Tarsnap: On-line Backups for the Truly Paranoid
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Tarsnap works on a prepaid utility-metered model. Subscribers deposit a minimum of $5.00 and are charged only for the storage and bandwidth they consume. Although the cost is higher than plain Amazon S3 service, it reflects both the cryptographic, compression and deduplication value-add of Tarsnap. At the time of this writing, Tarsnap costs 30 cents per gigabyte-month for storage and 30 cents per gigabyte transmitted.
This cost may make Tarsnap infeasible for large, whole-server terabyte-size backups. However, it is ideal for critical, sensitive files that must be durable, available and safe in the event an attacker succeeded in compromising them. With no minimum charge or monthly fees, Tarsnap is very economical for small data sets or for data that compresses well. Some examples:
Backing up 100MB of files with 10% daily change rate for a month would cost only 30 cents.
A gigabyte that is backed up weekly with a 20% change rate would cost $1.40 a month.
Tarsnap bills based on attodollars (quintillionths of a dollar) to avoid profiting through rounding. This means your account balance is tracked to 18 decimal places. This is not just "pay by the drink" cloud pricing—it's practically "pay by the atom". Some users find that a small deposit lasts them months or years.
One of Tarsnap's best features is how easy it is to script.
The ability to put a
tarsnap cf command into a shell script makes
use in cron jobs very straightforward, which encourages unattended,
automated backups—the best kind.
Crucially, Tarsnap also supports a division of responsibilities. You can use the tarsnap-keymgmt tool to create keyfiles with limited authority. You may have one keyfile that lives on your server with permission to create archives, but not the authority to delete them. A master key with full privileges could be kept off-site, so that if attackers were to compromise your server, they would be unable to destroy your backups.
To get started with Tarsnap, register at tarsnap.com, deposit some funds into your account, and download the client.
The client is available only as source, but the straightforward
./configure ; make install process is very easy. The client is
supported on all major Linux distributions (as well as BSD-based systems).
Take a quick peek at the download page to make sure you have the required
operating system packages, as some of the development packages are not
installed in typical Linux configurations.
If you are using a firewall, be aware that Tarsnap communicates via TCP on port 9279.
There are only two critical configuration items: the location of your keyfile and the location of your Tarsnap cache. Both are set in /usr/local/etc/tarsnap.conf. A tarsnap.conf.example is provided, and you probably can just copy the example as is. It defines your Tarsnap key as /root/tarsnap.key and your cache directory as /usr/local/tarsnap-cache, which will be created if it doesn't exist. The cachedir is a small state-tracking directory that lets Tarsnap keep track of backups.
Next, register your machine as follows. In this case, I'm setting up Tarsnap service for a machine called helicarrier. The e-mail address and password are the ones I used when I signed up for service with Tarsnap:
# tarsnap-keygen --keyfile /root/tarsnap.key ↪--user firstname.lastname@example.org --machine helicarrier Enter tarsnap account password: #
I have a directory I'd like to back up with Tarsnap:
# ls -l /docs total 2092 -rw-rw---- 1 andrew 1833222 Jun 14 16:38 2011 Tax Return.pdf -rw------- 1 andrew 48568 Jun 14 16:41 andrew_passwords.psafe3 -rw------- 1 tina 14271 Jun 14 16:42 tina_passwords.psafe3 -rw-rw-r-- 1 andrew 48128 Jun 14 16:41 vacation_hotels.doc -rw-rw-r-- 1 andrew 46014 Jun 14 16:35 vacation_notes.doc -rw-rw-r-- 1 andrew 134959 Jun 14 16:44 vacation_reservation.pdf
To back up, I just tell Tarsnap what name I want to call my archive ("docs.20120701" in this case) and which directory to back up. There's no requirement to use a date string in the archive name, but it makes versioning straightforward, as you'll see:
# tarsnap cf docs.20120701 /docs tarsnap: Removing leading '/' from member names Total size Compressed size All archives 2132325 1815898 (unique data) 2132325 1815898 This archive 2132325 1815898 New data 2132325 1815898
In my tarsnap.conf, I enabled the
print-stats directive, which gives
the account report shown. Note the compression, which reduces storage
costs and improves cryptographic security. The "compressed
size" of the
"unique data" shows how much data is actually stored at
Tarsnap, and you
pay only for the compressed size.
The next day, I back up docs again to "docs.20120702". If I haven't made many changes, the backup will proceed very quickly and use little additional space:
# tarsnap cf docs.20120702 /docs tarsnap: Removing leading '/' from member names Total size Compressed size All archives 4264650 3631796 (unique data) 2132770 1816935 This archive 2132325 1815898 New data 445 1037
As you can see, although the amount of data for "all archives" has grown, the actual amount of "unique data" has barely increased. Tarsnap is smart enough to avoid backing up data that has not changed.
Andrew Fabbro is a senior technologist living in the Portland, Oregon, area. He's used Linux since Slackware came on floppies and presently works for Con-way, a Fortune 500 transportation company.
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