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? Confused, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg) -- 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
