Easy Automated Snapshot-Style Backups with Linux and Rsync
Contents
- Abstract
- Motivation
- Using
rsync to make a backup
- Basics
- Using the
--delete flag
- Be lazy: use
cron
- Incremental backups with
rsync
- Review of hard links
- Using
cp -al
- Putting it all together
- I'm used to
dump or tar! This seems backward!
- Isolating the backup from the rest of the system
- The easy (bad) way
- Keep it on a separate partition
- Keep that partition on a separate disk
- Keep that disk on a separate machine
- Making the backup as read-only as possible
- Bad:
mount/unmount
- Better:
mount read-only most of the time
- Tempting but it doesn't seem to work: the 2.4 kernel's
mount --bind
- My solution: using NFS on localhost
- Extensions: hourly, daily, and weekly snapshots
- Keep an extra script for each level
- Run it all with
cron
- Known bugs and problems
- Appendix: my actual configuration
- Listing one:
make_snapshot.sh
- Listing two:
daily_snapshot_rotate.sh
- Sample output of
ls -l /snapshot/home
- References
This document describes a method for generating automatic rotating
"snapshot"-style backups on a Unix-based system, with specific examples
drawn from the author's GNU/Linux experience. Snapshot backups are a
feature of some high-end industrial file servers; they create the
illusion of multiple full (up to ownership/permission) backups per
day without the space or processing overhead. All of the snapshots are
read-only, and are accessible directly by users as special system
directories. It is often possible to store several hours, days, and even
weeks' worth of snapshots with slightly more than 2x storage. This method,
while not as space-efficient as some of the proprietary technologies (which,
using special copy-on-write filesystems, can operate on slightly more than
1x storage), makes use of only standard file utilities and the common rsync program, which is installed by
default on most Linux distributions. Properly configured, the method can
also protect against hard disk failure, root compromises, or even back up a
network of heterogeneous desktops automatically.
Note: what follows is the original sgvlug DEVSIG announcement.
Ever accidentally delete or overwrite a file you were working on? Ever lose
data due to hard-disk failure? Or maybe you export shares to your
windows-using friends--who proceed to get outlook viruses that twiddle a
digit or two in all of their .xls files. Wouldn't it be nice if there were
a /snapshot directory that you could go back to, which had complete
images of the file system at semi-hourly intervals all day, then daily
snapshots back a few days, and maybe a weekly snapshot too? What if every
user could just go into that magical directory and copy deleted or
overwritten files back into "reality", from the snapshot of choice, without
any help from you? And what if that /snapshot directory were
read-only, like a CD-ROM, so that nothing could touch it (except maybe root,
but even then not directly)?
Best of all, what if you could make all of that happen automatically, using
only one extra, slightly-larger, hard disk? (Or one extra
partition, which would protect against all of the above except disk
failure).
In my lab, we have a proprietary NetApp file server which provides that sort of
functionality to the end-users. It provides a lot of other things too, but
it cost as much as a luxury SUV. It's quite appropriate for our heavy-use
research lab, but it would be overkill for a home or small-office
environment. But that doesn't mean small-time users have to do without!
I'll show you how I configured automatic, rotating snapshots on my $80 used
Linux desktop machine (which is also a file, web, and mail server) using
only a couple of one-page scripts and a few standard Linux utilities that
you probably already have.
I'll also propose a related strategy which employs one (or two, for the
wisely paranoid) extra low-end machines for a complete, responsible,
automated backup strategy that eliminates tapes and manual labor and makes
restoring files as easy as "cp".
The rsync utility is a very well-known piece of GPL'd software,
written originally by Andrew Tridgell and Paul Mackerras. If you have a
common Linux or UNIX variant, then you probably already have it installed;
if not, you can download the source code from rsync.samba.org. Rsync's specialty is
efficiently synchronizing file trees across a network, but it works fine on
a single machine too.
Basics
Suppose you have a directory called source, and you want to
back it up into the directory destination. To accomplish that,
you'd use:
rsync -a source/ destination/
(Note: I usually also add the -v (verbose) flag too so that
rsync tells me what it's doing). This command is equivalent
to:
cp -a source/* destination/
except that it's much more efficient if there are only a few differences.
