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simplesnap is a simple way to send ZFS snapshots across a network. Although it can serve many purposes, its primary goal is to manage backups from one ZFS filesystem to a backup filesystem also running ZFS, using incremental backups to minimize network traffic and disk usage.
simplesnap is FLEXIBLE; it is designed to perfectly compliment snapshotting tools, permitting rotating backups with arbitrary retention periods. It lets multiple machines back up a single target, lets one machine back up multiple targets, and keeps it all straight.
simplesnap is EASY; there is no configuration file needed. One ZFS property is available to exclude datasets/filesystems. ZFS datasets are automatically discovered on machines being backed up.
simplesnap is SAFE; it is robust in the face of interrupted transfers, and needs little help to keep running.
simplesnap is SECURE; unlike many similar tools, it does not require full root access to the machines being backed up. It runs only a small wrapper as root, and the wrapper has only three commands it implements.
Besides the above, simplesnap:
Does one thing and does it well. It is designed to be used with a snapshot auto-rotator on both ends (such as zfSnap). simplesnap will transfer snapshots made by other tools, but will not destroy them on either end.
Requires ssh public key authorization to the host being backed up, but does not require permission to run arbitrary commands. It has a wrapper to run on the backup host, written in bash, which accepts only three operations and performs them simply. It is suitable for a locked-down authorized_keys file.
Creates minimal snapshots for its own internal purposes, generally leaving no more than 1 or 2 per dataset, and reaps them automatically without touching others.
Is a small program, easily audited. In fact, most of the code is devoted to sanity-checking, security, and error checking.
Automatically discovers what datasets to back up from the remote. Uses a user-defined zfs property to exclude filesystems that should not be backed up.
Logs copiously to syslog on all hosts involved in backups.
Intelligently supports a single machine being backed up by multiple backup hosts, or onto multiple sets of backup media (when, for instance, backup media is cycled into offsite storage)
simplesnap's operation is very simple.
The simplesnap program runs on the machine that stores the backups -- we'll call it the backuphost. There is a restricted remote command wrapper called simplesnapwrap that runs on the machine being backed up -- we'll call it the activehost. simplesnapwrap is never invoked directly by the end-user; it is always called remotely by simplesnap.
With simplesnap, the backuphost always connects to the activehost -- never the other way round.
simplesnap runs in the backuphost, and first connects to the simplesnapwrap on the activehost and asks it for a list of the ZFS datasets ("listfs" operation). simplesnapwrap responds with a list of all ZFS datasets that were not flagged for exclusion.
Next, simplesnap connects back to simplesnapwrap once for each dataset to be backed up -- the "sendback" operation. simplesnap passes along to it only two things: the setname and the dataset (filesystem) name.
simplesnapwrap looks to see if there is an existing simplesnap snapshot corresponding to that SETNAME. If not, it creates one and sends it as a full, non-incremental backup. That completes the job for that dataset.
If there is an existing snapshot for that SETNAME, simplesnapwrap creates a new one, constructing the snapshot name containing a timestamp and the SETNAME, then sends an incremental, using the oldest snapshot from that setname as the basis for zfs send -I.
After the backuphost has observed zfs receive exiting without error, it contacts simplesnapwrap once more and requests the "reap" operation. This cleans up the old snapshots for the given SETNAME, leaving only the most recent. This is a separate operation in simplesnapwrap ensuring that even if the transmission is interrupted, still it will be OK in the end because zfs receive -F is used, and the data will come across next time.
The idea is that some system like zfSnap will be used on both ends to make periodic snapshots and clean them up. One can use careful prefix names with zfSnap to use different prefixes on each activehost, and then implement custom cleanup rules with -F on the holderhost.
This section will describe how a first-time simplesnap user can get up and running quickly. It assumes you already have simplesnap installed and working on your system. If not, please follow the instructions in the INSTALL.txt file in the source distribution.
As above, I will refer to the machine storing the backups as the "backuphost" and the machine being backed up as the "activehost".
First, on the backuphost, as root, generate an ssh keypair that will be used exclusively for simplesnap.
ssh-keygen -t rsa -f ~/.ssh/id_rsa_simplesnap
When prompted for a passphrase, leave it empty.
Now, on the activehost, edit or create a file called ~/.ssh/authorized_keys. Initialize it with the content of ~/.ssh/id_rsa_simplesnap.pub from the backuphost. (Or, add to the end, if you already have lines in the file.) Then, at the beginning of that one very long line, add text like this:
(I broke that line into two for readability, but this must all be on a single line in your file.)
The 22.214.171.124 is the IP address that connections from the backuphost will appear to come from. It may be omitted if the IP is not static, but it affords a little extra security. The line will wind up looking like:
command="/usr/sbin/simplesnapwrap",from="126.96.36.199", no-port-forwarding,no-X11-forwarding,no-pty ssh-rsa AAAA....
