On 05/03/12 15:08, Alex Crow wrote: > On 05/03/12 14:45, Alan Brown wrote: >> On 05/03/12 14:24, Gael Guilmin wrote: >>> To gain space on the LT0. >> Don't bother. >> >> 1: Compression maxes out at about 30MB/s >> >> 2: LTO has inbuilt hardware compression which is "good enough" and a >> _LOT_ faster than any CPU you can throw at the task. >> >> The _only_ use for compression is on slow WAN links and when writing to >> disk - and for the latter I'd consider using compressing FSes such as >> ZFS in preference to using Bacula compression. >> > > What about when you are encrypting? You have to do the compression in > Bacula as once you've encrypted the data it can no longer be compressed > by the drive (eg for LTO < LTO4 where the drive cannot encrypt.)
Encryption programs generally compress as well in order to increase entropy - so any external compression routines will just make things slower for no gain. As soon as you're doing things which slow down the data flow you have to put up with painfully slow (for me) throughput and you _MUST_ use disk spooling to prevent tape drive shoeshining(*). Multithreading might help but it's generally only available for block-based compression algorithms such as bzip2 (multithreaded decompression isn't available for Pbzip) and that's not going to help much on a stream-based setup such as bacula without major rewrites. Encrypted setups are fine, but bear in mind that 2% of data losses are down to loss of keys - and while a damaged enencrypted backup might be partially recoverable its' highly likely that damage on an encrypted one will result in 100% loss. (*) Anything which results in uncompressed data being presented to the tape drive at a minimum throughput of less than 200MB/s will result in shoeshining (even though the actual tape rate is less than this, if the data is compressaable it needs to feed the drive fast enough to avoid the tape stopping or slowing down. Highly compressable data may require much faster minimum throughputs.) Even slow spool disks are a problem. You can test this by measuring throughput to /dev/null ------------------------------------------------------------------------------ Try before you buy = See our experts in action! The most comprehensive online learning library for Microsoft developers is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, Metro Style Apps, more. Free future releases when you subscribe now! http://p.sf.net/sfu/learndevnow-dev2 _______________________________________________ Bacula-users mailing list Bacula-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bacula-users
