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.
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
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