On Fri, Apr 29, 2016 at 2:38 PM, Kamil Cholewiński <[email protected]> wrote: > Silly, tangentially related question, perhaps someone knows an answer: > > Is there a considerable performance impact to be expected when using an > odd number of disks in RAID[56] setups? > > I mean, e.g. with RAID5, one disk stores parity data, so in a 3-disk > setup, a 512-byte data block is split between two devices. In a 5-disk > setup, or in a 6-disk RAID6 setup, similarly the data chunk is split > between four physical devices, so 512/4=nice number. What about > situations where 512/3, 512/5, etc? > > Am I making sense or garbage?
If you look into softraid_raid5.c you will see that actual write I/O is done to only 2 drives: one data and one parity chunk. The thing is that both data and parity chunks change based on the disk block position and data length. I really recommend to see the code as it is nicely commented. Look for sr_raid5_rw and sr_raid5_write. In case of RAID6 this is more complicated, but write I/O goes to 3 drives (1 data + 2 parities) depending again on disk block position and data length.
