> the "perfect" max load should be the number of cpus you have,
> so a quad-core server can sustain a load of 4 without any
> problem... after that number, the higher the load, the higher
> will be the performance lost
>
> I dont agree here at all. backuppc is not generally CPU bound. If your
target is to max out a quad core then you really need to beef up your
storage devices. Im talking 10 spindles of SAS/SCSI at 15000 RPMS in RAID10
before backuppc is going to peg a 2.4Ghz quad core.
> a high load and still having cpu idle is a sign of IO load.
> high system cpu usage is a sign of the kernel spending too
> much time managing the IO
>
> for IO load can be 99/100%, but only as long the IO wait
> doesnt increase alot... this values depends of the HD type,
> raid layout, number of disks, bus, etc
>
> in modern HDs (sata, scsi, sas) IO waits between 5-20 are
> normal... 100 is high load...if you see times above 150,
> that HD is probably getting too much load and is slowing
> down a lot the rest of the system (but again, this values
> depend of your OS/config/FS/HD layout)
>
> dont forget that writes usually put more load on a HD than
> reads, the HDs cache helps a lot, but usually write cache is
> not enabled
>
> use iostat (like iostat -kx 30 ) to monitor the HD loads
> and see how the io load and wait is going... sporadic
> high loads/times are normal, specially in small check times,
> but sustained high loads/times are a sign of problems
>
> if you want to try to get a little more from your HDs and
> the data isnt critical in case of a powerloss (usually
> backups arent, next backup would "fix" the bad data), you
> can enable the write cache.
>
> be aware that at least xfs doesnt like to lose data with
> powerloss+write cache, its format assumes that data hits
> the HD when it tells and can miss behave if the cache
> just decided to only really write half of the data before
> the powerloss. that is why is highly recommended to use xfs
> with a UPS and do a controlled shutdown in case of powerloss
>
>
You make a lot more sence here, but I think you overestimate CPU usage.
backuppc is so IO bound that after your get a 2Ghz+ Dual core and 2GB RAM
you can pretty much blame your disks for slow performance. I have a dual
core 2Ghz Opteron with 2GB of ram and 8 drives in a linux raid10 and hard
disk speed is still my bottleneck. I run 4 concurrent backups on that
machine and it does give high system load numbers but still handles the
desktops in the office faster than 3 concurrent while 5 concurrent takes
quite a bit longer to complete. filesystem choice and io scheduler do make a
difference but faster disks is the only real cure.
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