Just to whet your appetite, here's a way to do the same thing as in the
example above, but with destination on a remote machine, over a
secure shell:
rsync -a -e ssh source/ username@remotemachine.com:/path/to/destination/
Trailing Slashes Do Matter...Sometimes
This isn't really an article about rsync, but I would like to
take a momentary detour to clarify one potentially-confusing detail about
its use. You may be accustomed to commands that don't care about trailing
slashes. For example, if a and b are two
directories, then cp -a a b is equivalent to cp -a a/
b/. However, rsync does care about the
trailing slash, but only on the source argument. For example, let
a and b be two directories, with the file
foo initially inside directory a. Then this
command:
rsync -a a b
produces b/a/foo, whereas this command:
rsync -a a/ b
produces b/foo. The presence or absence of a trailing slash on
the destination argument (b, in this case) has no effect.
Using the --delete flag
If a file was originally in both source/ and
destination/ (from an earlier rsync, for example),
and you delete it from source/, you probably want it to be
deleted from destination/ on the next rsync.
However, the default behavior is to leave the copy at
destination/ in place. Assuming you want rsync to
delete any file from destination/ that is not in
source/, you'll need to use the --delete flag:
rsync -a --delete source/ destination/
Be lazy: use cron
One of the toughest obstacles to a good backup strategy is human nature; if
there's any work involved, there's a good chance backups won't happen.
(Witness, for example, how rarely my roommate's home PC was backed up before
I created this system). Fortunately, there's a way to harness human
laziness: make cron do the work.
To run the rsync-with-backup command from the previous section every morning
at 4:20 AM, for example, edit the root cron table: (as root)
crontab -e
Then add the following line:
20 4 * * * rsync -a --delete source/ destination/
Finally, save the file and exit. The backup will happen every morning at
precisely 4:20 AM, and root will receive the output by email. Don't copy
that example verbatim, though; you should use full path names (such as
/usr/bin/rsync and /home/source/) to remove any
ambiguity.
Since making a full copy of a large filesystem can be a time-consuming and
expensive process, it is common to make full backups only once a week or
once a month, and store only changes on the other days. This technique is
called making "incremental" backups, and is supported by the venerable old
dump and tar utilities, along with many others.
However, if you don't have to use tape as your backup medium, it is both
possible and vastly more efficient to perform incremental backups with
rsync.
The most common way to do it is by using the rsync -b
--backup-dir= combination. I have seen examples of that usage here, but I won't
discuss it further, because there is a better way. If you're not familiar
with hard links, though, you should start with the following review first.
Review of hard links
We usually think of a file's name as being the file itself, but really the
name is a hard link. A given file can have more than one hard link to
itself--for example, a directory has at least two hard links: the directory
name and . (for when you're inside it). It also has one hard
link from each of its sub-directories (the .. file inside each
one). If you have the stat utility installed on your machine,
you can find out how many hard links a file has (along with a bunch of other
information) with the command:
stat filename
Hard links aren't just for directories--you can create more than one link to
a regular file too. For example, if you have the file a, you
can make a link called b:
ln a b
Now, a and b are two names for the same file, as
you can verify by seeing that they reside at the same inode (the inode
number will be different on your machine):
ls -i a
232177 a
ls -i b
232177 b
So ln a b is roughly equivalent to cp a b, but
there are several important differences:
- The contents of the file are only stored once, so you don't use twice
the space.
- If you change
a, you're changing b, and
vice-versa.
- If you change the permissions or ownership of
a, you're
changing those of b as well, and vice-versa.
- If you overwrite
a by copying a third file on top of it, you
will also overwrite b, unless you tell cp to unlink
before overwriting. You do this by running cp with the
--remove-destination flag. Notice that rsync
always unlinks before overwriting!!. Note, added 2002.Apr.10: the
previous statement applies to changes in the file contents only, not
permissions or ownership.
But this raises an interesting question. What happens if you
rm one of the links? The answer is that rm is a
bit of a misnomer; it doesn't really remove a file, it just removes that one
link to it. A file's contents aren't truly removed until the number of
links to it reaches zero. In a moment, we're going to make use of that
fact, but first, here's a word about cp.
