On Mon, Oct 12, 2009 at 7:33 PM, Mike Snitzer <snitzer@xxxxxxxxx> wrote: > On Mon, Oct 12, 2009 at 11:00 AM, Chris Worley <worleys@xxxxxxxxx> wrote: >> On Mon, Oct 12, 2009 at 8:22 AM, Mike Snitzer <snitzer@xxxxxxxxx> wrote: >>> On Sun, Oct 4, 2009 at 3:40 PM, Chris Worley <worleys@xxxxxxxxx> wrote: >>>> When you mirror two devices, they may not have the same performance. >>>> For example, if one device is local and the other remote. >>>> >>>> There's nothing you can do about writes, as everything needs written. >>>> >>>> But, reads shouldn't be split 50/50: they should be balanced depending >>>> relative performance of each mirror to optimize overall read >>>> performance. >>> >>> And MD already has the ability to flag the slow paths as write-mostly, see: >>> >>> mdadm ... --write-mostly >>> >>> There is also '--write-behind' support for devices that have been >>> tagged as 'write-mostly'. >> >> Will "write-mostly" optimize the "read", such that both devices will >> be accessed in proportion to their performance? >> >> The man page says: >> >> "...and means that the ’md’ driver will avoid reading from these >> devices if at all possible." >> >> Even a slow device can take some of the load and increase performance. > > AFAIK, with current MD raid1, reads are only balanced across all > non-'write-mostly' drives. Having raid1's reads be issued proportional > to each underlying devices' performance is not currently possible. > > I'd imagine that the per-bdi dirty-page accounting of each underlying > device could be used to train raid1's reads (score each raid1 member > over time) but I'm not aware of any effort toward that end. Off the top of my head: the user could control this w/ a per-device read queue depth. Set it small for the remote and large for the local. Let the use optimize for their case. Chris > > Mike > -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
