Thanks to Alan Rollow and Nick Hill for their comments!
I originally wrote:
Hi.
We're in the process of configuring an AlphaServer 2100A (5/250) to be a
BRS database machine, and are pondering the following:
- are the HSZ Array Controllers worth the $$$, and does DU3.2+
deal well with them?
- do folks find that FWD disks really improve performance to the degree
that the theortical numbers would indicate?
Per usual, money is at the root of these questions, and while we're not
trying to be *cheap*, we do need to be efficient...
Thanks.
Pat Wilson
paw_at_dartmouth.edu
And the replies (headers edited):
------- Message 1
Date: Mon, 5 Feb 1996 10:26:39 -0700
From: alan_at_nabeth.cxo.dec.com (Alan Rollow - Dr. File System's Home for Wayward Inodes.)
Message-Id: <9602051726.AA19941_at_nabeth.cxo.dec.com>
To: paw_at_northstar.dartmouth.edu
Subject: Re: HW configuration: HSZ and FWD?
The bandwidth performance of the HSZ40 isn't anything special (good,
but not great). Under the right load the through-put performance is
impressive; a read and write cache will do that. The value of the
HSZ40 is:
o The ability to connect LOTS of disks. In a single controller,
non-redundant power supply configuration, an HSZ40 can allow
access to 42 disks, using up to 4 target IDs on a single SCSI
bus. In a dual-controller or dual power supply configuration
it supports 36 disks. Using 4.3 GB disks this is 154.8 GB to
180.6 GB. A SCSI adapter using simple disks is limited to 7
disks (30.1 GB). (Digital UNIX doesn't currently support wide
addressing).
o Controller failover. In a dual controller configuration, one
controller will take over, if the other fails.
o Striping
o RAID-5
o Mirroring
o Hot spares for RAID-5 and Mirroring.
o The ability to clone a device.
o The ability to turn a plain disk into a mirrored unit
(or the members of a stripe set) without having to take
the unit offline and backup/restore the data.
If you only need a few GB and don't need any of the above features,
then a few direct connected disks should be sufficient. Using LSM
for striping is bound to be as good or better (across multiple
controllers) than any controller based striping. Eventually,
Digital UNIX will support wide addressing and it will possible
to put more (wide) disks on a FWD adapter. Of course, it will
also be possible to put more HSZ40s on the same adapter with
more addresses available.
The value of FWD is:
Fast - Devices can transfer data at (up to) 10 MB/sec rather
than (up to) 5 MB/sec. This can allow a single fast
device to be fast, or allow more medium speed devices
to run at speed, without saturating the bus.
Wide - Allows wide devices to transfer 16 bits of data instead
the usual 8 per clock. As with fast, it allows these
devices to be faster.
Differential - Single ended SCSI, is limited a cable length of 6 meters
for slow SCSI and 3 meters for fast SCSI. Some marginal
configurations may only work at shorter lengths. Differential
SCSI on the other hand allows 25 meters. This just improves
the configuration potential, but when you might only have
a meter or two to play with connecting up a few device it
makes a big difference.
------- Message 2
Date: Mon, 5 Feb 1996 18:26:40 GMT
From: Nick Hill - RAL CISD Systems Group <NMH1_at_axprl1.rl.ac.uk>
Reply-To: n.m.hill_at_rl.ac.uk
To: paw_at_northstar.dartmouth.edu
Subject: Re: HW configuration: HSZ and FWD?
Well...
I have some HSZ 40 controllers which I have had working fine with DU3.2c. DU
knows about them, ie has diskatb entries and SCSI cam entries for them. You
have to make the devices by hand in /dev as MAKEDEV does not handle SCSI LUN
devices.
I do not have the RAID license on these controllers so cannot give any
information about RAID 5 or shadowsets. The striping you get with the default
HSZ OS works O.K. though.
I also have PCI FWD controllers with FWD disks connected and these work
really well. Using wide versions of DEC rz29 4Gb drives you can get up to
6600kb/s write rate and 6800kb/s read rate sustained transfer for a single disk.
With three disks on the same bus then each write at around 6300kb/s and read
at 6800kb/s in parallel down the same bus. These rates are as reported by the
monitor program available from gatekeeper.dec.com all measurements on a
Alphaserver 8400 with kzpsa pci fwd scsi controllers.
So which to use? If you want to off load RAID functionality from the host to
hardware then the HSZ is probably a good bet, RAID 5 will probably work
better done via an HSZ with 32mb of read and 32mb of write cache than in
software on the host. If you want more than around 6.5Mb/s from a disk then
you need to look at striping as a single FWD disk is not going to do that for
you. If numbers of busses are limited then a stripe set on six busses behind
an HSZ will probably be better than striping via LSM. If you have say 6 FWD
SCSI busses then striping via LSM may be better. DEC claim up to 14Mb/s for a
stripe set behind an HSZ40b. Certainly for a single FWD bus configuration
then an HSZ40 stripe set using 6 rz29 disks one per HSZ bus would give better
performance than 6 wide rz29 disks on the same bus using LSM.
- -
Nick Hill
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Computing and Information Systems Department
Rutherford Appleton Laboratory
Chilton
Didcot Internet: nmh1_at_axprl1.rl.ac.uk
Oxfordshire Janet: nmh1_at_uk.ac.rl.axprl1
OX11 0QX DECnet: omni:.uk.ac.rl.axprl1::nmh1
ENGLAND 20054::nmh1
WWW:
http://www.cis.rl.ac.uk/people/nmh1/contact.html
Tel: +44 (0)1235-445598
Fax: +44 (0)1235-446626
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
------- Message 3
To: Pat Wilson <paw_at_northstar.dartmouth.edu>
Subject: Re: HW configuration: HSZ and FWD?
In-Reply-To: Your message of "Mon, 05 Feb 96 12:39:48 EST."
<199602051739.MAA15664_at_phibes.dartmouth.edu>
Date: Mon, 05 Feb 96 12:13:19 -0700
For bare disks that support fast vs. slow I haven't
seen much to suggest a difference. Part of problem
is that the bare disks aren't fast enough to push into
anything beyond 5 MB/sec. As long as this is the case,
the value of fast is that it allows a transfer to take
less time on the bus, letting some other device transfer
a decent speed.
For wide, I have seen some performance tests (Digital
internal I'm afraid) that suggest there is a difference.
Unfortunately, the people that run those tests use such
low level tests that they're useless for seeing how a
real system will behave. They're good for marketing
glossies, but not much else.
On a subsystem like the HSZ40, wide support probably
buys more transfers as opposed to much more bandwidth.
------- End of Messages
Received on Wed Feb 07 1996 - 00:53:10 NZDT