Scratch Disk on RAID system

SS
Posted By
Steven_Scotten
Oct 9, 2008
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463
Replies
11
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Closed
I have one RAID: hardware RAID card, four 15,000 RPM SAS drives, configured as RAID5. This is my system volume. I am also using it as Photoshop swap.

I know that with drive systems that don’t multitask as well as SCSI it is critical that a separate drive be specified for swap. Perhaps this is less important for SCSI but still very important.

Here’s the thing: I have four drives on the same RAID controller. The striping is supposed to make writing to the drives faster. Lots faster. The RAID controller is already sending commands to write to and read from all four drives.

Would taking one of my drives off the RAID5 system volume to make it a dedicated PS swap volume really give me a performance boost? Really???

I’m skeptical. All the read/write commands are still being managed by the same controller and all the writes are still going to the same four drives. The only difference is that I’ll have created a second drive, substantially slower than the system drive because it won’t have the benefit of striping.

Perhaps an external drive or RAID would help, but even then I think I’d still be looking at a performance hit because I’d have to go through the FW bus to move the data instead of having the controller right on the system bus.

Am I thinking about this totally wrong, or does the conventional wisdom only hold true for IDE/ATA drives?
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Ann_Shelbourne
Oct 9, 2008
I haven’t worked with a RAID set-up, but my guess is that designating a separate volume for Scratch is still the best practice.

Perhaps it would be a good idea to try taking one of your drives off the RAID5 system volume to make it a dedicated PS Scratch and see if it is an improvement.
PF
Peter_Figen
Oct 9, 2008
RAID Level 5 with four drives should use three drives for data and one for parity that is used to rebuild when you have a drive failure. It should be faster than a single drive, but not as fast as a RAID Level 0 array. I think if you’re after the fastest scratch performance you’ll want to add a RAID 0 setup externally. I’d do an eSATA external enclosure with at least two drives striped together and use it only for scratch, remembering that with RAID 0 if you lose one drive you lose all the data – not a huge deal if it’s just scratch. I’ve been using RAID arrays for at least the last dozen years for scratch, first with SCSI and now with SATA. The performance increase is incremental, but if you are hitting scratch regularly, it can make a small difference. I like the G-Tech enclosures – very robust, beautiful and have great fans for cooling.
AW
Allen_Wicks
Oct 10, 2008
What I said in your other post:

I do not know why CS2 is outperforming CS3 on a MP. I have the opposite experience. [How does RAM report out during your tests?]

DO NOT BUY THE FW DRIVE, FW is much slower than the internal SATA or if you were to choose an external eSATA drive. Instead add a Raptor drive internally in the extra optical drive slot and put the System and apps there, with scratch on your RAID array. If you choose not to use the optical bay slot use an eSATA-connected drive.

I agree that you should not partition the RAID. Personally I find the 4 limited bays of a MP inadequate for me to sacrifice all 4 bays for RAID5 so I have two in RAID0 plus a System/apps drive and an on-site backup drive.

I did get 8GB of RAM; would have gotten more, but I read that it won’t matter until CS goes 64-bit in CS5 at the earliest.

Not true, you will see improved performance with more than 8 GB RAM. Also, do a forum search on "Bigger Tiles" and also on "Force VM Buffering" as one or both will probably be appropriate for your large files.
SS
Steven_Scotten
Oct 10, 2008
Allen, thank you. I’ll put at least another 8GB on my shopping list, but it sounds like I should take care of the hard drives first.

You suggest I put a Raptor in my second optical bay. That’s appealing, but confusing. Apple told me that I could have *either* SAS *or* SATA drives but not both. I assumed that they put the RAID card where the SATA drive controller would have been. Am I wrong about that? Do I still have a connector somewhere to hook up a fifth drive (be it SAS or SATA)? I know there’s a second old-style IDE cable for the optical drive, but I’m guessing I’m not going to find a high-performance drive with that connector.

I chose RAID5 because I figured I wanted everything on one volume and I did not want to risk RAID0 for my system disk. I’ve been told that RAID5 is purty darn fast (Apple claims 251MB/s read, 196MB/s write for RAID5 and 304MB/s read, 256MB/s write for RAID0… either of those versus the 125MB/s read claim of the SAS drives not in a RAID is a big step up.) Having the stability of RAID5 seemed prudent.
R
Ram
Oct 10, 2008
Having the stability of RAID5

You are aware that RAID drives multiply the risk of losing data, aren’t you?
JJ
John Joslin
Oct 10, 2008
Surely you don’t mean all RAID configurations with that remark?
SS
Steven_Scotten
Oct 10, 2008
Having the stability of RAID5

You are aware that RAID drives multiply the risk of losing data, aren’t you?

