preseed

d-i partman-auto/method         string raid
d-i partman-auto/disk           string  /dev/sdy /dev/sdac
d-i partman-md/device_remove_md boolean true
d-i partman-md/confirm          boolean true
d-i partman-lvm/confirm         boolean true
d-i partman-lvm/device_remove_lvm   boolean true
d-i partman/confirm             boolean true
d-i partman/confirm_nooverwrite boolean true 
d-i partman-auto-lvm/new_vg_name    string  vg0
d-i partman-auto-raid/recipe    string  \
    1 2 0 lvm - /dev/sdy1#/dev/sdac1 .
d-i partman-auto/expert-recipe  string  \
    multiraid ::                        \
        256000 256000 300000 raid       \
            $lvmignore{ }               \
            $primary{ }                 \
            method{ raid }              \
        .                               \
        512 1024 1024 ext3              \
            $defaultignore{ }           \
            $lvmok{ }                   \
            in_vg{ vg0 }                \
            lv_name{ lv_boot }          \
            method{ format }            \
            format{ }                   \
            use_filesystem{ }           \
            filesystem{ ext3 }          \
            mountpoint{ /boot }         \
        .                               \
        1024 4096 4096 linux-swap       \
            $defaultignore{ }           \
            $lvmok{ }                   \
            in_vg{ vg0 }                \
            lv_name{ lv_swap }          \
            method{ swap }              \
            format{ }                   \
        .                               \
        8192 245760 245760 ext4         \
            $defaultignore{ }           \
            $lvmok{ }                   \
            in_vg{ vg0 }                \
            lv_name{ lv_root }          \
            method{ format }            \
            format{ }                   \
            use_filesystem{ }           \
            filesystem{ ext4 }          \
            mountpoint{ / }             \
        .
d-i partman-partitioning/confirm_write_new_label    boolean true
d-i partman/choose_partition    select finish

syslog

Jul 22 23:48:56 apt-install: Queueing package mdadm for later installation
Jul 22 23:48:56 partman-auto-raid: Selected spare count: 0
Jul 22 23:48:56 partman-auto-raid: Spare devices count: 0
Jul 22 23:48:56 partman-auto-raid: mdadm: cannot open /dev/sdy1: Device or resource busy
Jul 22 23:48:56 partman-auto-raid: Error creating array /dev/md1
Jul 22 23:48:58 main-menu[473]: (process:16373): WARNING: could not open /lib/modules/3.8.0-19-generic/modules.builtin: No such file or directory
Jul 22 23:48:58 main-menu[473]: (process:16373): Raid devices count: 2
Jul 22 23:48:58 main-menu[473]: (process:16373): /lib/partman/choose_partition/25md/choices: 
Jul 22 23:48:58 main-menu[473]: (process:16373): line 9: 
Jul 22 23:48:58 main-menu[473]: (process:16373): can't open /var/lib/partman/outfifo: no such file
Jul 22 23:48:58 main-menu[473]: (process:16373): 
Jul 22 23:48:58 main-menu[473]: (process:16373): /lib/partman/choose_partition/30lvm/choices: 
Jul 22 23:48:58 main-menu[473]: (process:16373): line 10: 
Jul 22 23:48:58 main-menu[473]: (process:16373): can't open /var/lib/partman/outfifo: no such file
Jul 22 23:48:58 main-menu[473]: (process:16373): 
Jul 22 23:48:58 main-menu[473]: (process:16373): /lib/partman/choose_partition/35crypto/choices: 
Jul 22 23:48:58 main-menu[473]: (process:16373): line 7: 
Jul 22 23:48:58 main-menu[473]: (process:16373): can't open /var/lib/partman/outfifo: no such file
Jul 22 23:48:58 main-menu[473]: (process:16373): 
Jul 22 23:48:58 main-menu[473]: (process:16373): /lib/partman/choose_partition/60partition_tree/choices: line 5: can't open /var/lib/partman/outfifo: no such file