Using cp -al
In the previous section, it was mentioned that hard-linking a file is
similar to copying it. It should come as no surprise, then, that the
standard GNU fileutils cp command comes with a -l
flag that causes it to create (hard) links instead of copies (it doesn't
hard-link directories, though, which is good; you might want to think about
why that is). Another handy switch for the cp command is
-a (archive), which causes it to recurse through directories
and preserve file owners, timestamps, and access permissions.
Together, the combination cp -al makes what appears to
be a full copy of a directory tree, but is really just an illusion that
takes almost no space. If we restrict operations on the copy to adding or
removing (unlinking) files--i.e., never changing one in place--then the
illusion of a full copy is complete. To the end-user, the only differences
are that the illusion-copy takes almost no disk space and almost no time to
generate.
2002.05.15: Portability tip: If you don't have GNU cp installed
(if you're using a different flavor of *nix, for example), you can use
find and cpio instead. Simply replace
cp -al a b with
cd a && find . -print | cpio -dpl ../b. Thanks to
Brage Førland for that tip.
Putting it all together
We can combine rsync and cp -al to create what
appear to be multiple full backups of a filesystem without taking multiple
disks' worth of space. Here's how, in a nutshell:
rm -rf backup.3
mv backup.2 backup.3
mv backup.1 backup.2
cp -al backup.0 backup.1
rsync -a --delete source_directory/ backup.0/
If the above commands are run once every day, then backup.0,
backup.1, backup.2, and backup.3 will
appear to each be a full backup of source_directory/ as it
appeared today, yesterday, two days ago, and three days ago,
respectively--complete, except that permissions and ownerships in old
snapshots will get their most recent values (thanks to J.W. Schultz for
pointing this out). In reality, the extra storage will be equal to the
current size of source_directory/ plus the total size of the
changes over the last three days--exactly the same space that a full plus
daily incremental backup with dump or tar would
have taken.
I'm used to dump or tar! This seems backward!
The dump and tar utilities were originally
designed to write to tape media, which can only access one file at a time.
If you're used to their style of incremental backup, rsync
might seem backward. I hope that the following example will help make the
differences clearer.
Suppose that on a particular system, backups were done on Monday night,
Tuesday night, and Wednesday night, and now it's Thursday.
With dump or tar, the Monday backup is the big
("full") one. It contains everything in the filesystem being backed up. The
Tuesday and Wednesday "incremental" backups would be much smaller, since
they would contain only changes since the previous day. At some point
(presumably next Monday), the administrator would plan to make another full
dump.
With rsync, in contrast, the Wednesday backup is the big one. Indeed, the
"full" backup is always the most recent one. The Tuesday directory
would contain data only for those files that changed between Tuesday and
Wednesday; the Monday directory would contain data for only those files that
changed between Monday and Tuesday.
A little reasoning should convince you that the rsync way is
much better for network-based backups, since it's only necessary to
do a full backup once, instead of once per week. Thereafter, only the
changes need to be copied. Unfortunately, you can't rsync to a tape, and
that's probably why the dump and tar incremental
backup models are still so popular. But in your author's opinion, these
should never be used for network-based backups now that rsync
is available.
If you take the simple route and keep your backups in another directory on
the same filesystem, then there's a very good chance that whatever damaged
your data will also damage your backups. In this section, we identify a few
simple ways to decrease your risk by keeping the backup data separate.
The easy (bad) way
In the previous section, we treated /destination/ as if it were
just another directory on the same filesystem. Let's call that the easy
(bad) approach. It works, but it has several serious limitations:
- If your filesystem becomes corrupted, your backups will be corrupted
too.
- If you suffer a hardware failure, such as a hard disk crash, it might be
very difficult to reconstruct the backups.
- Since backups preserve permissions, your users--and any programs or
viruses that they run--will be able to delete files from the backup. That
is bad. Backups should be read-only.
- If you run out of free space, the backup process (which runs as root)
might crash the system and make it difficult to recover.
- The easy (bad) approach offers no protection if the root account is
compromised.
Fortunately, there are several easy ways to make your backup more robust.
Keep it on a separate partition
If your backup directory is on a separate partition, then any corruption in
the main filesystem will not normally affect the backup. If the backup
process runs out of disk space, it will fail, but it won't take the rest of
the system down too. More importantly, keeping your backups on a separate
partition means you can keep them mounted read-only; we'll discuss that in
more detail in the next chapter.