(Again, this should all be on one huge line.)
If there are any ZFS datasets you do not want to be backed up, set
on the activehost
on. For instance:
zfs set org.complete.simplesnap:exclude=on tank/junkdata
Now, back on the backuphost, you should be able to run:
ssh -i ~/.ssh/id_rsa_simplesnap activehost
say yes when asked if you want to add the key to the known_hosts file. At this point, you should see output containing:
"simplesnapwrap: This program is to be run from ssh."
If you see that, then simplesnapwrap was properly invoked remotely.
Now, create a ZFS filesystem to hold your backups. For instance:
zfs create tank/simplesnap
I often recommend compression for simplesnap datasets, so:
zfs set compression=lz4 tank/simplesnap
(If that gives an error, use compression=on instead.)
Now, you can run the backup:
simplesnap --host activehost --setname mainset --store tank/simplesnap --sshcmd "ssh -i /root/.ssh/id_rsa_simplesnap"
You can monitor progress in /var/log/syslog. If all goes well, you will see filesystems start to be populated under tank/simplesnap/host.
Now, go test that you have the data you expected to: look at your STORE filesystems and make sure they have everything expected. Test repeatedly over time that you can restore as you expect from your backups.
Most people will always use the same SETNAME. The SETNAME is used to track and name the snapshots on the remote end. simplesnap tries to always leave one snapshot on the remote, to serve as the base for a future incremental.
In some situations, you may have multiple bases for incrementals. The two primary examples are two different backup servers backing up the same machine, or having two sets of backup media and rotating them to offsite storage. In these situations, you will have to keep different snapshots on the activehost for the different backups, since they will be current to different points in time.
All simplesnap options begin with two dashes (`--'). Most take a parameter, which is to be separated from the option by a space. The equals sign is not a valid separator for simplesnap.
The normal simplesnap mode is backing up. An alternative check mode is available, which requires fewer parameters. This mode is described below.
Normally, simplesnap automatically obtains a list of
datasets to back up from the remote, and backs up all of
them except those that define the
property. With this option, simplesnap does not bother
to ask the remote for a list of datasets, and instead
backs up only the one precise
DATASET given. For now, ignored when
--check is given, but that may change in
the future. It would be best to not specify this option
with --check for now.
Do not back up, but check existing backups. If any
datasets' newest backup is older than
TIMEFRAME, print an error and
exit with a nonzero code. Scans all hosts unless a
specific host is given with
parameter is in the format given to GNU date(1); for
--check "30 days ago". Remember to enclose it in quotes
if it contains spaces.
Valid only with
--backupdataset, gives a
specific destination for the backup, whith may be outside
the STORE. The STORE
must still exist, as it is used for storing lockfiles and
Gives the name of the host to back up. This is both passed to ssh and used to name the backup sets.
In a few situations, one may not wish to use the same name for both. It is recommend to use the Host and HostName options in ~/.ssh/config to configure aliases in this situation.
Specifies that the host being backed up is local to the machine. Do not use ssh to contact it, and invoke the wrapper directly.
Gives the command to use to connect to the remote host.
Defaults to "ssh". It may be used to select an
alternative configuration file or keypair. Remember to
quote it per your shell if it contains spaces. For example:
--sshcmd "ssh -i /root/.id_rsa_simplesnap". This command
is ignored when
--check is given.
Gives the backup set name. Can just be a made-up word if multiple sets are not needed; for instance, the hostname of the backup server. This is used as part of the snapshot name.
Gives the ZFS dataset name where the data will be stored. Should not begin with a slash. The mountpoint will be obtained from the ZFS subsystem. Always required.
Gives the path to simplesnapwrap (which must be on the
remote machine unless
--local is given).
Not usually relevant, since the
command parameter in
~root/.ssh/authorized_keys gives the
path. Default: "simplesnapwrap"
Since simplesnap stores backups in standard ZFS datasets, you can use standard ZFS tools to obtain information about backups. Here are some examples.
Try something like this:
# zfs list -r -d 1 tank/store NAME USED AVAIL REFER MOUNTPOINT tank/store 540G 867G 34K /tank/store tank/store/host1 473G 867G 32K /tank/store/host1 tank/store/host2 54.9G 867G 32K /tank/store/host2 tank/store/host3 12.2G 867G 31K /tank/store/host3
Here, you can see that the total size of the simplesnap data is 540G - the USED value from the top level. In this example, host1 was using the most space -- 473G -- and host3 the least -- 12.2G. There is 867G available on this zpool for backups.
