I have a raid 5 (software) with 5x2TB drives. I encrypted the raid with cryptsetup and put an ext4-partition on top.

In the beginning opening and mounting the raid took less than 10 seconds, now (for a few weeks) mounting alone takes 1:30 minutes and the cpu stays around 93% the whole time:

The output of "time sudo mount /dev/mapper/8000 /media/8000" is:

real    1m31.952s
user    0m0.008s
sys     1m25.229s

At the same time only one line is added to /var/log/syslog:

kernel: [ 2240.921381] EXT4-fs (dm-1): mounted filesystem with ordered data mode. Opts: (null)

My Ubuntu-version is "12.04.1 LTS" and no updates are pending.

I checked the partition with fsck, but it says that all is ok.

The "cryptsetup luksOpen" command only takes a few seconds.

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I also tried changing the raid-bitmap (as it was suggested in some forum) but it did not change the behaviour.

sudo mdadm --grow /dev/md0 -b internal

and

sudo mdadm --grow /dev/md0 -b none

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I had the idea that it might be the hardware being slow, but a read test with "sudo hdparm -t /dev/md0" spit out values between 62 and 159 MB/sec:

Timing buffered disk reads: 382 MB in  3.00 seconds = 127.14 MB/sec
Timing buffered disk reads: 482 MB in  3.02 seconds = 159.62 MB/sec
Timing buffered disk reads: 190 MB in  3.03 seconds =  62.65 MB/sec
Timing buffered disk reads: 474 MB in  3.02 seconds = 157.12 MB/sec

Although I think it is strange that the read rate jumps by more than 100% - could that mean something?

The speed test when reading from the mapped (decrypted) device shows similar behavior, although it is of course much slower. "sudo hdparm -t /dev/mapper/8000":

Timing buffered disk reads:  56 MB in  3.02 seconds =  18.54 MB/sec
Timing buffered disk reads: 122 MB in  3.09 seconds =  39.43 MB/sec
Timing buffered disk reads: 134 MB in  3.02 seconds =  44.35 MB/sec

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The output of a verbose mount "mount -vvv /dev/mapper/8000 /media/8000" does not help much:

mount: fstab path: "/etc/fstab"
mount: mtab path:  "/etc/mtab"
mount: lock path:  "/etc/mtab~"
mount: temp path:  "/etc/mtab.tmp"
mount: UID:        0
mount: eUID:       0
mount: spec:  "/dev/mapper/8000"
mount: node:  "/media/8000"
mount: types: "(null)"
mount: opts:  "(null)"
mount: you didn't specify a filesystem type for /dev/mapper/8000
       I will try type ext4
mount: mount(2) syscall: source: "/dev/mapper/8000", target: "/media/8000", filesystemtype: "ext4", mountflags: -1058209792, data: (null)

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Any idea where I could find additional information on why mounting takes so long, or what additional tests I could run?