[Discuss] SAS drives

[dsr at tao.merseine.nu (Dan Ritter)]

On Wed, Aug 03, 2011 at 06:56:24AM -0400, Daniel Feenberg wrote:
> 
> I have been experimenting with a Promise 4650 SAS controller and 2
> 2.5" Seagate SAS drives and as someone who has never used a SAS
> drive before I have been surprised by a couple of things.
> 
> 1) The connector on the drive is a single connector for power and data
> and incompatible with the connectors used by 2.5" SATA drives. The
> controller came with  cables that connect power to a Molex connector
> and data to the controller. Perhaps this is why power supplies
> continue to have so many molex connectors? The controller end of the
> cable is the
> same as a SATA connector, as the controller can handle SATA drives also.

This is bizarre. SATA and SAS use the same cabling, with the
proviso that drives may take SATA power or Molex power or
provide connectors for both (and only one is required).

If you place a SATA power connector next to a SATA data
connection, as specified, you can get a cable which has a single
connector that does both, in which case the other end will split
out to a Molex connector for the power supply. Is that what
you're seeing? It should be completely compatible with SATA.

There are other connectors for internal and external multilane
cables, ranging from simple plugs to a fairly complicated
SFF8088 connector.


> 2) Reviews of various SAS controllers on Newegg speak about
> "directional SAS cables" and "SAS expanders", but I haven't been
> able to find out anything about them.

The cables are only directional in the sense that you must make
sure of plug compatibility when two different plugs are used,
and that a 90 degree bend on the connector may be more
advantageous at the drive side (or not).

A SAS expander is a smart adapter that allows multiple (up to 4) drives to
be connected to a single (non-multilane) cable. Bandwidth is
shared, so this is peachy for rotating media but not necessarily
a good idea for the latest SSDs.


> 6) Reviewers on Newegg of SAS controllers often complain about long
> boot times, as though the SAS controller was interfering in some way.

They're probably talking about BIOS setup times.

> 7) Reviewers on Newegg of SAS controllers often complain about small
> controller caches, but I wonder if this is important if there is plenty
> of main memory.

Depends on what you're doing.

> Mostly, I'd like to figure out how to build a fast file server for large
> sequential files, and thought this might be a good approach. So far
> not so good.

Start with a good, supported controller. 3Ware and LSI are often
mentioned. Then use lots of spindles in RAID10 or RAID6,
depending on your fault tolerance.

http://blog.backblaze.com/ shows how to do a big storage system;
to build it fast, you can upgrade the controllers and disks.

To build it really fast and big, you need to have tiers: RAM, then SSD,
then fast disks (15K), then slow disks. 

Or, if you had an unlimited budget, store everything in RAM and
back it up to something more permanent every so often.

Depends on what you're doing.

-dsr-



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