Pegasus - RAID5 - Reads slower than write?

ogilloire

macrumors member
Original poster
Feb 22, 2014
40
0
Hi

I was checking speed of my RAID5 array with blackmagic disk speed test, it roughly indicates 300MB/s write vs. 120-130MB/s read?

I thought RAID5 was supposed to be faster in reading than writing!

Does anyone have a clue? How do the numbers look? Normal or lower than you would expect?
 

AidenShaw

macrumors P6
Feb 8, 2003
18,133
4,207
The Peninsula
Hi

I was checking speed of my RAID5 array with blackmagic disk speed test, it roughly indicates 300MB/s write vs. 120-130MB/s read?

I thought RAID5 was supposed to be faster in reading than writing!

Does anyone have a clue? How do the numbers look? Normal or lower than you would expect?
This effect is common, and can be the result of a poorly designed benchmark interacting with the cache on the RAID controller.

The Pegasus2 has a 512 MiB cache in the box. You can write hundreds of megabytes into the cache before the controller actually has to touch the disk.

Reading, however, has to move the disk heads and fetch the data. This means that (unless the read data is already in the cache) cold writes can be faster than cold reads if the amount of data isn't large enough to flush the cache.

Do explain your setup, though. T-Bolt/USB/T-Bolt2 ???
 

ogilloire

macrumors member
Original poster
Feb 22, 2014
40
0
Hi

Thanks for your replies !

Setup is Pegasus2 R4, and WD 2 x green + 1 x red disks over Thunderbolt 2.

The 2x green are those extracted from my previous WD Duo, the red is a recent addition to expand capacity as i moved from RAID1 to RAID5.

I don't think the disks are very fast themselves so that could be why. Also not sure if the WD red+green mix is very appropriate, i'd prefer to have all reds but i don't want to throw away good working disks either (or the money to buy the reds).
 

Stephent

macrumors member
Jan 31, 2012
92
0
Hi

Thanks for your replies !

Setup is Pegasus2 R4, and WD 2 x green + 1 x red disks over Thunderbolt 2.

The 2x green are those extracted from my previous WD Duo, the red is a recent addition to expand capacity as i moved from RAID1 to RAID5.

I don't think the disks are very fast themselves so that could be why. Also not sure if the WD red+green mix is very appropriate, i'd prefer to have all reds but i don't want to throw away good working disks either (or the money to buy the reds).
I have heard of issues when you mix dim types with the Pegasus so that could be contributing to your issue as well.
 

spaz8

macrumors 6502
Mar 3, 2007
407
8
WD greens are also not a good fit for a RAID/DAS.. they are all about saving power (and money) and go to sleep a lot. I had a Drobo full of greens and it killed the performance. Been replacing with Reds.

The WD reds are a better choice.

I have 4 Hitachi 7K4000's in the mail for my Pegaus 2 R4.
 

theSeb

macrumors 604
Aug 10, 2010
6,963
83
Poole, England
You only have 3 drives in there? Also, as mentioned, the WD Greens are not meant to be used in RAID and are pretty slow. In addition, make sure that you have checked for any firmware updates using the Promise app.

I have 4 x 1 TB drives in my Pegasus 1 R4. I get the following with Blackmagic Disk Speed test (half of the array is full)

516 MB/s read
312 MB/s write
 
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BayouTiger

macrumors 6502
Jul 24, 2008
437
206
New Orleans
I was seeing the same thing on my R6. I had always had my 30" display with the MiniDisplayport adapter daisy chained off the R6. When I moved the display to another TB port the Pegasus sped up substantially. I would not have thought that would have saturated the bus, and maybe there is another answer, but that's what I saw.
 

theSeb

macrumors 604
Aug 10, 2010
6,963
83
Poole, England
I was seeing the same thing on my R6. I had always had my 30" display with the MiniDisplayport adapter daisy chained off the R6. When I moved the display to another TB port the Pegasus sped up substantially. I would not have thought that would have saturated the bus, and maybe there is another answer, but that's what I saw.
I got similar results to the op when I bought my R4. The firmware update made a huge difference.
 

ogilloire

macrumors member
Original poster
Feb 22, 2014
40
0
Thanks for that! Your benchmarks makes for a good starting point.

I'll try to replace the greens with reds, and see wether i have the latest firmware indeed.

Screen is on a different thunderbolt port/bus so that should not be a problem.

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[/COLOR]
You only have 3 drives in there? Also, as mentioned, the WD Greens are not meant to be used in RAID and are pretty slow. In addition, make sure that you have checked for any firmware updates using the Promise app.

I have 4 x 1 TB drives in my Pegasus 1 R4. I get the following with Blackmagic Disk Speed test (half of the array is full)

516 MB/s read
312 MB/s write
Yes I use the 4th one as pass-thru to be my time machine drive.

What kind of drives are yours?

----------

I got similar results to the op when I bought my R4. The firmware update made a huge difference.

There does not seem to be a firmware update for pegasus2 though on their website.
 

BayouTiger

macrumors 6502
Jul 24, 2008
437
206
New Orleans
I was seeing the same thing on my R6. I had always had my 30" display with the MiniDisplayport adapter daisy chained off the R6. When I moved the display to another TB port the Pegasus sped up substantially. I would not have thought that would have saturated the bus, and maybe there is another answer, but that's what I saw.
I should mention that I meant writes faster than reads. My R6 is much faster than that. More like 500MB each way or more. Also makes a big difference the size chosen for the test.
 

ogilloire

macrumors member
Original poster
Feb 22, 2014
40
0
May I ask also what are your stripe sizes on your RAID arrays?

Pegasus utility chose a very large 1MB size by default, i'm not against it as most of my usage has to do with 40megs photos, just wondering how the stripe size could affect performance.
 

ogilloire

macrumors member
Original poster
Feb 22, 2014
40
0
I ended up changing my setup to have 2x3TB WD red in RAID0 configuration on the pegasus 2, deactivated write back cache and managed solid 250MB/s read and write speed, which is perfectly good for my needs.

The other two slots of the pegasus are WD greens for my time machine backups