Re: Complexity of toggling write-behind.

[Neil Brown <neilb@suse.de>]

On Wed, 13 Oct 2010 13:48:50 -0500
greg@enjellic.com 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
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