Write mostly flag

Write mostly flag

[Victor Garcia]

Hello all

I am trying to set up a raid 1 array using an SSD and a standard SAT
HDD drives. As I read, it's better to set the write-mostly flag to the
HDD partition of the array to keep SSD performance times.

This is the command I'm using:

# mdadm --create /dev/md1 -n 2 -l 1 --metadata=0.90 --bitmap=internal
/dev/nvme0n1p5 -W /dev/sda8 --write-behind

I expect seeing the (W) indicator in mdstat, but this is not showing:

# cat /proc/mdstat
Personalities : [raid1]
md1 : active raid1 sda8[1] nvme0n1p5[0]
      52400064 blocks [2/2] [UU]
      [>....................]  resync =  0.3% (175936/52400064)
finish=9.8min speed=87968K/sec

unused devices: <none>

 But if I force later on the flag by means of the sys FS handle, then
it does appear:

# cat /sys/block/md1/md/dev-sda8/state
in_sync
# echo writemostly > /sys/block/md1/md/dev-sda8/state
# cat /sys/block/md1/md/dev-sda8/state
in_sync,write_mostly
# cat /proc/mdstat
Personalities : [raid1]
md1 : active raid1 sda8[1](W) nvme0n1p5[0]
      52400064 blocks [2/2] [UU]
      [==>..................]  resync = 11.0% (5806336/52400064)
finish=9.8min speed=78966K/sec
      bitmap: 1/1 pages [4KB], 65536KB chunk

unused devices: <none>

How is it that the (W) is not showing in the first place?

BTW, this is the system being used:

# uname -a
Linux ubuntu 4.8.0-22-generic #24-Ubuntu SMP Sat Oct 8 09:15:00 UTC
2016 x86_64 x86_64 x86_64 GNU/Linu

Thanks and regards
Victor

Re: Write mostly flag

[Guoqing Jiang]

On 01/01/2017 03:37 PM, Victor Garcia wrote:
> Hello all
>
> I am trying to set up a raid 1 array using an SSD and a standard SAT
> HDD drives. As I read, it's better to set the write-mostly flag to the
> HDD partition of the array to keep SSD performance times.
>
> This is the command I'm using:
>
> # mdadm --create /dev/md1 -n 2 -l 1 --metadata=0.90 --bitmap=internal
> /dev/nvme0n1p5 -W /dev/sda8 --write-behind
>
> I expect seeing the (W) indicator in mdstat, but this is not showing:
>
> # cat /proc/mdstat
> Personalities : [raid1]
> md1 : active raid1 sda8[1] nvme0n1p5[0]
>        52400064 blocks [2/2] [UU]
>        [>....................]  resync =  0.3% (175936/52400064)
> finish=9.8min speed=87968K/sec
>
> unused devices: <none>
>
>   But if I force later on the flag by means of the sys FS handle, then
> it does appear:
>
> # cat /sys/block/md1/md/dev-sda8/state
> in_sync
> # echo writemostly > /sys/block/md1/md/dev-sda8/state
> # cat /sys/block/md1/md/dev-sda8/state
> in_sync,write_mostly
> # cat /proc/mdstat
> Personalities : [raid1]
> md1 : active raid1 sda8[1](W) nvme0n1p5[0]
>        52400064 blocks [2/2] [UU]
>        [==>..................]  resync = 11.0% (5806336/52400064)
> finish=9.8min speed=78966K/sec
>        bitmap: 1/1 pages [4KB], 65536KB chunk
>
> unused devices: <none>
>
> How is it that the (W) is not showing in the first place?

Seems only 0.9 metadata has the issue while 1.x is ok from my side,
maybe below changes could help you.

diff --git a/super0.c b/super0.c
index 938cfd95fa25..4ad591ba022e 100644
--- a/super0.c
+++ b/super0.c
@@ -915,6 +915,9 @@ static int write_init_super0(struct supertype *st)
                 while (Kill(di->devname, NULL, 0, -1, 1) == 0)
                         ;

+               if (di->disk.state & (1 << MD_DISK_WRITEMOSTLY))
+                       sb->disks[di->disk.number].state |= 
1<<MD_DISK_WRITEMOSTLY;
+
                 sb->disks[di->disk.number].state &= ~(1<<MD_DISK_FAULTY);

                 sb->this_disk = sb->disks[di->disk.number];

Regards,
Guoqing