I'm starting to get a collection of computers at home and to support them I have my "server" linux box running a RAID array.

Its currently mdadm RAID-1, going to RAID-5 once I have more drives (and then RAID-6 I'm hoping for). However I've heard various stories about data getting corrupted on one drive and you never noticing due to the other drive being used, up until the point when the first drive fails, and you find your second drive is also screwed (and 3rd, 4th, 5th drive).

Obviously backups are important and I'm taking care of that also, however I know I've previously seen scripts which claim to help against this problem and allow you to check your RAID while its running. However looking for these scripts again now I'm finding it hard to find anything which seems similar to what I ran before and I feel I'm out of date and not understanding whatever has changed.

How would you check a running RAID to make sure all disks are still preforming normally?

I monitor SMART on all the drives and also have mdadm set to email me in case of failure but I'd like to know my drives occasionally "check" themselves too.

  • Sounds like you're already on the right path, you just need to setup a cron to send you the results of smartctl for your drives.
    – laebshade
    Commented Jan 8, 2012 at 23:15

5 Answers 5


The point of RAID with redundancy is that it will keep going as long as it can, but obviously it will detect errors that put it into a degraded mode, such as a failing disk. You can show the current status of an array with mdadm --detail (abbreviated as mdadm -D):

# mdadm -D /dev/md0
       0       8        5        0      active sync   /dev/sda5
       1       8       23        1      active sync   /dev/sdb7

Furthermore the return status of mdadm -D is nonzero if there is any problem such as a failed component (1 indicates an error that the RAID mode compensates for, and 2 indicates a complete failure).

You can also get a quick summary of all RAID device status by looking at /proc/mdstat. You can get information about a RAID device in /sys/class/block/md*/md/* as well; see admin-guide/md.html in the kernel documentation. Some /sys entries are writable as well; for example you can trigger a full check of md0 with echo check >/sys/class/block/md0/md/sync_action.

In addition to these spot checks, mdadm can notify you as soon as something bad happens. Make sure that you have MAILADDR root in /etc/mdadm.conf (some distributions (e.g. Debian) set this up automatically). Then you will receive an email notification as soon as an error (a degraded array) occurs.

Make sure that you do receive mail send to root on the local machine (some modern distributions omit this, because they consider that all email goes through external providers — but receiving local mail is necessary for any serious system administrator). Test this by sending root a mail: echo hello | mail -s test root@localhost. Usually, a proper email setup requires two things:

  • Run an MTA on your local machine. The MTA must be set up at least to allow local mail delivery. All distributions come with suitable MTAs, pick anything (but not nullmailer if you want the email to be delivered locally).

  • Redirect mail going to system accounts (at least root) to an address that you read regularly. This can be your account on the local machine, or an external email address. With most MTAs, the address can be configured in /etc/aliases; you should have a line like

     root: djsmiley2k

    for local delivery, or

     root: [email protected]

    for remote delivery. If you choose remote delivery, make sure that your MTA is configured for that. Depending on your MTA, you may need to run the newaliases command after editing /etc/aliases.

  • Can you explain why nullmailer should not be used? Is it because of the reasons mentioned in unix.stackexchange.com/questions/1449/…? Which MTA would you recommend? Commented Jun 30, 2015 at 1:30
  • @CameronMartin Nullmailer only forwards mail to a remote machine, it doesn't do local delivery. You can use it if you have an SMTP server somewhere that accepts mail from your machine, but not if you're delivering the mail locally. I've edited my answer to clarify. Commented Jun 30, 2015 at 1:33
  • In madm v4 the device details option has moved; you need to run mdadm --misc -D /dev/md0 Commented May 2, 2021 at 23:51
  • @JonathanHolvey In what version, exactly? --misc is still optional in 4.1 and is still documented as such in Git (“Misc mode to be assumed”.). Commented May 3, 2021 at 9:05
  • Thanks for pointing this out @Gilles'SO-stopbeingevil'. It must have been user error, as omitting --misc is working fine for me now. Commented May 4, 2021 at 0:43