-r parameter to zfs
list requests a recursive report, but the
-d 1 parameter sets a maximum depth of 1
-- so you can see just the top-level hosts without all their
Let's say that you had the above example, and want to drill down into more detail. Perhaps, for instance, we continue the above example and drill down into host2:
# zfs list -r tank/store/host2 NAME USED AVAIL REFER MOUNTPOINT tank/store/host2 54.9G 867G 32K /tank/... tank/store/host2/tank 49.8G 867G 32K /tank/... tank/store/host2/tank/home 7.39G 867G 6.93G /tank/... tank/store/host2/tank/vm 42.4G 867G 30K /tank/... tank/store/host2/tank/vm/vm1 32.0G 867G 29.7G - tank/store/host2/tank/vm/vm2 10.4G 867G 10.4G - tank/store/host2/rpool 5.12G 867G 32K /tank/... tank/store/host2/rpool/misc 521M 867G 521M /tank/... tank/store/host2/rpool/host2-1 4.61G 867G 33K /tank/... tank/store/host2/rpool/host2-1/ROOT 317M 867G 312M /tank/... tank/store/host2/rpool/host2-1/usr 3.76G 867G 3.76G /tank/... tank/store/host2/rpool/host2-1/var 554M 867G 401M /tank/...
I've trimmed the "mountpoint" column here so it doesn't get too wide for the screen.
You see here the same 54.9G used as in the previous example, but now you can trace it down. There were two zpools on host2: tank and rpool. Most of the backup space -- 49.8G of the 54.9G -- is used by tank, and only 5.12G by rpool. And in tank, 42.4G is used by vm. Tracing it down, of that 42.4G used by vm, 32G is in vm1 and 10.4G in vm2. Notice how the values at each level of the tree include their descendents.
So in this example, vm1 and vm2 are zvols corresponding to virtual machines, and clearly take up a lot of space. Notice how vm1 says it uses 32.0G but in the refer column, it only refers to 29.7G? That means that the latest backup for vm2 used 29.7G, but when you add in the snapshots for that dataset, the total space consumed is 32.0G.
Let's look at an alternative view that will make the size consumed by snapshots more clear:
# zfs list -o space -r tank/store/host2 NAME AVAIL USED USEDSNAP USEDDS USEDCHILD .../host2 867G 54.9G 0 32K 54.9G .../host2/tank 867G 49.8G 0 32K 49.8G .../host2/tank/home 867G 7.39G 474M 6.93G 0 .../host2/tank/vm 867G 42.4G 50K 30K 42.4G .../host2/tank/vm/vm1 867G 32.0G 2.35G 29.7G 0 .../host2/tank/vm/vm1 867G 10.4G 49K 10.4G 0 .../host2/rpool 867G 5.12G 0 32K 5.12G .../host2/rpool/misc 867G 521M 51K 521M 0 .../host2/rpool/host2-1 867G 4.61G 51K 33K 4.61G .../host2/rpool/host2-1/ROOT 867G 317M 5.44M 312M 0 .../host2/rpool/host2-1/usr 867G 3.76G 208K 3.76G 0 .../host2/rpool/host2-1/var 867G 554M 153M 401M 0
(Again, I've trimmed some irrelevant columns from this output.)
The AVAIL and USED columns are the same as before, but now you have a breakdown of what makes up the USED column. USEDSNAP is the space used by the snapshots of that particular dataset. USEDDS is the space used by that dataset directly -- the same value as was in REFER before. And USEDCHILD is the space used by descendents of that dataset.
The USEDSNAP column is the easiest way to see the impact your retention policies have on your backup space consumption.
Let's take one example from before -- the 153M of snapshots in host2-1/var, and see what we can find.
# zfs list -t snap -r tank/store/host2/rpool/host2-1/var NAME USED AVAIL REFER ... .../var@host2-hourly-2014-02-11_05.17.02--2d 76K - 402M .../var@host2-hourly-2014-02-11_06.17.01--2d 77K - 402M .../var@host2-hourly-2014-02-11_07.17.01--2d 18.8M - 402M .../var@host2-daily-2014-02-11_07.17.25--1w 79K - 402M .../var@host2-hourly-2014-02-11_08.17.01--2d 156K - 402M .../var@host2-monthly-2014-02-11_09.01.36--1m 114K - 402M ...
In this output, the REFER column is the amount of data pointed to by that snapshot -- that is, the size of /var at the moment the snapshot is made. And the USED column is the amount of space that would be freed if just that snapshot were deleted.
