Skip to main content

Bandwidth with SAS expanders and RAID controllers




Having a RAID controller that has eight internal SATA3 lanes, you can get 6 Gb/s on all eight drives. What if I connect a 24 port SAS expander to an eight port RAID controller, do I still get max throughput of 8 x 6 Gb/s, or am I able to get 24 x 6 Gb/s, assuming the expander is rated for 6 Gb/s on all ports?



Of course the PCIe bandwidth is going to limit it, as well as the RAID controller but is this right theoritically speaking? PCIe 2.0 x8 has bandwidth of 4000 MB/s and PCIe 3.0 x8 has 7880 MB/s.



As an example, I was thinking of buying LSI MegaRAID 9271-8i for my home server. It has eight internal SATA 6 Gb/s lanes. With that one I am able to connect eight hard drives and they can work on their limits in terms of transfer rates because there is one 6 Gb/s lane for each drive available. But in the future the storage capacity might be too low. I thought I could just add a SAS expander, like Intel RES2SV240. It is a 24 port expander rated for 6 Gb/s per port. So do I get the full potential out of the expander, to have 6 Gb/s connection for all the possible 24 drives? If so, could I buy the 9271-4i (has only four internal SATA ports) and the Intel SAS expander to be able to connect up to 24 hard drives and have them work at their full speed?


Answer



Yes and no...



Think about this: Your disks will not perform at 6Gbps (unless they're SSDs). So some level of oversubscription is okay when you go to using a SAS expander.




A more common scenario is the use of an external JBOD storage enclosure. Those usually have 1 or 2 x 4-lane SAS connectors linking them to the main server. Let's assume 4 x 6Gbps, so 24Gbps total bandwidth. Things are definitely oversubscribed there, as you may have 24 disks linked at 6Gbps... but recall that most disks won't be able to achieve more than 1.5 or 2Gbps in practice, so that level of oversubscription is okay.



Remember, 6Gbps is just a link speed. You will not be able to achieve that through an expander to all connected disks, because the expander has upstream connections to a RAID controller. The RAID controller is the limiting factor here.



See:
Do SAS expanders work transparently with SAS controllers?

How exactly does a SAS SFF-8087 breakout cable work? + RAID/connection questions


Comments

Post a Comment

Popular posts from this blog

linux - Awstats - outputting stats for merged Access_logs only producing stats for one server's log

I've been attempting this for two weeks and I've accessed countless number of sites on this issue and it seems there is something I'm not getting here and I'm at a lost. I manged to figure out how to merge logs from two servers together. (Taking care to only merge the matching domains together) The logs from the first server span from 15 Dec 2012 to 8 April 2014 The logs from the second server span from 2 Mar 2014 to 9 April 2014 I was able to successfully merge them using the logresolvemerge.pl script simply enermerating each log and > out_putting_it_to_file Looking at the two logs from each server the format seems exactly the same. The problem I'm having is producing the stats page for the logs. The command I've boiled it down to is /usr/share/awstats/tools/awstats_buildstaticpages.pl -configdir=/home/User/Documents/conf/ -config=example.com awstatsprog=/usr/share/awstats/wwwroot/cgi-bin/awstats.pl dir=/home/User/Documents/parced -month=all -year=all...
Post a Comment
Read more

iLO 3 Firmware Update (HP Proliant DL380 G7)

The iLO web interface allows me to upload a .bin file ( Obtain the firmware image (.bin) file from the Online ROM Flash Component for HP Integrated Lights-Out. ) The iLO web interface redirects me to a page in the HP support website ( http://www.hp.com/go/iLO ) where I am supposed to find this .bin firmware, but no luck for me. The support website is a mess and very slow, badly categorized and generally unusable. Where can I find this .bin file? The only related link I am able to find asks me about my server operating system (what does this have to do with the iLO?!) and lets me download an .iso with no .bin file And also a related question: what is the latest iLO 3 version? (for Proliant DL380 G7, not sure if the iLO is tied to the server model)
Post a Comment
Read more

hp proliant - Smart Array P822 with HBA Mode?

We get an HP DL360 G8 with an Smart Array P822 controller. On that controller will come a HP StorageWorks D2700 . Does anybody know, that it is possible to run the Smart Array P822 in HBA mode? I found only information about the P410i, who can run HBA. If this is not supported, what you think about the LSI 9207-8e controller? Will this fit good in that setup? The Hardware we get is used but all original from HP. The StorageWorks has 25 x 900 GB SAS 10K disks. Because the disks are not new I would like to use only 22 for raid6, and the rest for spare (I need to see if the disk count is optimal or not for zfs). It would be nice if I'm not stick to SAS in future. As OS I would like to install debian stretch with zfs 0.71 as file system and software raid. I have see that hp has an page for debian to. I would like to use hba mode because it is recommend, that zfs know at most as possible about the disk, and I'm independent from the raid controller. For us zfs have many benefits, ...
Post a Comment
Read more