Ric Wheeler wrote: >> Don't those thingies usually have NV cache or backed by battery such >> that ORDERED_DRAIN is enough? > > All of the high end arrays have non-volatile cache (read, on power loss, > it is a promise that it will get all of your data out to permanent > storage). You don't need to ask this kind of array to drain the cache. > In fact, it might just ignore you if you send it that kind of request ;-) > > The size of the NV cache can run from a few gigabytes up to hundreds of > gigabytes, so you really don't want to invoke cache flushes here if you > can avoid it. > > For this class of device, you can get the required in order completion > and data integrity semantics as long as we send the IO's to the device > in the correct order. Thanks for clarification. >> The problem is that the interface between the host and a storage device >> (ATA or SCSI) is not built to communicate that kind of information >> (grouped flush, relaxed ordering...). I think battery backed >> ORDERED_DRAIN combined with fine-grained host queue flush would be >> pretty good. It doesn't require some fancy new interface which isn't >> gonna be used widely anyway and can achieve most of performance gain if >> the storage plays it smart. > > I am not really sure that you need this ORDERED_DRAIN for big arrays... ORDERED_DRAIN is to properly order requests from host request queue (elevator/iosched). We can make it finer-grained but we do need to put some ordering restrictions. -- tejun -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
