Re: [PATCH v2 0/3] raid1 balancing methods

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

On 21/10/24 22:32, waxhead wrote:
> Anand Jain wrote:
>> v2:
>> 1. Move new features to CONFIG_BTRFS_EXPERIMENTAL instead of 
>> CONFIG_BTRFS_DEBUG.
>> 2. Correct the typo from %est_wait to %best_wait.
>> 3. Initialize %best_wait to U64_MAX and remove the check for 0.
>> 4. Implement rotation with a minimum contiguous read threshold before
>>     switching to the next stripe. Configure this, using:
>>
>>          echo rotation:[min_contiguous_read] > /sys/fs/btrfs/<uuid>/ 
>> read_policy
>>
>>     The default value is the sector size, and the min_contiguous_read
>>     value must be a multiple of the sector size.
>>
>> 5. Tested FIO random read/write and defrag compression workloads with
>>     min_contiguous_read set to sector size, 192k, and 256k.
>>
>>     RAID1 balancing method rotation is better for multi-process workloads
>>     such as fio and also single-process workload such as defragmentation.
> 
> With this functionality added, would it not also make sense to add a 
> RAID0/10 profile that limits the stripe width, so a stripe does not 
> spawn more than n disk (for example n=4).
> 
 > On systems with for example 24 disks in RAID10, a read may activate 
12 > disks at the same time which could easily saturate the bus.
> 
> Therefore if a storage profile that limits the number of devices a 
> stripe occupy existed, it seems like there might be posibillities for 
> RAID0/10 as well.
> 
> Note that as of writing this I believe that RAID0/10/5/6 make the stripe 
> as wide as the number of storage devices available for the filesystem. 
> If I am wrong about this please ignore my jabbering and move on.

That's correct. I previously attempted to come up with a fix using
the device grouping method. If there's a convincing and more generic
way to specify how the devices should be grouped, we could consider
that.

Thanks, Anand