Re: [BackupPC-users] Tuning for disk contention

[higuita <higuita@GMX.net>]

Hi

On Fri, 4 Sep 2009 14:39:22 -0700, James Ward <je...@to...> wrote:
> What's a good IO/WAIT average to shoot for?  Load average?  Other  
> things I might not be thinking of?

	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

	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

good luck
higuita
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