On Monday September 12, [email protected] wrote:
> > No, I don't think so. The overhead would be substantial, so people
> > would be very unlikely to use it.
>
> There are situations where data integrity is far more important than speed.
> On AIX I usually use the Mirror Write Consistency and Write Verify
> options on my mirrored volumes that store data where integrity is more
> important than speed.
> I guess something like those options would also satisfy Lennert's
> needs, but I don't know if it's currently possible with the Linux LVM
> or elsewhere.
>
> You can read a bit about the MWC and WV options in AIX at :
> http://publib.boulder.ibm.com/infocenter/pseries/index.jsp?topic=/com.ibm.aix.doc/aixbman/prftungd/diskperf2.htm
Thanks for the link.
If I understand the (fairly brief) descriptions correctly:
Passive mirror-write-constancy has always been part of md/raid1
Active mirror-write-constancy is equivalent to the new
bitmap-write-intent support.
WV means read-after-write which we don't do, but might be useful.
However, I'm not 100% certain that WV would really be useful. Modern
drives will almost certainly return a read-after-write request out of
the drive's cache rather than going to the media. We would need some
way to tell the drive to ignore the cache for this read. I suspect
this is possible, but might not be trivial...
NeilBrown
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