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I have a Proliant DL380 G6 Server with 4 146GB 2.5" SAS drives.

In Ubuntu, (I'm using Webmin to manage), I see a single physical volume, as though there actually is only one disk.

If I run the command lshw -class disk -short I see:

/0/100/1/0/1.0.0        /dev/sda     disk     293GB LOGICAL VOLUME
/0/100/1d.7/1/8/0.0.0   /dev/sdb     disk     6001GB d2 TBT2 USB3
/0/3/0.0.0              /dev/cdrom   disk     DV-W28S-VS

At the moment, I cannot recall how I setup the HW raid controller, but it appears as though I've used all 4 internal drives in the LV.

QUESTION: By using the HW controller to manage RAID, am I sort of "tricking" the OS into thinking there is only one physical disk in the system?

FOLLOWUP: If that is the case, is this the better solution for RAID, or would it be better to manage raid inside of the OS?

Ultimately, what is the core difference between the two methods?

Jon Griffith
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2 Answers2

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Often "hardware RAID controllers" are nothing more that a small embedded computer (with PowerPC or ARM cores) running a stripped-down Linux kernel. Being closely intergrated, the hardware cards can manage the chassis LED, removing/replacing a disk, etc without the intervention of external software. Moreover, many RAID controllers provide a dedicated, ECC-protected write cache which is critical to obtain good performance. In the past, the offloading capability of the integrated card cores were valuable; nowadays, the advantage is basically non-existant.

The Linux MDRAID drives is very fast and reliable; moreover, using ledmon you can have automatically managed chassis LED. However, it remain somewhat less integrated: you not only need to manually create the raid arrays using mdadm, but automatic disk replacement has to be configured using mdadm.conf POLICY section (alternatively, you need to manually reinclude the replaced disk with mdadm manage --add). Finally, being completely software-based, it clearly does not include any dedicated, powerloss-proteced writeback caching.

Read here for more details.

To directly answer you questions:

  • with hardware RAID card, the OS only see the array volume as a single disk;
  • if your server has an hardware RAID card with powerloss protected writeback cache, use it (some old controllers does not even support raw disk passthrough).
shodanshok
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You are managing the RAID with the hardware controller thereby offloading the RAID overhead on to the controller freeing up some CPU cycles. In general, this is the 'best' way of managing a RAID. That being said, it really depends on your use case. If you will not be taxing the system, a software RAID is just fine.

You also want to find a utility called hpacucli. With that you will be able to probe the RAID controller for the status of the RAID and individual drives. For example hpacucli ctrl all show config See also https://www.thegeekstuff.com/2014/07/hpacucli-examples/ or another reference.

toppledwagon
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