Keep that partition on a separate disk
If your backup partition is on a separate hard disk, then you're also
protected from hardware failure. That's very important, since hard disks
always fail eventually, and often take your data with them. An entire
industry has formed to service the needs of those whose broken hard disks
contained important data that was not properly backed up.
Important: Notice, however, that in the event of hardware
failure you'll still lose any changes made since the last backup. For home
or small office users, where backups are made daily or even hourly as
described in this document, that's probably fine, but in situations where
any data loss at all would be a serious problem (such as where financial
transactions are concerned), a RAID system might be more appropriate.
RAID is well-supported under Linux, and the methods described in this
document can also be used to create rotating snapshots of a RAID system.
Keep that disk on a separate machine
If you have a spare machine, even a very low-end one, you can turn it into a
dedicated backup server. Make it standalone, and keep it in a physically
separate place--another room or even another building. Disable every single
remote service on the backup server, and connect it only to a dedicated
network interface on the source machine.
On the source machine, export the directories that you want to back up via
read-only NFS to the dedicated interface. The backup server can mount the
exported network directories and run the snapshot routines discussed in this
article as if they were local. If you opt for this approach, you'll only be
remotely vulnerable if:
- a remote root hole is discovered in read-only NFS, and
- the source machine has already been compromised.
I'd consider this "pretty good" protection, but if you're (wisely) paranoid,
or your job is on the line, build two backup servers. Then you can make
sure that at least one of them is always offline.
If you're using a remote backup server and can't get a dedicated line to it
(especially if the information has to cross somewhere insecure, like the
public internet), you should probably skip the NFS approach and use
rsync -e ssh instead.
It has been pointed out to me that rsync operates far more
efficiently in server mode than it does over NFS, so if the connection
between your source and backup server becomes a bottleneck, you should
consider configuring the backup machine as an rsync server instead of using
NFS. On the downside, this approach is slightly less transparent to users
than NFS--snapshots would not appear to be mounted as a system directory,
unless NFS is used in that direction, which is certainly another option (I
haven't tried it yet though). Thanks to Martin Pool, a lead developer of
rsync, for making me aware of this issue.
Here's another example of the utility of this approach--one that I use. If
you have a bunch of windows desktops in a lab or office, an easy way to keep
them all backed up is to share the relevant files, read-only, and mount them
all from a dedicated backup server using SAMBA. The backup job can treat
the SAMBA-mounted shares just like regular local directories.
In the previous section, we discussed ways to keep your backup data
physically separate from the data they're backing up. In this section, we
discuss the other side of that coin--preventing user processes from
modifying backups once they're made.
We want to avoid leaving the snapshot backup directory mounted
read-write in a public place. Unfortunately, keeping it mounted read-only
the whole time won't work either--the backup process itself needs write
access. The ideal situation would be for the backups to be mounted
read-only in a public place, but at the same time, read-write in a private
directory accessible only by root, such as /root/snapshot.
It's tempting to keep your backup partition mounted read-only as
/snapshot most of the time, but unmount that and remount it
read-write as /root/snapshot during the brief periods while
snapshots are being made. Don't give in to temptation!.
Bad: mount/umount
A filesystem cannot be unmounted if it's busy--that is, if some process is
using it. The offending process need not be owned by root to block an
unmount request. So if you plan to umount the read-only copy
of the backup and mount it read-write somewhere else,
don't--any user can accidentally (or deliberately) prevent the backup from
happening. Besides, even if blocking unmounts were not an issue, this
approach would introduce brief intervals during which the backups would seem
to vanish, which could be confusing to users.
Better: mount read-only most of the time
A better but still-not-quite-satisfactory choice is to remount the directory
read-write in place:
mount -o remount,rw /snapshot
[ run backup process ]
mount -o remount,ro /snapshot
Now any process that happens to be in /snapshot when the
backups start will not prevent them from happening. Unfortunately, this
approach introduces a new problem--there is a brief window of vulnerability,
while the backups are being made, during which a user process could write to
the backup directory. Moreover, if any process opens a backup file for
writing during that window, it will prevent the backup from being remounted
read-only, and the backups will stay vulnerable indefinitely.
