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

[Peter Lieven]

Am 14.04.2015 um 21:55 schrieb 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.

e) In iscsi_co_readv: If iscsilun->dpofua && (bs->open_flags & BDRV_O_NOCACHE) 
set FUA bit in Read 10/16.

This explicitly tells the iSCSI storage to bypass its read cache.

Peter