On Fri, Dec 18, 2009 at 07:37:20PM -0500, Matt Tehonica wrote:
> I have a 4 disk RAID5 using a 2048K chunk size and using XFS
> filesystem. Typical file size is about 2GB-5GB. I usually get around
> 50MB/sec transfer speed when writting files to the array. Is this
> typcial or is it below normal? A friend has a 20 disk RAID6 using the
> same filesystem and chunk size and gets around 150MB/sec. Any input on
> this??
You must be aware :
- each disk has physical limitations that depends from rpm
- that writing is slow on raid5 & raid6 .
With a raid5 device ,when writing a new block , you must :
- read the original block
- read the parity block
- compute the new parity
- write the new block
- write the new parity
With a raid6 device ,when writing a new block , you must :
- read the original block
- read the parity 1 block
- read the parity 2 block
- compute the new parity 1
- compute the new parity 2
- write the new block
- write the new parity 1
- write the new parity 2
With cache , you can have better performance somes times , that depends of usage of device
by the application .
Its'common to said that :
on raid 5 you have x4 penalty in writing
on raid 6 you have x6 penalty in writing
on raid 1/10 you have x2 penalty in writing
on raid 0 you have no penalty in writing
Its'common to said that :
one 15k rpm drive can do 180 random IO per second
one 10k rpm drive can do 140 random IO per second
one 7200 rpm drive can do 80 random IO per second
If you have perfomance problem you must have more drive, because more drive give you more iops .
If you have bad perfomance with raid5 during write you must try raid1 .
It's better to have many small disks that a big one , for perfomance .
But for Electricity consumption , it's different :
2,5 drive use less electricity that 3,5 drive
slower in RPM drive use lesser electricity
There no magic formula !
In your case :
you used 4 drives in raid5 , so when you write data , you have only the performance of one drive .
number_of_disk_in_the_array / 4 ( because you are in raid5 )
For your friend :
He used a 20 drives in raid6 , so when he write data , he have performance of 3.6 drives
number_of_disk_in_the_array / 6 ( because you are in raid6 )
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