Re: SYNCHRONIZE CACHE command from sd on close

[Douglas Gilbert <dgilbert@interlog.com>]

Christoph Hellwig wrote:
> On Mon, Feb 15, 2010 at 02:03:06PM +0100, Douglas Gilbert wrote:
>> Recently, judging from error reports reaching me
>> from smartmontools, sdparm and sg_start, something
>> changed in the sd driver associated with the
>> SYNCHRONIZE CACHE command it issues when a device
>> is closed.
>>
>> That only seems to happen when the device is opened
>> RW and it exposes a nasty difference between the
>> semantics of spinning up and down ATA disks compared
>> to SCSI disks.
> 
> The sd driver itself never sends a SYNCHRONIZE CACHE in
> response to access through the block device node, it is only
> sent for barrier requests, when hot-unplugging a scsi device,
> or when shutting down the system.
> 
> Now that has change recently is that we now send down a cache
> flush from the block layer when fsync is called on the block
> device node.  The kernel should never call that by itself when
> closing the device, but can you double check that the tools
> don't call fsync/fdatasync/msync or open the block device node
> using O_SYNC/O_DYSNC?

What about O_NONBLOCK (which stops a hang on open)?
The open code common to my utilities in Linux is
below.

Doug Gilbert


int
scsi_pt_open_device(const char * device_name, int read_only, int verbose)
{
     int oflags = O_NONBLOCK;

     oflags |= (read_only ? O_RDONLY : O_RDWR);
     return scsi_pt_open_flags(device_name, oflags, verbose);
}


int
scsi_pt_open_flags(const char * device_name, int flags, int verbose)
{
     int fd;

     if (verbose > 1) {
         if (NULL == sg_warnings_strm)
             sg_warnings_strm = stderr;
         fprintf(sg_warnings_strm, "open %s with flags=0x%x\n", device_name,
                 flags);
     }
     fd = open(device_name, flags);
     if (fd < 0)
         fd = -errno;
     return fd;
}