On 20 February 2012 15:20, Kevin Wolf <kw...@redhat.com> wrote: > Am 20.02.2012 15:18, schrieb Virtbie: >> Qemu manpage still says cache=writeback isn't safe, but I seem to find >> some discordant information reading around. >> >> Is the WCE+volatile flag exposed to the guest already? I am logging into >> a 2.6.38 guest and trying to find an indicator of disk cache in >> /sys/block/vda/device but I am not able to. There is an obscure very >> long "features" bitmask which I don't know what it shows. I think this >> mode is not safe until WBC is exposed, AFAIU, right? > > qemu exposes the WCE flag and if your guest handles it correctly (i.e. > it sends flush requests where needed), cache=writeback is safe. This is > true for recent Linux versions.
The thing that confuses me when this subject comes up is that "cache=writeback" is a property of the block layer, but the WCE flag is a SCSI parameter, right? How does this work on non-SCSI disks? Is there something that eg hw/sd.c should be doing to tell the block layer "writeback cache is safe/unsafe" ? -- PMM
