[dm-devel] Improving dm-mirror as a final year project

[nishant mungse]

On Tue, Jan 25, 2011 at 10:53 AM, Miklos Vajna <vmiklos at frugalware.org>wrote:

> Hi,
>
> I got the possibility to work on dm-mirror as a final year project at
> ULX, the Hungarian distributor of Red Hat.
>
> Get my feet wet, I created two small patches:
>
> 1) dm-mirror: allow setting ios count to 0
>
> Always read from the default_mirror in that case.
>
> 2) dm-mirror: allow setting the default mirror
>
> These can help in case one data leg of a mirror is a remote (iSCSI) one,
> so the default RR aproach is not optimal for reading. (One may set the
> ios count to 0, set the default mirror to the local one, and that will
> cause a read speedup.)
>
> I do not yet have the right to send those patches (I do this in
> university time, so the copyright is not mine), but I hope to be able to
> do so - to get them reviewed.
>
> So the final year project's aim is to improve "the fault tolerance and
> performance of dm-mirror". We (I and my mentors) have two ideas in that
> area, not counting the above patches:
>
> 1) Make the currently hardwired RR read approach modular, that would
> allow implementing for example a weighted RR algorithm. (In case one
> disk is two times faster than the other one, etc.)
>
> 2) From our experiments, it seems that in case the dm-mirror looses one
> of its legs and there is a write to the mirror, it gets converted to a
> linear volume. It would be nice (not sure how easy) to use the mirror
> log to mark the dirty blocks, so that the volume would not be converted
> to a linear one: once the other leg is back, it could be updated based
> on the mirror log.
>
> The question: what do you (who have much more experience with dm-mirror
> than me) think - are these reasonable goals? If not, what would you
> improve/change/add/remove to the above list?
>
> Thanks,
>
> Miklos
>
> PS: I'm not subscribed, please keep me in CC.
>
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hello all,
There is one doubt regarding the dm-raid1. As far as i know, in dm-raid1 the
data is written parallelly  on all the mirrors of mirrorset and if any of
the mirror fails to write the data then dm-mirror adds this mirror to fail
list by increasing the error count in "fail mirror" function in dm-raid1.
Actually my doubt is where this error count is decremented? i.e after kcpyd
or before and where exactly this error count is decremented?


Regards,
Nishant.
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