On Wed, 13 Oct 2010 13:48:50 -0500 greg@xxxxxxxxxxxx wrote: > Hi Neil/linux-raid, hope the week is going well for everyone. > > We implement a lot of geographically mirrored storage using Linux > RAID1 on the 'front-side' to access Linux servers running SCST for > storage backends. One of the issues we are addressing are methods to > reduce I/O latency from the initiator's perspective when various > housekeeping operations are being done on the Linux backends, ie. RAID > check operations/re-synchronizations. > > Write-mostly support is very helpful for reducing any read latencies. > To fully complete the loop it would be useful to leverage asynchronous > write-behind to eliminate write latency effects. We use write-intent > bitmaps on the initiators so we are already preparted for that. > > It is straight forward to toggle writemostly via the > /sys/mdN/md/rN/state pseudofiles but a review of the MD driver code in > at least 2.6.32.16 would suggest that write-behind is not readily > togglable. That may have changed in most recent kernels but I don't > remember seeing any changelogs with your 'git pull' requests which > would suggest that. > > Is this a function of support being a can of worms or just lack of > cycles? If it is the latter I would be willing to take some > directional suggestions and see if we could get a prototype > implementation up and running. In 2.6.33 and later you can write to /sys/block/mdX/md/bitmap/backlog When there is a bitmap and backlog is non-zero, writes to read-mostly devices a 'behind' writes (up to the limit of 'backlog'). NeilBrown > > Thanks much for any insight you may have. > > Best wishes for a productive remainder of the week. > > As always, > Dr. G.W. Wettstein, Ph.D. IDfusion.org > 4206 N. 19th Ave. Unified health identity architecture. > Fargo, ND 58102 > PH: 701-281-1686 > FAX: 701-281-3949 EMAIL: greg@xxxxxxxxxxxx > ------------------------------------------------------------------------------ > "We don't know the OS that God uses, but the Vatican uses Linux." > -- Sister Judith Zoebelein > Vatican Webmaster -- 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
