Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache

[Peter Lieven]

Am 14.04.2015 um 18:15 schrieb Paolo Bonzini:
>
> On 14/04/2015 08:49, Peter Lieven wrote:
>> Hi,
>>
>> Ronnie came up with an idea to reduce latency if !bs->enable_write_cache
>> for an iSCSI device.
>>
>> If !bs->enable_write_cache Qemu sends a flush after every single write.
>> What could be done is
>> the following:
>>
>> if (!bs->enable_write_cache)
>>  set FUA (force unit access) and DPO (disable page out) bits in every
>> write cmd
>>  make iscsi_co_flush a NOOP in this case.
>>
>> Your thoughts?
> Yes, that would work.  In fact I'm not even sure you need DPO.
>
> speed of cache=writethrough in general doesn't matter much, except if
> whoever runs the guest knows that the host has battery-backed cache.  In
> that case this trick would improve latency.  You could get the same with
> -drive file.cache.no-flush=on but this would just work.

You could, but this would not set the FUA bit. The DPO was Ronnies
idea. Ronnie? SBC-2 says about FUA and WRITE: "Write commands shall not return 
GOOD status until the logical blocks have actually been written on the media 
(i.e., the data is not write cached)."

Please correct me if I am wrong, but for iSCSI
cachemodes none, directsync, writethrough and none are identical.
cachemode unsafe avoids the explicit flush which is no good idea as
we all would agree.

cachemode writeback is the only one that enables bs->enable_write_cache.
I use it for our enterprise iSCSI SANs that have battery backup and I use it
only in conjunction with Virtio which falls back to writethrough if there is
no support for flushes from the guest.

What I would implement in the iSCSI driver is the following:

a) check support for DPOFUA in the MODE SENSE page. Store it in 
iscsilun->dpofua.
b) make iscsi_co_flush a NOOP if iscsilun->dpofua && !bs->enable_write_cache
c) In iscsi_co_writev
    If iscsilun->dpofua && !bs->enable_write_cache set the FUA bit in the write 
command.
d) in iscsi_co_write_zeroes
   If iscsilun->dpofua && !bs->enable_write_cache manually issue a 
iscsi_co_flush as the WRITESAME10/16 commands lack the FUA bit.

Peter