Re: [PATCH 2/2] btrfs: add read_mirror_policy parameter devid

[Edmund Nadolski <enadolski@suse.de>]

On 02/01/2018 01:12 AM, Anand Jain wrote:
> 
> 
> On 02/01/2018 01:26 PM, Edmund Nadolski wrote:
>> On 1/31/18 7:36 AM, Anand Jain wrote:
>>>
>>>
>>> On 01/31/2018 09:42 PM, Nikolay Borisov wrote:
>>>
>>>
>>>>>> So usually this should be functionality handled by the raid/san
>>>>>> controller I guess, > but given that btrfs is playing the role of a
>>>>>> controller here at what point are we drawing the line of not
>>>>>> implementing block-level functionality into the filesystem ?
>>>>>
>>>>>    Don't worry this is not invading into the block layer. How
>>>>>    can you even build this functionality in the block layer ?
>>>>>    Block layer even won't know that disks are mirrored. RAID
>>>>>    does or BTRFS in our case.
>>>>>
>>>>
>>>> By block layer I guess I meant the storage driver of a particular raid
>>>> card. Because what is currently happening is re-implementing
>>>> functionality that will generally sit in the driver. So my question was
>>>> more generic and high-level - at what point do we draw the line of
>>>> implementing feature that are generally implemented in hardware devices
>>>> (be it their drivers or firmware).
>>>
>>>   Not all HW configs use RAID capable HBAs. A server connected to a SATA
>>>   JBOD using a SATA HBA without MD will relay on BTRFS to provide all
>>> the
>>>   features and capabilities that otherwise would have provided by such a
>>>   presumable HW config.
>>
>> That does sort of sound like means implementing some portion of the
>> HBA features/capabilities in the filesystem.
>>
>> To me it seems this this could be workable at the fs level, provided it
>> deals just with policies and remains hardware-neutral.
> 
>  Thanks. Ok.
> 
>> However most
>> of the use cases appear to involve some hardware-dependent knowledge
>> or assumptions. 
> 
>> What happens when someone sets this on a virtual disk,
>> or say a (persistent) memory-backed block device? 
> 
>  Do you have any policy in particular ?

No, this is your proposal ;^)

You've said cases #3 thru #6 are illustrative only. However they make
assumptions about the underlying storage, and/or introduce potential for
unexpected behaviors. Plus they could end up replicating functionality
from other layers as Nikolay pointed out. Seems unlikely these would be
practical to implement.

Case #2 seems concerning if it exposes internal,
implementation-dependent filesystem data into a de facto user-level
interface. (Do we ensure the devid is unique, and cannot get changed or
re-assigned internally to a different device, etc?)

Thanks,
Ed


>> Case #6 seems to
>> open up some potential for unexpected interactions (which may be hard
>> to reproduce, esp. in error/recovery scenarios).
> 
>  Yep. Even the #1 pid based (current default) which motivated
>  me to provide the devid based policy.
> 
>> Case #2 takes a devid, but I notice btrfs_device::devid says, "the
>> internal btrfs device id".  How does a user obtain that internal value
>> so it can be set as a mount option?
> 
>   btrfs fi show -m
> 
> Thanks, Anand
> 
>> Thanks,
>> Ed
>>
>>
>>>>>>> ::
>>>>>>>>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>>>>>>>>> index 39ba59832f38..478623e6e074 100644
>>>>>>>>> --- a/fs/btrfs/volumes.c
>>>>>>>>> +++ b/fs/btrfs/volumes.c
>>>>>>>>> @@ -5270,6 +5270,16 @@ static int find_live_mirror(struct
>>>>>>>>> btrfs_fs_info *fs_info,
>>>>>>>>>              num = map->num_stripes;
>>>>>>>>>            switch(fs_info->read_mirror_policy) {
>>>>>>>>> +    case BTRFS_READ_MIRROR_BY_DEV:
>>>>>>>>> +        optimal = first;
>>>>>>>>> +        if (test_bit(BTRFS_DEV_STATE_READ_MIRROR,
>>>>>>>>> +                 &map->stripes[optimal].dev->dev_state))
>>>>>>>>> +            break;
>>>>>>>>> +        if (test_bit(BTRFS_DEV_STATE_READ_MIRROR,
>>>>>>>>> +                 &map->stripes[++optimal].dev->dev_state))
>>>>>>>>> +            break;
>>>>>>>>> +        optimal = first;
>>>>>>>>
>>>>>>>> you set optimal 2 times, the second one seems redundant.
>>>>>>>
>>>>>>>     No actually. When both the disks containing the stripe does not
>>>>>>>     have the BTRFS_DEV_STATE_READ_MIRROR, then I would just want to
>>>>>>>     use first found stripe.
>>>>>>
>>>>>> Yes, and the fact that you've already set optimal = first right after
>>>>>> BTRFS_READ_MIRROR_BY_DEV ensures that, no ? Why do you need to again
>>>>>> set
>>>>>> optimal right before the final break? What am I missing here?
>>>>>
>>>>>     Ah. I think you are missing ++optimal in the 2nd if.
>>>>
>>>> You are right, but I'd prefer you index the stripes array with
>>>> 'optimal'
>>>> and 'optimal + 1' and leave just a single assignment
>>>
>>>   Ok. Will improve that.
>>>
>>> Thanks, Anand
>>>
>>>
>>>>>
>>>>> Thanks, Anand
>>>>>
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