XFS/Linux Sanity check

[Stan Hoeppner]

On 5/3/2011 11:05 AM, Paul Anderson wrote:

> I'm still perfectly willing to buy good HW RAID cards, don't get me
> wrong, but their main benefit to me will be the battery backed cache,
> not the performance.

Good RAID cards have many more advantages than battery cache and 
performance.  One is moving a RAID card and its attached arrays from a 
failed host to a new one.  In the case of the hardware RAID card usually 
all that is required is loading the HBA driver and mounting the 
filesystem.  Such a move of an mdraid array is usually, well, not nearly 
as straightforward, to put it kindly.

> Keep in mind that it is hard to balance a HW RAID card across multiple
> SAS expanders -you can certainly get a -16e card of some sort, but
> then it does ALL of the I/O to those 4 expanders ALL of the time.

I'm note sure I know exactly what you mean here Paul. You seem to be 
talking about RAID card <-> drive chassis cabling flexibility and 
symmetrical bandwidth.  The following two SAS expander/switch products 
are likely worth a quick read:

http://www.intel.com/Products/Server/RAID-controllers/re-res2sv240/RES2SV240-Overview.htm
http://www.lsi.com/channel/products/switch/sas6160/index.html

Using an LSI 9260-4i single 8087 port RAID card, the Intel expander, and 
some 8087/8088 panel converters, one could attach *5* x 24 drive LSI 
620J SAS JBOD chassis for a total of 120 drives with equal bandwidth 
to/from all drives, about 2GB/s total bandwidth, RAID ASIC limited.  Few 
would want to connect 120 drives to such a single port RAID controller, 
but this example demonstrates that symmetry can be achieved across a 
large number of cascaded SAS expander ASICs (6 total) with a lot of drives.

-- 
Stan