HDDs won't spin down with hdparm command

Question

I am using Debian Jessie on a HP Proliant Microserver Gen8 with two Western Digital Red 3TB. No RAID or anything else. I wanted the HDDs to automatically spin down after two hours. Triggering it manually works fine with both commands

sudo hdparm -Y /dev/sda

sudo hdparm -y /dev/sda

so the output then is

user@NAS:~$ sudo hdparm -C /dev/sda

/dev/sda:
 drive state is:  standby

But when I use:

sudo hdparm -S 244 /dev/sda

the HDD won't go into any kind of standby mode. Note that sda is not the system drive. I mounted both HDDs via fstab like this:

UUID=xxx       /mnt/Stuff                                ext4    defaults,noatime,nodiratime        0       0

Any idea why this won't work? I also tried 10 seconds and 1 minute. Is there a way to determine if there is any process accessing the partitions on the HDDs? I do use a DLNA Server and Samba, but no access has been made by me during the test, of course.

Answer 1

Using iosnoop (available in the perf-tools-unstable package in Debian), you can trace activity on a specific device:

iosnoop -d 8,0

will show you what processes are accessing the device. That should allow you to determine what is waking your drives up.

To determine the device number to use, look at the major/minor device numbers for the nodes:

ls -l /dev/sd*

You can also run iosnoop without arguments to monitor all I/O.