Query about compression ratios

[Ailean]

Hi all

A few days ago my overnight backups started to fail and BE 12 is asking for another tape. I'm using LTO2 200/400 tapes with software compression and encryption.

I'm only backing up about 230GB of data so I was a little surprised to be running out of space. I checked that the compression was still turned one (it was) and had a look at the reports and noticed something odd about the compression ratios, several blocks of data were being displayed as 1.5:1 which seems fine, but one large block of dats (90GB) had a ratio of 0.6:1.

Does this mean that I'm using more than 90GB of tape to store my 90Gb of data? If so, any idea why? It would explain why I'm running out of space on my tape.

Thanks in advance.

[CraigV]

Hi Ailean,

Compression depends on a number of things:

1. Type of tapes using (200/400 tapes would see a minimum of 200GB being written at 1:1, and a maximum of 400GB at 2:1...you won't often get 2:1).
2. Type of data being backed up - some data list *.pst, some graphics files etc. are already compressed, and you will either not get ANY compression on them, or very minimal compression ratios.
3. Amount of data being backed up.

You're obviously getting no compression on that 90GB file, and as a result, it's a bit larger on tape.
You COULD look into using hardware compression (if it isn't already on), but it might make no difference. Best bet here would be to see what data you can leave off as unnecessary.

Laters!

[Ailean]

Thanks for your response.

I understand the compression issues, it's the 0.6:1 ratio that has me a bit puzzled. I can understand that the data might not be compressed, but if the compression ratio is 0.6:1 that means that the data is being stored 166% larger than it actually is. Overheads, formatting and headers are one thing, but 66% seems a bit much for that?

I'll take a look at the actual data tomorrow, it's comprised of general use files and folders (a shared drive in effect), so I'd have expected at least some compression.

[Ken_Putnam · Accepted solution]

that means that the data is being stored 166% larger than it actually is


if the data is already compressed, you can indeed increase the size by compressing again with a different algorithm

[Ailean]

Ahh, that might be it then.

Many thanks!