Yes. Precisely the reason to use RAID5.

Murphy’s law aside, drives don’t tend to fail at the same time. If I have data on four un-RAID-ed drives and one fails, I lose whatever was on that drive… maybe that’s 25% of my data.

If I have data on drives in a RAID0 configuration and one drive fails, I lose EVERYTHING.

If I have data on drives in a RAID5 configuration and one drive fails, I lose nothing. So long as I don’t lose two drives in the same week, even UPS ground should be able to get me a replacement before the second one goes.

Throw Murphy’s law in, and it doesn’t matter; I lose all my data no matter what I do. But given that drives *do* fail, RAID5 seems to be the most stable option.
P
PShock
Oct 10, 2008
(Striped) RAID benefits ….

OS / System … opening applications is faster (who cares?). Also OS swap performance but you may never notice this over a single drive.

Data storage … faster saves. (fairly important for large files.)

PS scratch … (substantial performance increase provided the RAID is dedicated to ONLY scratch)

You’re current setup focuses on "safety" but not performance. You’re better off with separate RAID 0 sets and implementing a thorough backup regime. NOTHING beats a good backup plan, including RAID 5.

-phil
SS
Steven_Scotten
Oct 10, 2008
Respectfully Phil, I think you’re talking about the way things go with IDE/ATA drives. If you have two drives, Photoshop can talk to one drive while OS processes talk to the other drive.

SCSI drives have always multitasked better, but that’s not even the point. The point is that here, all the requests, whether from Photoshop or the OS’s swap manager, all go through the same controller, which takes the requests, splits them up, and writes them to different disks. Set one drive apart from the rest and the controller is doing exactly the same work with less chance to optimize.

You’re also talking like RAID5 is slow and doesn’t utilize striping. RAID0 is only about 25% faster (given that all other variables are equal). That’s significant, but RAID5 is still twice as fast as un-RAID-ed drives.

I don’t pretend that using RAID5 is a substitute for backup. The difference is that with RAID5 I can have a drive die and continue to work while the replacement gets delivered. RAID0 or solo drives leave me dead for as long as it takes to get replacements. I guess I could just keep spare hard drives on hand, but that still involves restoring from backup rather than keeping on working.

I’m not trying to be argumentative… well, that’s not true, but I don’t mean to be a jerk. What you’ve said and my understanding of how hard drives and drive arrays work don’t mesh. I’m not interested in proving myself right but I mean to explain my understanding so that I can be shown where I am mistaken.

If having separate volumes on the same controller will avoid "entanglement" issues from different processes accessing the drives, then I should do some backing up and reformatting.
JS
Jeff_Schewe
Oct 10, 2008
If having separate volumes on the same controller will avoid "entanglement" issues from different processes accessing the drives, then I should do some backing up and reformatting.

Yes, it will…the RAID card on the PCI bus will not be a bottleneck to reading/writing to two separate physical partitions on the card but read/write to the same logical partition, even in a RAID will slow down. That’s why it’s suggested to have the boot drive (which by default will be used for system paging) and Photoshop’s scratch drive on separate physical volumes. If the system has to page and Photoshop’s scratch is also paging, you will see lots of nicely colored spinning beach balls.

And while RAID 5 is faster than no arrayed drives…it’s not really 2X, it’s up to 2X. But if you stripe (raid 0) across multiple drives, the more drives you add, the higher the speed factor will be (with a small degree of diminishing returns). So, in RAID 0, two are faster than 1, 3 are faster than 2 and 4 are faster than 3 with 4 being ALOT FASTER than 1…the parity drive has a very limiting effect on the overall speed.
AW
Allen_Wicks
Oct 10, 2008
You suggest I put a Raptor in my second optical bay. That’s appealing, but confusing. Apple told me that I could have *either* SAS *or* SATA drives but not both.

You and Apple are correct, I forgot you had SAS drives. The separate OS/apps drive that I recommend must also be SAS.

I disagree with folks who say the scratch drive must have nothing but scratch on it. The scratch drive or array must be large enough and should stay less than half full for speed, but IMO as long as the OS/apps are on another drive there is no major penalty having other data on the scratch drive. Some folks like to routinely erase the scratch drive but IMO with modern drives that is not all that necessary. However I have no specific test data to support my opinion.

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