On Sat, 07 Apr 2007 16:11:46 +0200, Krzysztof Halasa said: > > Think about it,... read speeds that are some FOUR times the physical > > disk read rate,... impossible without the use of compression (or > > something similar). > > It's really impossible with compression only unless you're writing > only zeros or stuff alike. I don't know what bonnie uses for testing > but real life data doesn't compress 4 times. Two times, sometimes, All depends on your data. From a recent "compress the old logs" job on our syslog server: /logs/lennier.cc.vt.edu/2007/03/maillog-2007-0308: 85.4% -- replaced with /logs/lennier.cc.vt.edu/2007/03/maillog-2007-0308.gz And it wasn't a tiny file either - it's a busy mailserver, the logs run to several hundred megabytes a day. Syslogs *often* compress 90% or more, meaning a 10X compression. > but then it will be typically slower than disk access (I mean read, > as write will be much slower). Actually, as far back as 1998 or so, I was able to document 20% *speedups* on an AIX system that supported compressed file systems - and that was from when a 133mz PowerPC 604e was a *fast* machine. Since then, CPUs have gotten faster at a faster rate than disks have, even increasing the speedup. The basic theory is that unless you're sitting close to 100%CPU, it is *faster* to burn some CPU to compress/decompress a 4K chunk of data down to 2K, and then move 2K to the disk drive, than it is to move 4K. It's particularly noticable for larger files - if you can apply the compression to remove the need to move 2M of data faster than you can move 2M of data, you win.
Attachment:
pgp1VIYc4GEVG.pgp
Description: PGP signature