Tempting but doesn't seem to work: the 2.4 kernel's mount --bind
Starting with the 2.4-series Linux kernels, it has been possible to mount a
filesystem simultaneously in two different places. "Aha!" you might think,
as I did. "Then surely we can mount the backups read-only in
/snapshot, and read-write in /root/snapshot at the
same time!"
Alas, no. Say your backups are on the partition /dev/hdb1. If
you run the following commands,
mount /dev/hdb1 /root/snapshot
mount --bind -o ro /root/snapshot /snapshot
then (at least as of the 2.4.9 Linux kernel), mount will report
/dev/hdb1 as being mounted read-write in
/root/snapshot and read-only in /snapshot, just as
you requested. Don't let the system mislead you!
It seems that, at least on my system, read-write vs. read-only is a property
of the filesystem, not the mount point. So every time you change the mount
status, it will affect the status at every point the filesystem is mounted,
even though neither /etc/mtab nor /proc/mounts
will indicate the change.
In the example above, the second mount call will cause both of
the mounts to become read-only, and the backup process will be unable to
run. Scratch this one.
Update: I have it on fairly good authority that this behavior is considered
a bug in the Linux kernel, which will be fixed as soon as someone gets
around to it. If you are a kernel maintainer and know more about this
issue, or are willing to fix it, I'd love to hear from you!
My solution: using NFS on localhost
This is a bit more complicated, but until Linux supports mount
--bind with different access permissions in different places, it
seems like the best choice. Mount the partition where backups are stored
somewhere accessible only by root, such as /root/snapshot.
Then export it, read-only, via NFS, but only to the same machine. That's as
simple as adding the following line to /etc/exports:
/root/snapshot 127.0.0.1(secure,ro,no_root_squash)
then start nfs and portmap from
/etc/rc.d/init.d/. Finally mount the exported directory,
read-only, as /snapshot:
mount -o ro 127.0.0.1:/root/snapshot /snapshot
And verify that it all worked:
mount
...
/dev/hdb1 on /root/snapshot type ext3 (rw)
127.0.0.1:/root/snapshot on /snapshot type nfs (ro,addr=127.0.0.1)
At this point, we'll have the desired effect: only root will be able to
write to the backup (by accessing it through /root/snapshot).
Other users will see only the read-only /snapshot directory.
For a little extra protection, you could keep mounted read-only in
/root/snapshot most of the time, and only remount it read-write
while backups are happening.
With a little bit of tweaking, we make multiple-level rotating snapshots.
On my system, for example, I keep the last four "hourly" snapshots (which
are taken every four hours) as well as the last three "daily" snapshots
(which are taken at midnight every day). You might also want to keep
weekly or even monthly snapshots too, depending upon your needs and your
available space.
Keep an extra script for each level
This is probably the easiest way to do it. I keep one script that runs
every four hours to make and rotate hourly snapshots, and another script
that runs once a day rotate the daily snapshots. There is no need to use
rsync for the higher-level snapshots; just cp -al from the appropriate
hourly one.
Run it all with cron
To make the automatic snapshots happen, I have added the following lines to
root's crontab file:
0 */4 * * * /usr/local/bin/make_snapshot.sh
0 13 * * * /usr/local/bin/daily_snapshot_rotate.sh
They cause make_snapshot.sh to be run every four hours on the
hour and daily_snapshot_rotate.sh to be run every day at 13:00
(that is, 1:00 PM). I have included those scripts in the appendix.
If you tire of receiving an email from the cron process every
four hours with the details of what was backed up, you can tell it to send
the output of make_snapshot.sh to /dev/null, like
so:
0 */4 * * * /usr/local/bin/make_snapshot.sh >/dev/null 2>&1
Understand, though, that this will prevent you from seeing errors if
make_snapshot.sh cannot run for some reason, so be careful with
it. Creating a third script to check for any unusual behavior in the
snapshot periodically seems like a good idea, but I haven't implemented it
yet. Alternatively, it might make sense to log the output of each run, by
piping it through tee, for example.
I know that listing my actual backup configuration here is a security risk;
please be kind and don't use this information to crack my site. However,
I'm not a security expert, so if you see any vulnerabilities in my setup,
I'd greatly appreciate your help in fixing them. Thanks!
I actually use two scripts, one for every-four-hours (hourly) snapshots, and
one for every-day (daily) snapshots. I am only including the parts of the
scripts that relate to backing up /home, since those are
relevant ones here.
