Hi, 在 2025/07/28 8:39, Damien Le Moal 写道:
md setting its io_opt to 64K*number of drives in the array is strange... It does not have to be that large since io_opt is an upper bound and not a "issue that IO size for optimal performance". io_opt is simply a limit saying: if you exceed that IO size, performance may suffer.
At least from Documentation, for raid arrays, multiple of io_opt is the prefereed io size to the optimal io performance, and for raid5, this is chunksize * data disks.
So a default of stride size x number of drives for the io_opt may be OK, but that should be bound to some reasonable value. Furthermore, this is likely suboptimal. I woulld think that setting the md array io_opt initially to min(all drives io_opt) x number of drives would be a better default.
For raid5, this is not ok, the value have to be chunksize * data disks, regardless of io_opt from member disks, otherwise raid5 have to issue additional IO from other disks to build xor data. For example: - write aligned chunksize to one disk, actually means read chunksize old xor data,then write chunksize data and chunksize new xor data. - write aligned chunksize * data disks, new xor data can be build directly without reading old xor data. Thanks, Kuai
