On 5/27/20 12:26 PM, Eric Sandeen wrote: > If the stripe width of a device is only 2x or 1x the stripe unit, there > is no parity disk on this device, and setting a larger log stripe unit > will not avoid any RMW cycles. However, a large log stripe unit does > have significant penalties for IO amplification because every log write > will be rounded up to the log stripe unit. > > This was recently highlighted by a user running bonnie++ in sync mode, > where the default RAID10 geometry of 256k/512k yielded results which > were 4x slower than a smaller log stripe unit. While bonnie++ may not > be the benchmark of choice, it does highlight this issue. > > Because a larger log stripe unit yields no RMW benefit on a device with > no parity disks, avoid setting in these cases. NAK I wasn't thinking about stacked parity raids like RAID50 (thanks Dave). So we cannot infer anything about parity disks from the top-level geometry, and so the proposed optimization is not valid. -Eric
