Hello,
I am relatively new to tape backups in general and I have recently become accustomed to using bacula, and I have a quick question about compression ratios/storage capacity on LTO tapes. I have an IBM 24-tape library with Sony Ultrium LTO-4 tapes (Rated: 800GB/1,600GB compressed). I recently set a job for a full backup of one of our servers that has a little over 3TB of disk capacity. A du -h of that server yields:
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
3.3T 2.2T 983G 70% /
My question relates to this: from a fresh install of bacula (5.0.3) I created a pool for monthly backups and ran a full backup of that server above. All the tapes were marked empty and the backup started by picking a tape from the designated pool and backed up successfully to it, but I fully expected it to span _two_ tapes with 1.6TB as the supposed maximum "rated" compression capacity for LTO-4 tapes. However it fit _all of it_ onto a single tape. Here is an excerpt from the job output:
Storage: "IBM_Autochanger" (From Job resource)
Scheduled time: 07-Feb-2012 10:26:00
Start time: 07-Feb-2012 10:26:02
End time: 07-Feb-2012 21:08:39
Elapsed time: 10 hours 42 mins 37 secs
Priority: 10
FD Files Written: 458,656
SD Files Written: 458,656
FD Bytes Written: 2,362,329,795,537 (2.362 TB)
SD Bytes Written: 2,362,399,227,266 (2.362 TB)
Rate: 61268.5 KB/s
Software Compression: None
VSS: no
Encryption: no
Accurate: no
Volume name(s): ML1038L4
Volume Session Id: 1
Volume Session Time: 1328631897
Last Volume Bytes: 2,364,166,103,040 (2.364 TB)
And 'list media' showed the pool as follows:
Pool: Monthly
+---------+------------+-----------+---------+-------------------+----------+--------------+---------+------+-----------+-----------+---------------------+
| MediaId | VolumeName | VolStatus | Enabled | VolBytes | VolFiles | VolRetention | Recycle | Slot | InChanger | MediaType | LastWritten |
+---------+------------+-----------+---------+-------------------+----------+--------------+---------+------+-----------+-----------+---------------------+
| 19 | ML1037L4 | Append | 1 | 64,512 | 0 | 31,536,000 | 1 | 19 | 1 | LTO-4 | 0000-00-00 00:00:00 |
| 20 | ML1047L4 | Append | 1 | 64,512 | 0 | 31,536,000 | 1 | 20 | 1 | LTO-4 | 0000-00-00 00:00:00 |
| 21 | ML1044L4 | Append | 1 | 64,512 | 0 | 31,536,000 | 1 | 21 | 1 | LTO-4 | 0000-00-00 00:00:00 |
| 22 | ML1041L4 | Append | 1 | 64,512 | 0 | 31,536,000 | 1 | 22 | 1 | LTO-4 | 0000-00-00 00:00:00 |
| 23 | ML1038L4 | Append | 1 | 2,364,166,103,040 | 2,365 | 31,536,000 | 1 | 23 | 1 | LTO-4 | 2012-02-07 21:08:17 |
+---------+------------+-----------+---------+-------------------+----------+--------------+---------+------+-----------+-----------+---------------------+
I actually calculated 2,364,166,103,040 bytes to be 2.15 TB, but either way this is much higher than the rated 1.6TB with the (theoretically) maximum compression, as I understand it. Not until I ran another relatively tiny backup afterward (around 90GB) of something else did it fill the first tape and start to write the remaining 80GB or so onto a second tape. The job output above says there was no software compression being used, and unless it is a default I have done nothing to enable (or disable) tape compression on the IBM Library itself.
Has anyone heard of getting more capacity out of an LTO-4 tape than it is rated for? Or are the byte amounts inflated, possibly, by artificially counting skipped-over file systems? I got several messages like "/boot is a different filesystem. Will not descend from / into /boot", but you would think that it wouldn't count those in the overall storage amount. I essentially just want to know if these figures are real, and if I'm just getting an awesome compression ratio, or if something else is going on.
Thanks!
Josh
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