geom_raid5 inclusion in HEAD? bsd at
Tue Nov 6 06:57:00 PST 2007

Hello list!

I was wondering if and when the excellent geom_raid5 module created by 
Arne Wörner would be committed to HEAD or releng_7. Right now there's 
gvinum with RAID5 support and geom_raid3. The latter is nice but only 
works with a 'strange' number of disks (3,5,9,17,33, etc) and has the 
disadvantages associated with RAID3 (no parallel access; all disks are 
involved in one single write operation), while gvinum offers poor RAID5 
write performance (~20MB/s in my testings).

geom_raid5, on the other hand, offers excellent performance due to write 
combining, that allows 2-phase requests to be converted into 1-phase 
requests. This is achieved by waiting for contiguous requests and 
splitting those into the 'full stripe', meaning: stripesize * ( 
number_of_disks - 1). This way, no parity information has to be read 
from any disk since all required data is already in memory. In normal 
english this means excellent sequential performance. In my own testings 
i have achieved over 400MB/s of RAID5 write performance using a simple 
"dd if=/dev/zero of=/dev/raid5/data bs=1m" of at least 10GB worth of 
data. raidtest benchmarks show that geom_raid5 scales nicely with the 
number of disks, though ofcourse RAID5's weak point is random write.

Considering this project has been around for quite a while, and sister 
project FreeNAS has been using it for more than a year in their official 
(stable) branch, i think it should become available in the official 
FreeBSD distribution as well. I don't know who decides on this, but i 
would like to get a discussion going on this matter.

Looking forward to any replies!

Kind regards,

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