Tape block size greater than MAXPHYS

[Shivaram Upadhyayula]

Hi,

It seems that currently any tape reads/writes greater than MAXPHYS
will fail. For example

cpi->maxio = 256 * 1024; /* Controller max io size 256K */

root at quadstorvtl # dd if=/dev/zero of=/dev/sa0 bs=256k count=1
sa0.0: request size=262144 > si_iosize_max=131072; cannot split request
sa0.0: request size=262144 > MAXPHYS=131072; cannot split request
dd: /dev/sa0: File too large
1+0 records in
0+0 records out
0 bytes transferred in 0.000390 secs (0 bytes/sec)

The first limitation comes from sys/cam/scsi/scsi_sa.c:saregister
        /*
         * If maxio isn't set, we fall back to DFLTPHYS.  Otherwise we take
         * the smaller of cpi.maxio or MAXPHYS.
         */
        if (cpi.maxio == 0)
                softc->maxio = DFLTPHYS;
        else if (cpi.maxio > MAXPHYS)
                softc->maxio = MAXPHYS;
        else
                softc->maxio = cpi.maxio;

softc limits maxio to MAXPHYS even if the controller supports a higher
maxio value. I tried removing the limitation which then led me to
reason for the actual reason for the limiation in
sys/kern/kern_physio.c:physio

        /*
         * If the driver does not want I/O to be split, that means that we
         * need to reject any requests that will not fit into one buffer.
         */
        if (dev->si_flags & SI_NOSPLIT &&
            (uio->uio_resid > dev->si_iosize_max || uio->uio_resid > MAXPHYS ||
            uio->uio_iovcnt > 1)) {

To maintain consistency of the block numbers SI_NOSPLIT has to be set,
but then to issue the entire request in a single bio the request size
will be limited to MAXPHYS.

Would is be correct to assume that the only way to increase the tape
block size for writes/reads is to increase MAXPHYS and recompile the
kernel ? (As of now on FreeBSD 10.1)

Regards,
Shivaram



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