Re: No syncing after crash. Is this a software raid bug?

[Mario 'BitKoenig' Holbe <Mario.Holbe@TU-Ilmenau.DE>]

Heinz Mauelshagen <mauelshagen@redhat.com> wrote:
> On Fri, Mar 03, 2006 at 03:30:29PM +0100, Mario 'BitKoenig' Holbe wrote:
>> But when a dirty page is modified while it's being accessed, it stays
>> dirty and gets cleaned (i.e. written to disk) later again, right?
> A mirroring driver (eg. the MD raid1 personality), will access the dirty
> page multiple times to store the data on multiple mirrors before the dirty
> flag will be cleared during endio processing.

So the dirty flag *is* cleared even if the page was modified while the
mirroring driver did access it?

> o write gets through to first mirror
> o page content gets changed
> o second write gets through to other mirror
...
> This is what I meant by well-behaved applications.
> The DBMS will write to such (eventually) inconsistent blocks
> *before* it'll read them back in hence removing the block-level inconsistency.
>> And couldn't this happen even on swap without reboot inbetween when a
>> page really needs to be read from disk?
> It shouldn't, because page-ins will follow page-outs first.
> Meanwhile the transient page table(s) will contain the disk address(es)
> of the respective page(s).

But given the rough scenario above there *are* page-outs first.
Wouldn't it be possible (in both cases: the DBMS as well as the swap
scenario), that the now cleaned page is read in later from the first
mirror and thus contains the old content which was written before it
got changed, written to the other mirror and got cleared?


regards
   Mario
-- 
<jv> Oh well, config
<jv> one actually wonders what force in the universe is holding it
<jv> and makes it working
<Beeth> chances and accidents :)