Re: BTRFS subvolume RAID level

[Anand Jain <anand.jain@oracle.com>]

On 12/3/19 7:27 AM, waxhead wrote:
> 
> 
> Anand Jain wrote:
>>
>>> I imagine that RAIDc4 for example could potentially give a grotesque 
>>> speed increase for parallel read operations once BTRFS learns to 
>>> distribute reads to the device with the least waitqueue / fastest 
>>> devices.
>>
>>   That exactly was the objective of the Readmirror patch in the ML.
>>   It proposed a framework to change the readmirror policy as needed.
>>
>> Thanks, Anand
> 
> Indeed. If I remember correctly your patch allowed for deterministic 
> reading from certain devices.
  It provides a framework to configure the readmirror policies. And the
  policies can be based on io-depth, pid, or manual for heterogeneous
  devices with different latency.

> As just a regular btrfs user the problem I 
> see with this is that you loose a "potential free scrub" that *might* 
> otherwise happen from often read data. On the other hand that is what 
> manual scrubbing is for anyway.

  Ha ha.

  When it comes to data and its reliability and availability we need
  guarantee and only deterministic approach can provide it.

  What you are asking for is to route the particular block to
  the device which was not read before so to avoid scrubbing or to
  make scrubbing more intelligent to scrub only old never read blocks
  - this will be challenging we need to keep history of block and the
  device it used for read - most likely a scope for the bpf based
  external tools but definitely not with in kernel. With in kernel
  we can create readmirror like framework so that external tool can
  achieve it.


Thanks, Anand