I use the NFS-to-localhost trick of exporting /root/snapshot
read-only as /snapshot, as discussed above.
The system has been running without a hitch for months.
Listing one: make_snapshot.sh
#!/bin/bash
# ----------------------------------------------------------------------
# mikes handy rotating-filesystem-snapshot utility
# ----------------------------------------------------------------------
# RCS info: $Id: make_snapshot.sh,v 1.6 2002/04/06 04:20:00 mrubel Exp $
# ----------------------------------------------------------------------
# this needs to be a lot more general, but the basic idea is it makes
# rotating backup-snapshots of /home whenever called
# ----------------------------------------------------------------------
# ------------- system commands used by this script --------------------
ID=/usr/bin/id;
ECHO=/bin/echo;
MOUNT=/bin/mount;
RM=/bin/rm;
MV=/bin/mv;
CP=/bin/cp;
TOUCH=/bin/touch;
RSYNC=/usr/bin/rsync;
# ------------- file locations -----------------------------------------
MOUNT_DEVICE=/dev/hdb1;
SNAPSHOT_RW=/root/snapshot;
EXCLUDES=/usr/local/etc/backup_exclude;
# ------------- the script itself --------------------------------------
# make sure we're running as root
if (( `$ID -u` != 0 )); then { $ECHO "Sorry, must be root. Exiting..."; exit; } fi
# attempt to remount the RW mount point as RW; else abort
$MOUNT -o remount,rw $MOUNT_DEVICE $SNAPSHOT_RW ;
if (( $? )); then
{
$ECHO "snapshot: could not remount $SNAPSHOT_RW readwrite";
exit;
}
fi;
# rotating snapshots of /home (fixme: this should be more general)
# step 1: delete the oldest snapshot, if it exists:
if [ -d $SNAPSHOT_RW/home/hourly.3 ] ; then \
$RM -rf $SNAPSHOT_RW/home/hourly.3 ; \
fi ;
# step 2: shift the middle snapshots(s) back by one, if they exist
if [ -d $SNAPSHOT_RW/home/hourly.2 ] ; then \
$MV $SNAPSHOT_RW/home/hourly.2 $SNAPSHOT_RW/home/hourly.3 ; \
fi;
if [ -d $SNAPSHOT_RW/home/hourly.1 ] ; then \
$MV $SNAPSHOT_RW/home/hourly.1 $SNAPSHOT_RW/home/hourly.2 ; \
fi;
# step 3: make a hard-link-only (except for dirs) copy of the latest snapshot,
# if that exists
if [ -d $SNAPSHOT_RW/home/hourly.0 ] ; then \
$CP -al $SNAPSHOT_RW/home/hourly.0 $SNAPSHOT_RW/home/hourly.1 ; \
fi;
# step 4: rsync from the system into the latest snapshot (notice that
# rsync behaves like cp --remove-destination by default, so the destination
# is unlinked first. If it were not so, this would copy over the other
# snapshot(s) too!
$RSYNC \
-va --delete --delete-excluded \
--exclude-from="$EXCLUDES" \
/home/ $SNAPSHOT_RW/home/hourly.0 ;
# step 5: update the mtime of hourly.0 to reflect the snapshot time
$TOUCH $SNAPSHOT_RW/home/hourly.0 ;
# and thats it for home.
# now remount the RW snapshot mountpoint as readonly
$MOUNT -o remount,ro $MOUNT_DEVICE $SNAPSHOT_RW ;
if (( $? )); then
{
$ECHO "snapshot: could not remount $SNAPSHOT_RW readonly";
exit;
} fi;
If you notice above, I have added an excludes list to the rsync
call. This is just to prevent the system from backing up garbage like web
browser caches, which change frequently (so they'd take up space in every
snapshot) but would be no loss if they were accidentally destroyed.
