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Re: [dm-devel] Re: possible regression by the barrier patch in 2.6.30-rc2



On Wed, 4 Nov 2009, Mikulas Patocka wrote:

> On Wed, 28 Oct 2009, Alasdair G Kergon wrote:
> 
> > Well let's go back to first principles.
> > 
> > There are two types of suspend.
> > 
> > (1) I don't care about the ordering of the I/O on the disk relative to
> > the suspend.  This one is easy: it's --noflush --nolockfs.
> > 
> > (2) I do want some control over the state of the device at the point
> > of the suspend.
> > 
> > Break this second case down.
> > If I have a filesystem, I require it to be consistent, so I require lockfs.
> > 
> > If the device belongs to a userspace database, I require it to be consistent,
> > so I must pause the database, at which point there is no further I/O being
> > issued to the device, then I suspend (and everything prior to this must be
> > flushed) and resume etc.  This can be either a "suspend with flush" OR the
> > flush could have been issued prior to the suspend (and any decent database
> > would have done that).
> 
> In this database case, you have to pause the database, the pause procedure 
> will wait until all I/O finishes and won't submit new I/O. When the pause 
> procedure finishes, the database has no I/O in flight, so it doesn't 
> matter if you use flush or noflush suspend.
> 
> The reason is that there may be another I/O midlayers between the database 
> and the device mapper. So, if the database submits I/O, it doesn't have to 
> immediatelly arrive to the device mapper. If you paused the database 
> (without waiting for complete I/Os) and then issued "flush" suspend, the 
> I/O may still be pending somewhere above the suspended device, then the 
> device finishes flush suspend, then the I/O arrives and waits until 
> unsuspend.
> 
> I'm somehow starting to think that "flush" suspend is not needed at all 
> and all suspends may be "noflush". Do you have any counterexamples?
> 
> Mikulas

Anyway, suspend unconditionally purges all I/Os from target drivers. All 
that this "noflush" flag does is that it allows the I/O to be held and 
requeued for new dm table incarnation, if the target requests it with 
DM_MAPIO_REQUEUE (dm-raid1 and dm-multipath do it). If no target returns 
DM_MAPIO_REQUEUE, then "flush" and "noflush" suspend is equivalent.

Question: could "noflush" be the default behavior and could the "flush" 
flag be dropped? I think yes.

Mikulas


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