Re: [PATCH RFC 4/4] btrfs: introduce new read_policy round-robin

Anand Jain
2021-01-20T03:20:21Z

On 20/1/21 3:41 am, Josef Bacik wrote:
> On 1/11/21 4:41 AM, Anand Jain wrote:
>> Add round-robin read policy to route the read IO to the next device in
>> the
>> round-robin order. The chunk allocation and thus the stripe-index follows
>> the order of free space available on devices. So to make the round-robin
>> effective it shall follow the devid order instead of the stripe-index
>> order.
>>
>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>> --
>> RFC because: Provides terrible performance with the fio tests.
>> I am not yet sure if there is any io workload or a block layer
>> tuning that shall make this policy better. As of now just an
>> experimental patch.
>>
>
> Just drop this one, if we can't find a reason to use it then don't
> bother adding the code.  The other options have real world valuable
> uses, so stick with those.  Thanks,
>

Yep. I will drop this patch in the next iteration.

The low performance is attributed to the low number of read IO
mergers in the block layer. The consecutive blocks in my test case
(fio random read) were read from the other disk, so the block layer
lost the opportunity to merge the IOs.

Thanks, Anand

> Josef
>