Number of drives.

[digrouz]

Hello,

I have a NAS with 4k block sized drives. I thinking about reinstall it with freebsd and zfs. I've read some stuff talking about the number of drives needed to get good performances with RAIDZ. Something like with 4 drives it's bad but with 3 or 5 it's good.

As i understand, the performance issues with 4K disks isn’t just partition alignment, but also an issue with RAID-Z’s variable stripe size.
RAID-Z basically works to spread the 128KiB recordsizie upon on its data disks. That would lead to a formula like:
128KiB / (nr_of_drives – parity_drives) = maximum (default) variable stripe size
Let’s do some examples:
3-disk RAID-Z = 128KiB / 2 = 64KiB = good
4-disk RAID-Z = 128KiB / 3 = ~43KiB = BAD!
5-disk RAID-Z = 128KiB / 4 = 32KiB = good
9-disk RAID-Z = 128KiB / 8 = 16KiB = good

4-disk RAID-Z2 = 128KiB / 2 = 64KiB = good
5-disk RAID-Z2 = 128KiB / 3 = ~43KiB = BAD!
6-disk RAID-Z2 = 128KiB / 4 = 32KiB = good
10-disk RAID-Z2 = 128KiB / 8 = 16KiB = good

Can anyone told me more about it? Did anyone experiment this kind of configuration? As my nas as 4 drives and 1 esata port, would the esata drop the performances of the array?

 

[Sebulon]

I´ve also been thinking about those numbers a while back; about why you subtract the parity?

Also, if the stripe size doesn´t fit, why not just change the stripe size?

/Sebulon

 

[vermaden]

@digrouz

The answer for that would be making tests with these configurations and comparing benchmarks results. It there are any 'anomalies' in scaling of those configurations, then this issue is true, if not and benchmarks are quite linear, then its not an issue.