Listing two: daily_snapshot_rotate.sh
#!/bin/bash
# ----------------------------------------------------------------------
# mikes handy rotating-filesystem-snapshot utility: daily snapshots
# ----------------------------------------------------------------------
# RCS info: $Id: daily_snapshot_rotate.sh,v 1.2 2002/03/25 21:53:27 mrubel Exp $
# ----------------------------------------------------------------------
# intended to be run daily as a cron job when hourly.3 contains the
# midnight (or whenever you want) snapshot; say, 13:00 for 4-hour snapshots.
# ----------------------------------------------------------------------
# ------------- system commands used by this script --------------------
ID=/usr/bin/id;
ECHO=/bin/echo;
MOUNT=/bin/mount;
RM=/bin/rm;
MV=/bin/mv;
CP=/bin/cp;
# ------------- file locations -----------------------------------------
MOUNT_DEVICE=/dev/hdb1;
SNAPSHOT_RW=/root/snapshot;
# ------------- the script itself --------------------------------------
# make sure we're running as root
if (( `$ID -u` != 0 )); then { $ECHO "Sorry, must be root. Exiting..."; exit; } fi
# attempt to remount the RW mount point as RW; else abort
$MOUNT -o remount,rw $MOUNT_DEVICE $SNAPSHOT_RW ;
if (( $? )); then
{
$ECHO "snapshot: could not remount $SNAPSHOT_RW readwrite";
exit;
}
fi;
# step 1: delete the oldest snapshot, if it exists:
if [ -d $SNAPSHOT_RW/home/daily.2 ] ; then \
$RM -rf $SNAPSHOT_RW/home/daily.2 ; \
fi ;
# step 2: shift the middle snapshots(s) back by one, if they exist
if [ -d $SNAPSHOT_RW/home/daily.1 ] ; then \
$MV $SNAPSHOT_RW/home/daily.1 $SNAPSHOT_RW/home/daily.2 ; \
fi;
if [ -d $SNAPSHOT_RW/home/daily.0 ] ; then \
$MV $SNAPSHOT_RW/home/daily.0 $SNAPSHOT_RW/home/daily.1; \
fi;
# step 3: make a hard-link-only (except for dirs) copy of
# hourly.3, assuming that exists, into daily.0
if [ -d $SNAPSHOT_RW/home/hourly.3 ] ; then \
$CP -al $SNAPSHOT_RW/home/hourly.3 $SNAPSHOT_RW/home/daily.0 ; \
fi;
# note: do *not* update the mtime of daily.0; it will reflect
# when hourly.3 was made, which should be correct.
# now remount the RW snapshot mountpoint as readonly
$MOUNT -o remount,ro $MOUNT_DEVICE $SNAPSHOT_RW ;
if (( $? )); then
{
$ECHO "snapshot: could not remount $SNAPSHOT_RW readonly";
exit;
} fi;
Sample output of ls -l /snapshot/home
total 28
drwxr-xr-x 12 root root 4096 Mar 28 00:00 daily.0
drwxr-xr-x 12 root root 4096 Mar 27 00:00 daily.1
drwxr-xr-x 12 root root 4096 Mar 26 00:00 daily.2
drwxr-xr-x 12 root root 4096 Mar 28 16:00 hourly.0
drwxr-xr-x 12 root root 4096 Mar 28 12:00 hourly.1
drwxr-xr-x 12 root root 4096 Mar 28 08:00 hourly.2
drwxr-xr-x 12 root root 4096 Mar 28 04:00 hourly.3
Notice that the contents of each of the subdirectories of
/snapshot/home/ is a complete image of /home at
the time the snapshot was made. Despite the w in the directory
access permissions, no one--not even root--can write to this directory; it's
mounted read-only.
As-written, the snapshot system above does not properly maintain old
ownerships/permissions; if a file's ownership or permissions are changed in
place, then the new ownership/permissions will apply to older snapshots as
well. This is because rsync does not unlink files prior to
changing them if the only changes are ownership/permission. Thanks to J.W.
Schultz for pointing this out. This is not a problem for me, but slightly
more complicated workarounds are possible
Apparently, a bug in some Linux kernels between 2.4.4 and 2.4.9 causes
mv to update timestamps; this may result in inaccurate
timestamps on the snapshot directories. Thanks to Claude Felizardo for
pointing this problem out. He was able to work around the problem my
replacing mv with the following script:
MV=my_mv;
...
function my_mv() {
REF=/tmp/makesnapshot-mymv-$$;
touch -r $1 $REF;
/bin/mv $1 $2;
touch -r $REF $2;
/bin/rm $REF;
}
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