[Bug 42939] New: hv_storvsc - write cache enabled by default and cannot be disabled

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https://bugzilla.kernel.org/show_bug.cgi?id=42939

           Summary: hv_storvsc - write cache enabled by default and cannot
                    be disabled
           Product: SCSI Drivers
           Version: 2.5
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Other
        AssignedTo: scsi_drivers-other@xxxxxxxxxxxxxxxxxxxx
        ReportedBy: bugzilla-kernel-org-sux2k0@xxxxxxxxx
        Regression: No


dmesg output:

[    8.080401] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled,
supports DPO and FUA

Question: how safe is my (database) data in the case of a host machine crash or
a VM crash?

There is no battery-backed writecache installed in the host, just some cheap
and slow SATA disks.

Another curiosity occurs when running the fio iometer benchmark on the same
machine: it gives me 5.000 to 6.000 I/O random access operations per second
when using the para-virt hv_storvsc driver, but only the expected 200 I/O ops
when using ata_piix via emulation.The kernel in question is version 3.2.11.

There doesn't seem to be any way to disable the para-virt device's write-cache.

Kernel version is mainline 3.2.11.

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