How to format RAID1 correctly

[Eric Sandeen]

On 9/23/14 7:46 PM, Helmut Tessarek wrote:
> The information provided in the FAQ and on several web sites is not really
> useful regarding RAID1.
> 
> According to the FAQ (entry 35):
> The correct options to format a RAID1 (2 disks) with 64k chunk size is:
> mkfs.xfs -d su=64k -d sw=1 /dev/mapper/data

I don't see that text in the faq... where is it?  I see:

> So if your RAID controller has a stripe size of 64KB, and you have a RAID-6 with 8 disks, use
> 
>  su = 64k
>  sw = 6 (RAID-6 of 8 disks has 6 data disks)

but!  your raid doesn't have a 64k stripe, so that doesn't apply.

> But it also states that it would be automatically detected and used correctly,
> yet
> mkfs.xfs /dev/mapper/data
> yields a different result:
> 
> [root at atvie01s ~]# mkfs.xfs -f /dev/mapper/data
> 
> meta-data=/dev/mapper/data       isize=256    agcount=4,
> agsize=244173876 blks
>          =                       sectsz=4096  attr=2, projid32bit=1
>          =                       crc=0        finobt=0
> data     =                       bsize=4096   blocks=976695504, imaxpct=5
>          =                       sunit=0      swidth=0 blks

no geometry because md0 raid1 doesn't export any stripe geometry.

> Formel 1:
> 
> [root at atvie01s ~]# mkfs.xfs -f -d su=64k -d sw=1 /dev/mapper/data
> 
> meta-data=/dev/mapper/data       isize=256    agcount=32,
> agsize=30521728 blks
>          =                       sectsz=4096  attr=2, projid32bit=1
>          =                       crc=0        finobt=0
> data     =                       bsize=4096   blocks=976695296, imaxpct=5
>          =                       sunit=16     swidth=16 blks

You specified it, so mkfs obeyed.

> naming   =version 2              bsize=4096   ascii-ci=0 ftype=0
> log      =internal log           bsize=4096   blocks=476902, version=2
>          =                       sectsz=4096  sunit=1 blks, lazy-count=1
> realtime =none                   extsz=4096   blocks=0, rtextents=0
> 
> 
> Another inconsistency is that RAID1 doesn't use striping, so the chunk size
> should be irrelevant in the first place.

agreed - but I don't see anything in the faq about raid1 stripes.
Am I missing something?

> So what is ultimately the correct way to format a RAID1?

for software md raid over individual disks, bare mkfs should do the right
thing.

-Eric