Re: TGT and DPO,FUA

TGTD supports if the client sets these flags, but TGTD does not
advertize that FUA/DPO is available.
Since TGTD does not do any caching internally but always performs
system calls to read/write the underlying data, I
do not know if it would be useful to advertize "we support DPO/FUA
(but we actually don't do any caching)"

This support is communicated via the device specific byte in mode-sense.

https://github.com/fujita/tgt/blob/master/usr/sbc.c#L120

We currently signal back to the initiator only if the device is WP
(write protected)


If you want to have TGTD tell the initiator that FUA/DPO are available
we would need to add

if (mode6)
  data[2] |= 0x10;
else
  data[3] |= 0x10;


I see no reason to not report DPO/FUA support back to the initiator so
I can send a patch to the list later that does this change unless
someone objects.

regards
ronnie sahlberg



On Sun, Nov 9, 2014 at 1:12 AM, Xi Tom <[email protected]> wrote:
> Hi,
>
> I am using tgt as iscsi server, and found linux kernel said that a tgt
> scsi target does not support DPO and FUA:
>
> [ 4671.673196] scsi8 : iSCSI Initiator over TCP/IP
> [ 4671.928530] scsi 8:0:0:0: RAID              IET      Controller
>   0001 PQ: 0 ANSI: 5
> [ 4671.928800] scsi 8:0:0:0: Attached scsi generic sg5 type 12
> [ 4671.929454] scsi 8:0:0:1: Direct-Access     IET      VIRTUAL-DISK
>   0001 PQ: 0 ANSI: 5
> [ 4671.930878] sd 8:0:0:1: Attached scsi generic sg6 type 0
> [ 4671.932074] sd 8:0:0:1: [sde] 6144000 512-byte logical blocks:
> (3.14 GB/2.92 GiB)
> [ 4671.932273] sd 8:0:0:1: [sde] Write Protect is off
> [ 4671.932277] sd 8:0:0:1: [sde] Mode Sense: 69 00 00 08
> [ 4671.932420] sd 8:0:0:1: [sde] Write cache: enabled, read cache:
> enabled, doesn't support DPO or FUA
>
>
> Why does not tgt support these two features ? but I have found its
> source code handle
> DPO and FUA flag in its bs_rdwr.c.
>
> Regards,
> Tom
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