You can force a check of the entire array while it's online. For example, to check the array on /dev/md0, run as root:

echo check > /sys/block/md0/md/sync_action

I also have a cron job that runs the following command once a month:

tar c /dir/of/raid/filesystem > /dev/null

It’s not a thorough check of the drive itself, but it does force the system to periodically verify that (almost) every file can be read successfully off the disk. Yes, some files are going to be read out of memory cache instead of disk. But I figure that if the file is in memory cache, then it’s successfully been read off disk recently, or is about to be written to disk, and either of those operations will also uncover drive errors. Anyway, running this job tests the most important criterion of a RAID array (“Can I successfully read my data?”) and in the three years I’ve been running my array, the one time I had a drive go bad, it was this command that discovered it.

One little warning is that if your filesystem is big, then this command is going to take a long time; my system takes about 6hr/TiB. I run it using ionice so that the rest of the system doesn’t grind to a halt during the drive check:

ionice -c3 tar c /dir/of/raid/filesystem > /dev/null
  • Note that ionice will only work if you use the (default) CFQ I/O scheduler.
    – Totor
    Commented Nov 2, 2013 at 17:48
  • So this may be obvious to most, but it's not to me - how does running a script whose output is redirected to devnull actually notify you of something? Is it the case that if "tar" encounters any errors these will get propagated up to the mdadm daemon which will (presumably) send you an email?
    – ljwobker
    Commented Sep 4, 2015 at 3:09
  • My question to you stharward, is how do you pick up on the tar errors if it's being run from a cron job? Where does that output? I would have thought you'd add a redirection for stderr to a file that could be monitored periodically or the tail of it being printed to console of opening a terminal window :)
    – Madivad
    Commented Nov 13, 2015 at 16:31
  • 1
    @ljwobker Sorry for reviving an old thread. I think the intent of the tar command here is to attempt to read the entire contents of the volume. This would verify the entire volume is still readable and give md a chance to detect a bad disk.
    – mikepj
    Commented May 23, 2017 at 16:54
  • 1
    If it's coming from a cronjob, cron will normally send all it's output directly to the mailto= location, if one is set, otherwise to root. However... I'm wondering if dd would be better than tar, for lower overhead? Commented Jun 14, 2018 at 12:43

the Debian and Ubuntu 'mdadm' package contains the file


which in turns the first sunday of each month will run the command

/usr/share/mdadm/checkarray --cron --all --idle --quiet

that will check all your arrays for consistency (unless you set AUTOCHECK to false in /etc/default/mdadm ). A report will be sent to the 'root' user (make sure you receive such emails).

  • This doesn't appear to be in any /etc/cron.*/ anymore in debian 12.
    – reukiodo
    Commented Sep 29, 2023 at 17:07
  • it was substituted by /lib/systemd/system/mdcheck_start.timer
    – am70
    Commented Oct 3, 2023 at 18:49

I use this simple function to check /proc/mdstat:

#Health of RAID array
raid() { awk '/^md/ {printf "%s: ", $1}; /blocks/ {print $NF}'  /proc/mdstat; }
  • neat! i like the style :) .. this is exactly what I was looking for to put into MOTD :) thanks!
    – jirislav
    Commented May 12, 2018 at 13:19

I added this to my prompt by creating /etc/profile.d/raid_status.sh:

raid_prompt() { awk '/^md/ {printf "%s: ", $1}; /blocks/ {print $NF}'  /proc/mdstat | awk '/\[U+\]/ {print "\033[32m" $0 "\033[0m"}; /\[.*_.*\]/ {print "\033[31m" $0 "\033[0m"}'; }

Then setting my PS1 to:

PS1='$(raid_prompt)\n[\u@\h \W]\$ '

This prints the array name and member status in green if all members are up. If a member fails, then it's printed in red.

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