> system was up and then executed "/etc/init.d/lvm2 start". For that
> case, I
> now get reasonable looking results for /dev/disk/by-id/
>
> irwin at raven> ls -l /dev/disk/by-id
> total 0
> lrwxrwxrwx 1 root root 9 2007-12-11 17:45 ata-ST3500630AS-6QG16YA9 ->
> ../../sdb
> lrwxrwxrwx 1 root root 10 2007-12-11 17:45
> ata-ST3500630AS-6QG16YA9-part1 ->
> ../../sdb1
> lrwxrwxrwx 1 root root 10 2007-12-11 17:45
> ata-ST3500630AS-6QG16YA9-part2 ->
> ../../sdb2
> lrwxrwxrwx 1 root root 10 2007-12-11 17:45
> ata-ST3500630AS-6QG16YA9-part5 ->
> ../../sdb5
> lrwxrwxrwx 1 root root 9 2007-12-11 17:33 ata-ST3500630AS-9QG3XJPX ->
> ../../sda
> lrwxrwxrwx 1 root root 10 2007-12-11 17:33
> ata-ST3500630AS-9QG3XJPX-part1 ->
> ../../sda1
> lrwxrwxrwx 1 root root 10 2007-12-11 17:33
> ata-ST3500630AS-9QG3XJPX-part2 ->
> ../../sda2
> lrwxrwxrwx 1 root root 10 2007-12-11 17:33
> ata-ST3500630AS-9QG3XJPX-part5 ->
> ../../sda5
> lrwxrwxrwx 1 root root 9 2007-12-11 17:45 scsi-S_6QG16YA9 -> ../../sdb
> lrwxrwxrwx 1 root root 10 2007-12-11 17:45 scsi-S_6QG16YA9-part1 ->
> ../../sdb1
> lrwxrwxrwx 1 root root 10 2007-12-11 17:45 scsi-S_6QG16YA9-part2 ->
> ../../sdb2
> lrwxrwxrwx 1 root root 10 2007-12-11 17:45 scsi-S_6QG16YA9-part5 ->
> ../../sdb5
> lrwxrwxrwx 1 root root 9 2007-12-11 17:33 scsi-S_9QG3XJPX -> ../../sda
> lrwxrwxrwx 1 root root 10 2007-12-11 17:33 scsi-S_9QG3XJPX-part1 ->
> ../../sda1
> lrwxrwxrwx 1 root root 10 2007-12-11 17:33 scsi-S_9QG3XJPX-part2 ->
> ../../sda2
> lrwxrwxrwx 1 root root 10 2007-12-11 17:33 scsi-S_9QG3XJPX-part5 ->
> ../../sda5
>
> If I use vgdisplay --verbose to find the physical volumes assocated with
> my raven and raven_backup, the former is /dev/sda5 while the latter
> is /dev/sdb5 as expected.
>
> Now if I turn off the external drive (note, I did not mount any
> filesystems
> on that drive for this session) and re-run /etc/init/lvm2, all
> continues to
> be well. The raven_backup volume group is gone, and the raven volume
> group
> is associated with /dev/sda5. All the /dev/sdb devices are gone, but the
> /dev/sda devices are still there. mounted (as expected, but I believe
> this
> was wrong in the previous case since /dev/sda1 did not exist), and "ls -l
> /dev/disk/by-id" continues to show all the ../../sda* results but the
> corresponding ../../sdb* results are dropped as expected. Finally,
> "mount"
> says /boot is mounted on /dev/sda1 (as expected, and also correct this
> time
> since sda is associated with the internal drive because the boot occurred
> without the external drive turned on).
>
> I have just replaced /dev/sda1 and /dev/sdb1 by the appropriate
> persistent
> id's in /etc/fstab (the /dev/mapper/ ones seem to be persistent from the
> above inadvertent experiments so I will leave those as is). As a
> result of
> this change, it should no longer matter whether the external drive is
> off or
> on when I boot.
>
> Thanks, Steven, for helping me understand the importance of persistent
> disk
> id's, and thanks again for pointing me to the above udev rule reference.
>
> Alan
> __________________________
> Alan W. Irwin
>
> Astronomical research affiliation with Department of Physics and
> Astronomy,
> University of Victoria (astrowww.phys.uvic.ca).
>
> Programming affiliations with the FreeEOS equation-of-state
> implementation
> for stellar interiors (freeeos.sf.net); PLplot scientific plotting
> software
> package (plplot.org); the libLASi project (unifont.org/lasi); the
> Loads of
> Linux Links project (loll.sf.net); and the Linux Brochure Project
> (lbproject.sf.net).
> __________________________
>
> Linux-powered Science
> __________________________
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