Dual Disk SATA - question for the hard disk gurus

[Spin]

Gurus,

On a machine with two SATA drives, is there any performance gain by running
applications off of the second SATA disk? I thought I understood that SATA
was serial in-line technology, that being said, if all the reads and writes
have to come through the same controller, then perhaps running applications
off of the second SATA disk might actually result in a DECREASE in total
system performance?

However, someone corrected me to say that each drive is directly connected
to a SATA port on the motherboard and that they do not connect in a
Master/Slave set up on the same cable. This means, to me, that throughput
to each drive is independent of the other and that running applications off
of the second SATA disk will result in an INCREASE in system performance.
Is this correct?

 

[Rod Speed]

Gurus,

None of those here, just us rather badly behaved animals.

On a machine with two SATA drives, is there any performance gain by running applications off of the second SATA disk?

Not with a decent modern OS and enough physical ram.

I thought I understood that SATA was serial in-line technology, that being said, if all the reads and writes have to
come through the same controller, then perhaps running applications off of the second SATA disk might actually result
in a DECREASE in total system performance?

Nope, essentially because where the app is loaded from just affects
the app startup time much, and it shouldnt be doing much else unless
there is a shortage of physical ram. If there is a shortage of physical
ram, you may well be swapping stuff out to the swap file when the app
is starting, but since each SATA drive has its own connection to the
controller, even that volume isnt being shared over the one cable when
you have two physical drives.

And the best fix for that situation is to have enough physical ram so it doesnt need to swap.

However, someone corrected me to say that each drive is directly
connected to a SATA port on the motherboard and that they do not connect in a Master/Slave set up on the same cable.

Thats correct.

This means, to me, that throughput to each drive is independent of the other and that running applications off of the
second SATA disk will result in an INCREASE in system performance. Is this correct?

Only if there isnt enough physical ram, so memory contents are being
written out to the swap file at the same time that the app is being loaded.

And that only affects the app start time much, it shouldnt affect normal
ops once the app is loaded, because the swapping should be over by
then except in the most pathological situations with minimal physical ram.

 

[Mike Ruskai]

Gurus,

On a machine with two SATA drives, is there any performance gain by running
applications off of the second SATA disk? I thought I understood that SATA
was serial in-line technology, that being said, if all the reads and writes
have to come through the same controller, then perhaps running applications
off of the second SATA disk might actually result in a DECREASE in total
system performance?

However, someone corrected me to say that each drive is directly connected
to a SATA port on the motherboard and that they do not connect in a
Master/Slave set up on the same cable. This means, to me, that throughput
to each drive is independent of the other and that running applications off
of the second SATA disk will result in an INCREASE in system performance.
Is this correct?

The "Serial" in SATA refers to the technical nature of the drive's connection
only. It uses a single wire for each stream of bits (two and from the drive),
as opposed to PATA or SCSI, which use several wires to transmit several bits
simultaneously. Parallel is faster for the same signalling rate, but more
complicated. Using a serial protocol is much simpler, which makes it easier
to ramp the signal rate up.

It has nothing to do with how the controller works with each device. Two IDE
devices on a PATA channel must share the connection (poorly), but two IDE
channels with one device each are not sharing anything. SATA/SAS connections
simply don't have the ability to share channels. Each connector is a separate
channel.

While any given controller chip might not be able to handle more than X number
of devices, X will definitely be larger than two.

The upshot is, if you split I/O load over two different SATA drives,
performance will go up.