Re: [Iometer-devel] Questions about outstanding I/O Queue of Linux vs.TCQ/NCQ.

[Harder, D. W. <Dav...@gd...>]

Depends on the RAID, your RAID configuration, and your IOMeter profile.

For example, if you have 16 drives in a RAID-0 array and you have a
capable RAID controller (that can handle 256+ I/Os), then a small-block
random profile in IOMeter would typically result in roughly 16 I/Os to
each drive.

If you have a large-block sequential profile with the same hardware
configuration, the number of I/Os would be somewhere in between 16 and
256 with a tendency towards 256 (depending on the RAID controller).

-Dave-

-----Original Message-----
From: Liang Yang [mailto:yan...@ho...]=20
Sent: Friday, October 27, 2006 2:32 PM
To: Harder, David W.; iom...@li...;
iom...@li...
Subject: Re: [Iometer-devel] Questions about outstanding I/O Queue of
Linux vs.TCQ/NCQ.

Hi Dave,

Thanks for your answer. For the depth of TAG queue of SAS drives,
theoretically it could be 65535 as the SAS device can support up to that
many of initators. But in practice, 64 is the range most of SAS drives
are using.

As you mentioned the RAID stuff in your answer, I just have a question
for using outstanding I/Os on RAID volumes:
Suppose you have a RAID volume which is built on 16 SAS drives. When you
treat this RAID volume as single physcial drive and use IOMeter to do
some physcial drive performance testing. You set the queue depth in
IOMeter to 256. Does each drive behind this RAID volume get 256 I/O, or
the RAID controller card will divide 256 I/O between 16 drives and each
drive just get 16 I/Os?

Best regards,

Liang

----- Original Message -----
From: "Harder, David W." <Dav...@gd...>
To: <mul...@ho...>; <iom...@li...>;
<iom...@li...>
Sent: Friday, October 27, 2006 12:04 PM
Subject: RE: [Iometer-devel] Questions about outstanding I/O Queue of
Linux vs.TCQ/NCQ.


1) You need one queue in order to populate the other.  In other words,
if your application does not tell the OS to issue multiple asynchronous
I/O (or allow the OS to do this implicitly for you), then the block
device will never have more than one I/O in it's queue.

2) I might be wrong, but I believe it is possible for block devices to
support queue depths much greater than 32 or 64.  For example, you could
have a block device that is actually a SAS-SAS (or SAS-SATA) RAID 16
drives behind it. (It might even be indistinguishable from a SAS drive
as far as the host is concerned.)

3) IMHO, I doubt there are any measurable negative effects of having an
application I/O queue that is greater than the block device's I/O queue.
(i.e. If you have a block device with a queue size of 64 and your
application queues 70 I/Os, the performance will not be any worse than
if your application queued 64.)

-Dave-

-----Original Message-----
From: iom...@li...
[mailto:iom...@li...] On Behalf Of Liang
Yang
Sent: Friday, October 27, 2006 1:35 PM
To: iom...@li...;
iom...@li...
Subject: [Iometer-devel] Questions about outstanding I/O Queue of Linux
vs.TCQ/NCQ.

Hi,
Under Linux, we can issue different number of outstanding I/O to the
block device, e.g. the queue depth can be set from 1 to 256 from
IOMeter. However, the hard disk drive itself may have its own queue
(e.g. NCQ--Native tagged command queue  for SATA drives and TCQ--tagged
command queue for SAS/SCSI drives). A typical queue depth for NCQ is 32
and TCQ is 64. The outstanding I/O queue should be controlled by the
block device driver and RAID controller card/SAS HBA firmware. The
NCQ/TCQ should be controlled by the hard disk drive firmware.

I have two questions here:
If outstanding I/O queue has different queue depth from NCQ/TCQ, will
this affect the I/O performance, e.g. the Linux outstanding I/O queue is
much longer than the TCQ/NCQ in the hard disk drive?

Why do we need to maintain two queues here? Does the duplicate queue
cause additional overhead?

Could anyone give me some explanation about this?

Thanks,

Liang


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