Re: [Non-DoD Source] Re: Slow initial resync in RAID6 with 36 SAS drives

On 9/1/21 9:22 AM, antlists wrote:

On 25/08/2021 11:28, Finlayson, James M CIV (USA) wrote:

quoted

I'm not a person necessarily in the "know", but I mess with these in 
udev  for SSDs:
SUBSYSTEM=="block", ACTION=="add|change", KERNEL=="md*", 
ATTR{md/sync_speed_max}="2000000",ATTR{md/group_thread_cnt}="64", 
ATTR{md/stripe_cache_size}="8192"

Guidance on why I change the values this way - educated "guess", as 
I'm an experienced practitioner at best and not someone that messes 
with the code......I've seen resync rates as high as 900MB/s 
sustained on my SSD mdraids, even though the SSDs should be able to 
sustain higher.....

Not raid, but I've picked up on comments (on LWN?) that certain 
file-systems are very much "single threaded" in their behaviour. I 
strongly suspect that md-raid predates common multi-cpu/multi-core 
systems, and I've heard somewhere that md-raid doesn't thread that well.

Yes and no, raid5 does support multi-threading if group_thread_cnt > 0, 
but IIUC, only one thread (mdx_resync) deals with resync IO.

Thanks,
Guoqing