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I just got my first QNAP NAS, used to work mostly with Synology before, and I'm currently trying to figure out how the system uses LVM. Mostly out of curiosity.

In my specific case, it's a 2 bay NAS that I'm setting up as RAID 1 "storage pool". Within that pool, I created a "thick volume".

These naming conventions do seem a bit confusing. When the "storage pool" gets created, QNAP seems to immediately create a LVM thin pool (i.e. a volume) across all of the free space. Any "thick" or "thin" volumes are then created within that pool.

Here is the output of vgs and lvs right after the creation of the storage pool and one 250GB thick volume: 

 # vgs
  VG   #PV #LV #SN Attr   VSize VFree
  vg1    1   3   0 wz--n- 1.81t    0

 # lvs
  LV    VG   Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  lv1   vg1  Vwi-aot--- 250.00g tp1         100.00
  lv544 vg1  -wi-------  18.54g
  tp1   vg1  twi-aot---   1.73t             14.13  0.03

After I created the volume, I tried to find out where it was mounted but neither mount nor df -h would show that lv1 was mounted. From the web interface, you could already interact with the shared folder in the lv, so clearly it had to be somewhere.

dmsetup ls --tree gives more info: 

# dmsetup ls --tree
cachedev1 (252:8)
 └─vg1-lv1 (252:7)
    └─vg1-tp1-tpool (252:4)
       ├─vg1-tp1_tmeta (252:0)
       │  └─ (9:1)
       ├─vg1-tp1_tierdata_2 (252:3)
       │  └─vg1-tp1_tierdata_2_fcorig (252:6)
       │     └─ (9:1)
       ├─vg1-tp1_tierdata_1 (252:2)
       └─vg1-tp1_tierdata_0 (252:1)

So, there is a device called cachedev1 in /dev/mapper and it's mounted on /share/CACHEDEV1_DATA. And within here, any "shared folders" will appear.

I don't have the knowledge to make sense of this configuration. What is /dev/mapper/cachedev1 exactly? How did lv1 become part of it? And what is the benefit of using it over just mounting the LVM volume directly?

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In brief, a mapped device is an activated LVM logical volume.

In your case, /dev/cachedev1 device is the activated vg1-lv1 logical volume.

There are several device mapping formats depending of the type of use of your logical volume: linear, striped, snapshot, thin, RAID...

I hope it helps.

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I'm not NAS user, hence I don't know any specific about QNAP NAS.
But I've been able to find out this piece of information, as a partial answer for question What is /dev/mapper/cachedev1 exactly?

The root of the HD installed in the NAS is /share/CACHEDEV1_DATA, which is mounted in a strange internal way

This is a quote from here.

As about other questions -- probably the best place to ask such questions is qnap forum.

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  • Thanks Alex. It's this „strange internal way“ I'd like to know more about. I'm sure someone proficient in configuring device mapper should be able to answer this. But yes, I may consider asking over at the Qnap forum if no one provides an answer here.
    – vic
    Apr 4, 2019 at 11:15
  • Wish you luck with that!
    – Alex Bender
    Apr 4, 2019 at 13:47

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