Bug#855871: mdadm: maximum of 27 block devices for RAID assembly

[Ian Bruce]

Package: mdadm
Version: 3.4-4
Severity: important

mdadm seems to have a limit of 27 block devices for a RAID array.

Is this limit documented anywhere?

Is it configurable?

I tried the solution suggested here, but it didn't help:

http://dev.bizo.com/2012/07/mdadm-device-or-resource-busy.html



# truncate -s 64M fs.{00..31}
# 
# ls
fs.00  fs.03  fs.06  fs.09  fs.12  fs.15  fs.18  fs.21  fs.24  fs.27  fs.30
fs.01  fs.04  fs.07  fs.10  fs.13  fs.16  fs.19  fs.22  fs.25  fs.28  fs.31
fs.02  fs.05  fs.08  fs.11  fs.14  fs.17  fs.20  fs.23  fs.26  fs.29
# 
# for x in fs.* ; do losetup --show -f $x ; done
/dev/loop0
/dev/loop1
/dev/loop2
/dev/loop3
/dev/loop4
/dev/loop5
/dev/loop6
/dev/loop7
/dev/loop8
/dev/loop9
/dev/loop10
/dev/loop11
/dev/loop12
/dev/loop13
/dev/loop14
/dev/loop15
/dev/loop16
/dev/loop17
/dev/loop18
/dev/loop19
/dev/loop20
/dev/loop21
/dev/loop22
/dev/loop23
/dev/loop24
/dev/loop25
/dev/loop26
/dev/loop27
/dev/loop28
/dev/loop29
/dev/loop30
/dev/loop31
#
# # *** RAID FAILURE *** :
# 
# mdadm --build /dev/md/md-test --level=linear --raid-devices=28 /dev/loop{0..27}
mdadm: ADD_NEW_DISK failed for /dev/loop27: Device or resource busy
# 
# # *** RAID SUCCESS *** :
# 
# mdadm --build /dev/md/md-test --level=linear --raid-devices=27 /dev/loop{0..26}
mdadm: array /dev/md/md-test built and started.
# 
# mdadm --detail /dev/md/md-test
/dev/md/md-test:
        Version : 
  Creation Time : Wed Feb 22 07:48:57 2017
     Raid Level : linear
     Array Size : 1769472 (1728.00 MiB 1811.94 MB)
   Raid Devices : 27
  Total Devices : 27

          State : clean 
 Active Devices : 27
Working Devices : 27
 Failed Devices : 0
  Spare Devices : 0

       Rounding : 64K

    Number   Major   Minor   RaidDevice State
       0       7        0        0      active sync   /dev/loop0
       1       7        1        1      active sync   /dev/loop1
       2       7        2        2      active sync   /dev/loop2
       3       7        3        3      active sync   /dev/loop3
       4       7        4        4      active sync   /dev/loop4
       5       7        5        5      active sync   /dev/loop5
       6       7        6        6      active sync   /dev/loop6
       7       7        7        7      active sync   /dev/loop7
       8       7        8        8      active sync   /dev/loop8
       9       7        9        9      active sync   /dev/loop9
      10       7       10       10      active sync   /dev/loop10
      11       7       11       11      active sync   /dev/loop11
      12       7       12       12      active sync   /dev/loop12
      13       7       13       13      active sync   /dev/loop13
      14       7       14       14      active sync   /dev/loop14
      15       7       15       15      active sync   /dev/loop15
      16       7       16       16      active sync   /dev/loop16
      17       7       17       17      active sync   /dev/loop17
      18       7       18       18      active sync   /dev/loop18
      19       7       19       19      active sync   /dev/loop19
      20       7       20       20      active sync   /dev/loop20
      21       7       21       21      active sync   /dev/loop21
      22       7       22       22      active sync   /dev/loop22
      23       7       23       23      active sync   /dev/loop23
      24       7       24       24      active sync   /dev/loop24
      25       7       25       25      active sync   /dev/loop25
      26       7       26       26      active sync   /dev/loop26
# 
# mdadm --stop /dev/md/md-test
mdadm: stopped /dev/md/md-test
# 
# losetup -d /dev/loop{0..31}
# 



-- System Information:
Debian Release: 9.0
  APT prefers testing
  APT policy: (990, 'testing'), (500, 'stable-updates'), (500, 'unstable'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 4.6.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_CA.utf8, LC_CTYPE=en_CA.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages mdadm depends on:
ii  debconf [debconf-2.0]  1.5.60
ii  libc6                  2.24-9
ii  lsb-base               9.20161125
ii  udev                   232-15

Versions of packages mdadm recommends:
ii  exim4-daemon-light [mail-transport-agent]  4.88-5
ii  kmod                                       23-2

mdadm suggests no packages.