Re: [PATCH 2/2] xfs: add 'discard_sync' mount flag

Jens Axboe
2018-04-30T23:02:57Z

On 4/30/18 4:59 PM, Eric Sandeen wrote:
> On 4/30/18 2:58 PM, Jens Axboe wrote:
>> On 4/30/18 1:57 PM, Eric Sandeen wrote:
>>> On 4/30/18 2:21 PM, Jens Axboe wrote:
>
> ...
>
>>> Ok, but I'm not sure how that precludes a block device tunable? You'd tune
>>> it for your workload, right?
>>
>> What kind of tunable are you thinking of? Right now we have one tunable,
>> which is the max discard size. Patch #1 actually helps make this do what
>> it should for sync discards, instead of just building a bio chain and
>> submitting that all at once.
>>
>>> Or is the concern that it could only be for the entire block device, and
>>> perhaps different partitions have different workloads?
>>>
>>> Sorry, caveman filesystem guy doesn't completely understand block devices.
>>
>> I just don't know what you are trying to tune :-)
>
> I'm wondering if "make discards synchronous" couldn't just be a block device
> tunable, rather than a filesystem mount option tunable.

See e-mail I just sent out. With that, you get that tunable bundled with
the discard_max_bytes tunable. If you leave it at the default, which is
generally very high, then you get huge discards. If you tune it down,
they become basically sync. I don't think we need a separate tunable
for this.

It's still per-caller, which is good enough for us. I don't want to
make it more complicated until people show up with a valid use case
that shows we need it strictly throttled per-device.

--
Jens Axboe