Re: [PATCH 6/6] writeback: throttle buffered writeback

[Jeff Moyer]

Jens Axboe <axboe@xxxxxx> writes:

> On 03/22/2016 02:12 PM, Jeff Moyer wrote:
>> Hi, Jens,
>>
>> Jens Axboe <axboe@xxxxxx> writes:
>>
>>> If the device has write back caching, 'wb_cache_delay' delays by
>>> this amount of usecs when a write completes before allowing more.
>>
>> What's the reason behind that?
>
> For classic write back caching, the cache can absorb a bunch of writes
> shortly, which means that the completion cost only shows a small part
> of the overall cost. This means that if we just throttle on
> completion, then when the device starts committing to media, then
> we'll end up starving other IO anyway. This knob is a way to attempt
> to tame that.

Ah, makes perfect sense.  Thanks!

-Jeff