On Wed, Apr 24 2013 at 8:12am -0400, Hannes Reinecke <hare@xxxxxxx> wrote: > On 04/24/2013 02:08 PM, Paolo Bonzini wrote: > > Il 24/04/2013 14:07, Hannes Reinecke ha scritto: > >> On 04/24/2013 01:17 PM, Paolo Bonzini wrote: > >>> Il 23/04/2013 22:07, James Bottomley ha scritto: > >>>> On Tue, 2013-04-23 at 15:41 -0400, Ric Wheeler wrote: > >>>>> For many years, we have used WCE as an indication that a device has a volatile > >>>>> write cache (not just a write cache) and used this as a trigger to send down > >>>>> SYNCHRONIZE_CACHE commands as needed. > >>>>> > >>>>> Some arrays with non-volatile cache seem to have WCE set and simply ignore the > >>>>> command. > >>>> > >>>> I bet they don't; they probably obey the spec. There's a SYNC_NV bit > >>>> which if unset (which it is in our implementation) means only sync your > >>>> non-NV cache. For a device with all NV, that equates to nop. > >>> > >>> Isn't it the other way round? > >>> > >>> SYNC_NV = 0 means "sync all your caches to the medium", and it's what we do. > >>> > >>> SYNC_NV = 1 means "sync volatile to non-volatile", and it's what Ric wants. > >>> > >>> So we should set SYNC_NV=1 if NV_SUP is set, perhaps only if the medium > >>> is non-removable just to err on the safe side. > >> > >> Or use 'WRITE_AND_VERIFY' here; that's guaranteed to hit the disk. > >> Plus it even has a guarantee about data consistency on the disk, > >> which the normal WRITE command has not. > > > > The point is to _avoid_ hitting the disk. :) > > > Ah. Really? > > Why do we discuss SYNCHRONIZE CACHE then? > I was under the impression that we're talking various 'barriers' > (or rather 'flush' nowadays) implementations. > Which require that some data needs to be written to disk before > continuing. > > Or did I somehow misread the thread? This thread was motivated by the fact that the storage is reporting WCE=1 and OracleDB (with ASM) is issuing SYNCHRONIZE CACHE (via REQ_FLUSH) which the array in question handles _very_ slowly (even though it is battery backed). So the question Ric had is: should we expose a new knob that allows admins to impose WCE=0 behavior (avoiding the SYNCHRONIZE CACHE). I'm concerned such a knob will be abused for the benefit of speed and all data integrity caution will get thrown to the wind (much like the nobarrier FS mount option). Mike -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