Note this important point: it is normal for the sum of the values in the USED column to be less than the space consumed by the snapshots of the datasets as reported by USEDSNAP in the previous example. The reason is that the USED column is the data unique to that one snapshot. If, for instance, 100MB of data existed on the system being backed up for three hours yesterday, each snapshot could very well show less than 100KB used, because that 100MB isn't unique to a particular snapshot. Until, that is, two of the three snapshots referncing the 100MB data are destroyed; then the USED value of the last one referencing it will suddenly jump to 100MB higher because now it references unique data.
One other point -- an indication that the last backup was successfully transmitted is the presence of a __simplesnap_...__ snapshot at the end of the list. Do not delete it.
The zfs diff command can let you see what changed over time -- either across a single snapshot, or across many. Let's take a look.
# zfs diff .../var@host2-hourly-2014-02-11_05.17.02--2d \ .../var@host2-hourly-2014-02-11_06.17.01--2d \ | sort -k2 | less M /tank/store/host2/rpool/host2-1/var/log/Xorg.0.log M /tank/store/host2/rpool/host2-1/var/log/auth.log M /tank/store/host2/rpool/host2-1/var/log/daemon.log ... M /tank/store/host2/rpool/host2-1/var/spool/anacron/cron.daily M /tank/store/host2/rpool/host2-1/var/spool/anacron/cron.monthly M /tank/store/host2/rpool/host2-1/var/spool/anacron/cron.weekly M /tank/store/host2/rpool/host2-1/var/tmp
Here you can see why there was just a few KB of changes in that snapshot: mostly just a little bit of logging was happening on the system. Now let's inspect the larger snapshot:
# zfs diff .../var@host2-hourly-2014-02-11_07.17.01--2d \ .../var@host2-daily-2014-02-11_07.17.25--1w \ | sort -k2 | less M /tank/store/host2/rpool/host2-1/var/backups + /tank/store/host2/rpool/host2-1/var/backups/dpkg.status.0 - /tank/store/host2/rpool/host2-1/var/backups/dpkg.status.0 + /tank/store/host2/rpool/host2-1/var/backups/dpkg.status.1.gz R /tank/store/host2/rpool/host2-1/var/backups/dpkg.status.1.gz -> /tank/store/host2/rpool/host2-1/var/backups/dpkg.status.2.gz R /tank/store/host2/rpool/host2-1/var/backups/dpkg.status.2.gz -> /tank/store/host2/rpool/host2-1/var/backups/dpkg.status.3.gz ... M /tank/store/host2/rpool/host2-1/var/cache/apt R /tank/store/host2/rpool/host2-1/var/cache/apt/pkgcache.bin.KdsMLu -> /tank/store/host2/rpool/host2-1/var/cache/apt/pkgcache.bin
Here you can see some file rotation going on, and a temporary file being renamed to permanent. Normal daily activity on a system, but now you know what was taking up space.
Any backup scheme should be tested carefully before being relied upon to serve its intended purpose. This item is not simplesnap-specific, but pertains to every backup solution: test that you are backing up the data you expect to before you need it.
In order to account for various situations that could lead to divergence of filesystems, including the simple act of mounting them, simplesnap always uses zfs receive -F. Any local changes you make to the simplesnap store datasets will be lost at any time. If you need to make local changes there, it is best to copy them elsewhere.
Since simplesnap sends all snapshots, it is possible that locally-created snapshots made outside of your rotation scheme will also be sent to your backuphost. These may not be automatically reaped there, and may stick around. An example at the end of the cron.daily.simplesnap.backuphost file included with simplesnap is one way to check for these. They could automatically be reaped with zfs destroy as well, but this must be carefully tuned to local requirements, so an example of doign that is intentionally not supplied with the distribution.
Ordinarily, an interrupted transfer is no problem for simplesnap. However, the very first transfer of a dataset poses a bit of a problem, since the simplesnap wrapper can't detect failure in this one special case. If your first transfer gets interrupted, simply zfs destroy the __simplesnap_...__ snapshot on the activehost and rerun. NEVER DESTROY __simplesnap SNAPSHOTS IN ANY OTHER SITUATION!
If, by way of the
property or the
--datasetdest parameters, you do not request a
parent dataset to be backed up, but do request a descendent
dataset to be backed up, you may get an error on the first
dataset tree leading to the destination location for that
dataset has not yet been created. simplesnap performs only
the narrow actions you request. Running an appropriate
zfs create command will rectify the
zfSnap (1), zfs (8).
The simplesnap homepage: https://github.com/jgoerzen/simplesnap
The examples included with the simplesnap distribution, or on its homepage.
The zfSnap package compliments simplesnap perfectly. Find it at https://github.com/graudeejs/zfSnap.
This software and manual page was written by John Goerzen
granted to copy, distribute and/or modify this document under
the terms of the GNU General Public License, Version 3 any
later version published by the Free Software Foundation. The
complete text of the GNU General Public License is included in
the file COPYING in the source distribution.