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 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
