Accessing discs connected to H/W RAID controller over SG interface

Accessing discs connected to H/W RAID controller over SG interface

[Tom Crane]

We have a system whose RAID array has performance problems.  We see I/O 
maxing out at throughputs far below the spec of the controller and even 
well below that of individual discs attached to it.

The RAID controller from Areca is configured with 2-off RAID volumes and 
has 4-off 6TB discs attached to it.  I would like to be able to access the 
4 discs individually, e.g. to be able to dd them to /dev/null to check on 
their performances.  I know we could JBOD the discs and do S/W RAID but 
this is a production system and in any case the point of having a H/W RAID 
controller is to avoid this!

This shows the configuration,

# lsscsi -gs
[0:0:0:0]    disk    Areca    ARC-1226-VOL#000 R001  /dev/sda   /dev/sg0    199GB
[0:0:0:1]    disk    Areca    ARC-1226-VOL#001 R001  /dev/sdb   /dev/sg1   11.8TB
[0:0:16:0]   process Areca    RAID controller  R001  -          /dev/sg2        -


I know that some access to the individual discs attached is possible 
because the smartmontools utility smartctl can do it, e.g.,

# smartctl -i -d areca,1 /dev/sg2
smartctl 7.0 2018-12-30 r4883 [x86_64-linux-3.10.0-1160.80.1.el7.x86_64] (local build)
Copyright (C) 2002-18, Bruce Allen, Christian Franke, 
www.smartmontools.org

=== START OF INFORMATION SECTION ===
Model Family:     Western Digital Red
Device Model:     WDC WD60EFRX-68L0BN1
etc...

I was hoping to use 'sg_dd if=<???> of=/dev/null' or similar. I could not 
find what I was looking for at https://sg.danny.cz/sg/ or elsewhere.  Is 
this possible? Can anyone give me some pointers?

Many thanks
Tom Crane

-- 
Tom Crane, Digital Electronics Engineer, Dept. Physics, Royal Holloway, University of London, Egham Hill,
Egham, Surrey, TW20 0EX, England.
Email:  T.Crane@rhul.ac.uk

Re: Accessing discs connected to H/W RAID controller over SG interface

[Tom Crane]

On Mon, 12 Feb 2024, Tom Crane wrote:

> We have a system whose RAID array has performance problems.  We see I/O 
> maxing out at throughputs far below the spec of the controller and even well 
> below that of individual discs attached to it.
>
> The RAID controller from Areca is configured with 2-off RAID volumes and has 
> 4-off 6TB discs attached to it.  I would like to be able to access the 4 
> discs individually, e.g. to be able to dd them to /dev/null to check on their 
> performances.  I know we could JBOD the discs and do S/W RAID but this is a 
> production system and in any case the point of having a H/W RAID controller 
> is to avoid this!
>
> This shows the configuration,
>
> # lsscsi -gs
> [0:0:0:0]    disk    Areca    ARC-1226-VOL#000 R001  /dev/sda   /dev/sg0 
> 199GB
> [0:0:0:1]    disk    Areca    ARC-1226-VOL#001 R001  /dev/sdb   /dev/sg1 
> 11.8TB
> [0:0:16:0]   process Areca    RAID controller  R001  -          /dev/sg2 
> -
>
>
> I know that some access to the individual discs attached is possible because 
> the smartmontools utility smartctl can do it, e.g.,
>
> # smartctl -i -d areca,1 /dev/sg2
> smartctl 7.0 2018-12-30 r4883 [x86_64-linux-3.10.0-1160.80.1.el7.x86_64] 
> (local build)
> Copyright (C) 2002-18, Bruce Allen, Christian Franke, www.smartmontools.org
>
> === START OF INFORMATION SECTION ===
> Model Family:     Western Digital Red
> Device Model:     WDC WD60EFRX-68L0BN1
> etc...
>
> I was hoping to use 'sg_dd if=<???> of=/dev/null' or similar. I could not 
> find what I was looking for at https://sg.danny.cz/sg/ or elsewhere.  Is this 
> possible? Can anyone give me some pointers?
>
> Many thanks
> Tom Crane>

I have not yet had any follow-up to my query above.  Would anyone be kind 
enought to give me some comment/pointers?

If there is a more appropriate forum for my query or if the information I 
am looking for is documented somewhere, could someone point me to it?

Thanks
Tom Crane

-- 
Tom Crane, Digital Electronics Engineer, Dept. Physics, Royal Holloway, University of London, Egham Hill,
Egham, Surrey, TW20 0EX, England.
Email:  T.Crane@rhul.ac.uk

Re: Accessing discs connected to H/W RAID controller over SG interface

[Martin K. Petersen]

Tom,

> The RAID controller from Areca is configured with 2-off RAID volumes
> and has 4-off 6TB discs attached to it.  I would like to be able to
> access the 4 discs individually, e.g. to be able to dd them to
> /dev/null to check on their performances.

It really depends on the RAID controller whether it is possible to
directly access physical disks. Most controllers disallow accessing any
drives that are part of a logical RAID volume.

Those controllers that expose physical disks will typically only present
them as generic SCSI devices for purposes of health monitoring and drive
firmware updates. The controller firmware will often only allow a few
carefully curated SCSI commands to be passed through to the physical
drive.

The reason you can query the drives through smartctl is that the latter
has special code which knows how to address individual drives behind an
areca interface. But querying SMART data is not the same as being able
to send or receive arbitrary SCSI commands.

-- 
Martin K. Petersen	Oracle Linux Engineering