Re: [PATCH 0/2] Policy to balance read across mirrored devices

[Peter Becker <floyd.net@gmail.com>]

This is all clear. My question referes to "use the lower devid disk
containing the stripe"

2018-01-31 10:01 GMT+01:00 Anand Jain <anand.jain@oracle.com>:
>  When a stripe is not present on the read optimized disk it will just
>  use the lower devid disk containing the stripe (instead of failing back
>  to the pid based random disk).

Use only one disk (the disk with the lowest devid that containing the
stripe) as fallback should be not a good option imho.
Instead of it should still be used the pid as fallback to distribute
the workload among all available drives.

[stripe to use] = [preffer stripes present on read_mirror_policy
devids] > [fallback to pid % stripe count]

Perhaps I'm not be able to express myself in English or did I misunderstand you?

2018-01-31 15:26 GMT+01:00 Anand Jain <anand.jain@oracle.com>:
>
>
> On 01/31/2018 06:47 PM, Peter Becker wrote:
>>
>> 2018-01-31 10:01 GMT+01:00 Anand Jain <anand.jain@oracle.com>:
>>>
>>>   When a stripe is not present on the read optimized disk it will just
>>>   use the lower devid disk containing the stripe (instead of failing back
>>>   to the pid based random disk).
>>
>>
>> Is this a good behavior? beause this would eliminate every performance
>> benefit of the pid base random disk pick if the requested stripe is
>> not present on the read optimized disk.
>> Wouldn't it be better to specify a fallback and use the pid base
>> random pick as default for the fallback.
>>
>> For example:
>>
>> RAID 1 over 4 disk's
>>
>> devid | rpm | size
>> ------------------------
>> 1 | 7200 rpm | 3 TB
>> 2 | 7200 rpm | 3 TB
>> 3 | 5400 rpm | 4 TB
>> 4 | 5400 rpm | 4 TB
>>
>> mount -o read_mirror_policy=1,read_mirror_policy=2
>>
>> Cases:
>> 1. if the requested stripe is on devid 3 and 4 the algorithm should
>> choise on of both randomly to incresse performance instead of read
>> everytime from 3 and never from 4
>> 2. if the requested stripe is on devid 1 and 3, all is fine ( in case
>> of the queue deep of 1 isn't mutch larger then the queue deep of 3 )
>> 3. if the requested stripe is on devid 1 and 2, the algorithm should
>> choise on of both randomly to incresse performance instead of read
>> everytime from 1 and never from 2
>
>>
>>
>> And all randomly picks of a device should be replaced by a heuristic
>> algorithm wo respect the queue deep and sequential reads in the
>> future.
>
>
>  This scenario is very well handled by the pid/heuristic based
>  read load balancer, pid based read load balancer is by default still,
>  Tim has written IO load based read balancer which can be set using
>  this mount option when all integrated together, and it needs
>  experiments to see if it can be by default replacing the pid method.
>  Further as of now we don't do allocation grouping, so if you have two
>  ssd and two hd in a RAID1 its not guaranteed that allocation will
>  always span across a SSD and a HD, so there is bit of randomness
>  in the allocation itself.
>
> Thanks, Anand