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Re: [Bacula-users] Bacula causing high disk-io on clients
2015-06-27 09:21:44
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On 27/06/15 10:45, Dmitri Maziuk wrote:
Compressing data on the client means fewer bytes to send over the
wire. Block-level compression like bzip2 tends to be completely
cpu-bound and anything bigger than a cellphone tends to have
plenty of cycles to spare.
Not entirely true and certainly not accurate for Gb/s and faster
links.
Individual CPU cores haven't changed much in terms of speed for
nearly a decade and most compression algorithms are single threaded
when fed from stdin (even pigz and pbzip). On top of that you're
facing memory <-> CPU bottlenecks.
The result when running gzip compression over a 1Gb/s link is that
you'll usually achieve a maximum end-to-end rate of 30MB/s, vs the
wire speed of 110MB/s ( b = bit, B=BYTE, aka o = octet if you're
french )
bzip is significantly worse. Even LZO gives little gain at 1Gb/s in
exchange for making your CPU run hot and drawing more power than you
need to.
At 10Gb/s, LZO is a significant bottleneck.
The takeaway: Unless you're backing up over a WAN (and a slow or
overloaded one at that), wireside compression is not a net win.
We have 300 people on our campus linking to the main site at 1Gb/s
and whilst backing up machines on the main site to our satellite one
we still achieve faster backups without compression than with it.
The other side of compression is that it's a huge CPU drain. I
already have enough problems with people rebooting windows systems
mid-backup without adding even more incentive for them to hit the
Big Red Button.
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