Storage Forums: Tcq, Raid, Scsi, And Sata - Storage Forums

StorageReview takes the first of several comprehensive looks at tagged command queuing and its implications for the desktop as well as the server world. Does TCQ rate? How does it mesh with RAID arrays? Is SATA TCQ as effective as SCSI TCQ? Find the answers to these questions and more in SR's latest!

TCQ, RAID, SCSI, and SATA

In a single user environment would a task such as copying or moving 20,000 files (say 20GB total) all at once benefit from command queing?

rustybx, on Jun 24 2004, 02:28 PM, said:

No. OS-level caching strategies dominate in such a scenario... TCQ exerts little effect.

Eugene, on Jun 24 2004, 12:03 PM, said:

I started reading your article and after a few lines I have to disagree!

TCQ (the 'parallel ATA tagged command queueing') does _not_ need a special controller! It requires a TCQ capable hard disc and a TCQ capable (SW) driver. That's a very big difference to NCQ (Sata native command queueing). Here the drive and the controller interact without SW-assistance. With NCQ you can start a bunch of DMAs (with different address and length) and the Controller and the Drive will serve this request until completion and a raised interrupt. With TCQ you can issue several Read/Write commands to the drive and, when the drive completed one request, you can 'arm' the Controller with the corresponding address and length. The CPU still has to do something for each of the issued commands.

That's my understanding and in my opinion the biggest advantage of NCQ!

Correct me if I'm wrong...

- berndl

Quote

Considering that the driver comes with the controller, I think this is a rather silly point, no offence. Why bother splitting this hair? In all practicality you have to buy a controller that supports TCQ.

Besides SR, notes that the differences between the S150 TX4 and the TX4200 lie only in the firmware realms. This implies to me that NCQ can be enabled in firmware/drivers as well.


As for you performance concerns I think they are unwarranted.
1. ATA TCQ uses DMA to return the answered requests, just like NCQ, the CPU utilization isn't going to be different.

2. If you're taking issue with managing the registers for the commands and tags, or the overhead of the actual tagging I don't think there is enough information available to distinguish between the ATA TCQ and NCQ. Is your concern that ATA TCQ implementations use main memory space for the registers to store the commands instead of registers on the controller? I think it is likely that the implementation of NCQ on the TX4200 is just as 'software' an implementation of TCQ as ATA TCQ is on the same controller.

Hello,
I was wondering if there would be a PDF version available for this article, specifically the RAID article. I found it very informative but I would definitely like to keep a hardcopy of it to look through without having to go link by link.

It is interesting that TCQ seems to be a disadvantage at low queue depths. Perhaps the optimal solution for TCQ enabled drives would be to have the controller monitor the queue depth and serve requests on first-in first-out basis until the queue depth justifies turning on TCQ.

Are there any good utilities to monitor queue depth during real-world usage ? It would seem that TCQ is a no-brainer on a server with a large number of users, but what about a server with only 10 users. If performance is of the utmost importance, it might be better in this case to choose some Raptors with a non TCQ controller over a SCSI solution. The Win2k performance monitor has an object called Current Disk Queue Length. Is that an accurate representation ?

This was a good article - not many people ask these kinds of question.

The obvious followup question is:

When will the WD740 article/benchmarks be updated with the new (MUCH) slower results? I seem to recall something like this before with WD. Is this right or am I recalling incorrectly?

Does StorageReview update this or continue with the vendor-supplied drives?

I'd be very interested in StorageReview's take on one of the 3ware 9000 series SATA controllers. Have you tried to get your hands on one of these beasts?

ravton, on Jun 24 2004, 08:46 PM, said:

A quick check of the SR performance database should show that there are currently two results of the WD740GD. Results from the 00FLA1 version will eventually become the only results that exist in the performance database. Would immediate deletion of the full set of old results have been in the best interest of readers? How else would readers be able to assess the "MUCH" slower results for themselves? Hell if I was that paranoid I'd assume immediate deletion of the old results was a coverup.