linux-ext4 - Re: [PATCH v2 0/4] Write-hint for FS journal

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Date:   Wed, 30 Jan 2019 19:24:39 +0530
From:   Kanchan Joshi <joshi.k@...sung.com>
To:     Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>
Cc:     Keith Busch <keith.busch@...el.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "jack@...e.com" <jack@...e.com>, "tytso@....edu" <tytso@....edu>,
        "prakash.v@...sung.com" <prakash.v@...sung.com>,
        Jens Axboe <axboe@...com>
Subject: Re: [PATCH v2 0/4] Write-hint for FS journal


On Wednesday 30 January 2019 05:43 AM, Dave Chinner wrote:
> On Tue, Jan 29, 2019 at 11:07:02AM +0100, Jan Kara wrote:
>> On Mon 28-01-19 16:24:24, Keith Busch wrote:
>>> On Mon, Jan 28, 2019 at 04:47:09AM -0800, Jan Kara wrote:
>>>> On Fri 25-01-19 09:23:53, Keith Busch wrote:
>>>>> On Wed, Jan 09, 2019 at 09:00:57PM +0530, Kanchan Joshi wrote:
>>>>>> Towards supporing write-hints/streams for filesystem journal.
>>>>>>                                                                                  
>>>>>> Here is the v1 patch for background -
>>>>>> https://marc.info/?l=linux-fsdevel&m=154444637519020&w=2
>>>>>>                                                                                  
>>>>>> Changes since v1:
>>>>>> - introduce four more hints for in-kernel use, as recommended by Dave chinner
>>>>>>    & Jens axboe. This isolates kernel-mode hints from user-mode ones.
>>>>>
>>>>> The nvme driver disables streams if the controller doesn't support
>>>>> BLK_MAX_WRITE_HINT number of streams, so this series breaks the feature
>>>>> for controllers that only support up to 4.
>>>>
>>>> Right. Do you know if there are such controllers? Or are you just afraid
>>>> that there could be?
>>>
>>> I've asked around, and the concensus I received is all currently support
>>> at least 8, but they couldn't say if that would be true for potential
>>> lower budget products. Can we implement a reasonable fallback to use
>>> what's available?
>>
>> OK, thanks for input. So probably we should just map kernel stream IDs to 0
>> if the device doesn't support them. But that probably means we need to
>> propagate number of available streams up from NVME into the block layer so
>> that this can be handled reasonably seamlessly. Jens, Kanchan?
> 
> Yeah, that's basically what I said we needed to do when this was
> last discussed. i.e. that the block layer needed to know how many
> streams the hardware had and map the 4 "kernel internal" hints
> appropriately to what he device supports.
> 
> e.g. if the device only supports 4 hints, then it needs to map the
> kernel hints either to zero. If it supports less than 8 streams,
> then they need otbe mapped into the hints above index 5. If there
> are N streams, then they need to be mapped to the hints {N-3,N}
> 
> And, to top it all off, there needs to be guards so that if we want
> to grow the userspace hints to more than 4 hints, they don't crash
> into ranges the kernel is already reserving because of limited
> device range support.
> 
> Nothing is ever simple....
> 
Thanks all for feedback.
user-hints, when they reach to kernel via fcntl path, are sanity-checked 
(rw_hint_valid function).
Currently streams are enabled when nvme driver is made to run with 
"streams =1" option, while stream users always pass some write-hint, 
without bothering whether streams (and how many of those) are 
operational or not. This keeps configuration simple for stream users. 
Second, block layer does not translate write-hint to stream-number, 
rather it is done inside nvme driver. I suppose I should keep both these 
properties intact.
And considering all the suggestions, this is the plan for V3 -

[In block layer]
1. Introduce one macro "KERN_WRITE_HINT_MIN" which will take the value 
"user_hint_cnt + 1".
FS code will use this value (onwards) to define their own streams.

2. Introduce another macro "BLK_MAX_KERNEL_WRITE_HINTS" which will be 
set to 4 for now.

[In nvme driver]
1. Continue working as before if device supports just 4 streams. All 
these streams are used by user-hints, and kernel-hints are translated to 0.

2. If device supports any more than 4 streams, those will be mapped to 
serve kernel-hints, starting from KERN_WRITE_HINT_MIN onwards.
For example, if device has 6 streams, four streams (numbers = 1,2,3,4) 
will be used to serve user-hints and two streams ( numbers = 65535, 
65534) will be used to serve first two kernel hints. Other kernel-hints 
get mapped to 0. OTOH, if device has 10 streams, first four kernel-hints 
will be mapped to non-zero values (65535 to 65532) and anything else 
would get turned to 0.


Let me know if this sounds fine?


Thanks,
Kanchan