>>> Using a Dell R720 head unit, plus a bunch of Dell MD1200 JBODs dual pathed
>>> to a couple of LSI SAS switches.
>> How many HBA's in the R720?
> We have qty 2 LSI SAS 9201-16e HBA's (Dell resold).
Sounds similar in approach to the Aberdeen product another sender referred to,
with SAS switch layout:
One concern I had is that I compared our SuperMicro JBOD with 40x 4TB drives
in it, connected via a dual-port LSI SAS 9200-8e HBA, to the same pool layout
on a 40-slot server with 40x SATA drives in it. But the server uses no SAS
expanders, instead using SAS-to-SATA octopus cables to connect the drives
directly to three internal SAS HBA's (2x 9201-16i's, 1x 9211-8i).
What I found was that the internal pool was significantly faster for both
sequential and random I/O than the pool on the external JBOD.
My conclusion was that I would not want to exceed ~48 drives on a single
8-port SAS HBA. So I thought that running the I/O of all your hundreds
of drives through only two HBA's would be a bottleneck.
LSI's specs say 4800MBytes/sec for an 8-port SAS HBA, but 4000MBytes/sec
for that card in an x8 PCIe-2.0 slot. Sure, the newer 9207-8e is rated
at 8000MBytes/sec in an x8 PCIe-3.0 slot, but it still has only the same
8 SAS ports going at 4800MBytes/sec.
Yes, I know the disks probably can't go that fast. But in my tests
above, the internal 40-disk pool measures 2000MBytes/sec sequential
reads and writes, while the external 40-disk JBOD measures at 1500
to 1700 MBytes/sec. Not a lot slower, but significantly slower, so
I do think the number of HBA's makes a difference.
At the moment, I'm leaning toward piling six, eight, or ten HBA's into
a server, preferably one with dual IOH's (thus two PCIe busses), and
connecting dual-path JBOD's in that manner.
I hadn't looked into SAS switches much, but they do look more reliable
than daisy-chaining a bunch of JBOD's together. I just haven't seen
how to get more bandwidth through them to a single host.
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