Re: mdadm "hang", 100% CPU usage when trying to create RAID-1 array with external bitmap

[Neil Brown <neilb@suse.de>]

On Sun, 18 Jul 2010 15:09:38 -0400
"John Stoffel" <john@stoffel.org> wrote:

> >>>>> "Neil" == Neil Brown <neilb@suse.de> writes:
> 
> Neil> On Wed, 14 Jul 2010 12:53:45 +0200
> Neil> Tomasz Chmielewski <mangoo@wpkg.org> wrote:
> 
> >> I'm trying to create RAID-1 with external bitmap with this command:
> >> 
> >> mdadm --create /dev/md11 --level=1 --bitmap=/mnt/src/bitmap --raid-devices 2 /dev/storage/origin missing
> >> 
> >> 
> >> /dev/storage/origin is 1.2 TB; is placed on "stacked" block devices like this:
> >> 
> >> RAID-5 -> dm-crypt -> LVM-2 -> /dev/storage/origin
> >> 
> >> 
> >> /mnt/src/bitmap is technically the same stacked device, except the last LVM volume:
> >> 
> >> RAID-5 -> dm-crypt -> LVM-2 -> /dev/storage/src -> /mnt/src/bitmap
> >> 
> >> (although I tried to keep bitmap outside of this RAID-5 device, and it was still hanging).
> >> 
> >> 
> >> When started with "strace", mdadm hangs with 100% CPU usage after opening /mnt/src/bitmap:
> >> 
> >> (...)
> >> rename("/var/run/map.new", "/var/run/map") = 0
> >> flock(3, LOCK_UN)                       = 0
> >> close(3)                                = 0
> >> munmap(0x6ff40000, 4096)                = 0
> >> unlink("/var/run/map.lock")             = 0
> >> ioctl(4, 0x800c0910, 0x77f5f89c)        = 0
> >> fstat64(4, {st_mode=S_IFBLK|0600, st_rdev=makedev(9, 1), ...}) = 0
> >> fstat64(4, {st_mode=S_IFBLK|0600, st_rdev=makedev(9, 1), ...}) = 0
> >> ioctl(4, 0x800c0910, 0x77f5f7cc)        = 0
> >> ioctl(4, 0x40480923, 0x77f5f854)        = 0
> >> access("/mnt/src/bitmap", F_OK)         = -1 ENOENT (No such file or directory)
> >> open("/mnt/src/bitmap", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
> >> 
> >> 
> 
> Neil> Hmm... that's rather embarrassing.
> Neil> This patch should fit it.  Or you can explicitly set a bitmap chunk size -
> Neil> aim for several megabyte.
> 
> Neil> diff --git a/bitmap.c b/bitmap.c
> Neil> index beef2dc..44a8677 100644
> Neil> --- a/bitmap.c
> Neil> +++ b/bitmap.c
> Neil> @@ -373,7 +373,7 @@ int CreateBitmap(char *filename, int force, char uuid[16],
> Neil>  		 */
> Neil>  		chunksize = DEFAULT_BITMAP_CHUNK;
> Neil>  		/* <<20 for 2^20 chunks, >>9 to convert bytes to sectors */
> Neil> -		while (array_size > (chunksize << (20-9)))
> Neil> +		while (array_size > ((unsigned long long)chunksize << (20-9)))
> Neil>  			chunksize <<= 1;
> Neil>  	}
>  
> How about also replacing the magic numbers 20 and 9 with usefully
> named constants as well?   I realize the comment tells you what's
> going on here, but I just boggle a bit when I see 20-9 in the source.
> :-)

If you sent that suggestion as a patch it would be much more likely to be
applied (it might anyway, but still...)
:-)

NeilBrown