Filipe Brandenburger wrote:
Hi,
On Thu, Sep 24, 2009 at 08:48, nate centos@linuxpowered.net wrote:
RAID 6 has a pretty good performance hit vs RAID 5 due to the extra parity disk, on some arrays the performance hit is even greater as the array calculates parity twice, NetApp I think has as good a RAID 6 implementation as there is, though they can't get around writing the parity information to two different disks.
Actually, NetApp tries to attenuate that impact.
As the mirroring is coupled with the filesystem, NetApp uses an intelligent algorithm that tries to fill all the blocks in a whole stripe before writing to the disks, that way it only has to calculate the parity of data in RAM and can flush to all disks at the same time. That's why in most cases they do not have the "read before write" impact of RAID-5 and RAID-6. That's also why they have RAID-4 instead of RAID-5, they say that the parity disk will not be a hot spot as in most cases all the disks will be written at the same time. Well, at least that's what they say...
I believe ZFS implements similar ideas. I guess that's one of the patents for which NetApp tried to sue Sun a couple of years ago when ZFS first came out.
Not quite. ZFS does things the other way around. "RAID-Z is a data/parity scheme like RAID-5, but it uses dynamic stripe width. " From your description, NetApp massages data to fit the stripe. ZFS's raidz massages the stripe to fit the data.