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Read Striping


britcowboy

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Hi,

 

I'm trying to get my head around Read Striping. In theory this should make it act a bit like a RAID 1 right, combining the two drives to get an improved read speed? I'm not expecting 2x read speeds, but at least 1.5x or so of one of the disks read speeds should make sense right?

 

However when I run CrystalMark on Drivepool, I find that it's basically limited to the speed of one of the drives - why isn't it combining the two drives to get a read speed that's above the speed of one drive on it's own? I care more about read speed because I can always improve write speed with a SSD cache.

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Probably because the file thats created for the test is not duplicated in time for you to see x2 read speeds if you do not have an ssd cache enabled

 

I just tested my pool which has an ssd cache and i get 800+ read and 225+ write - i have real time duplication on so that limits the write speed which is what i see in the "real world" when copying over my network although it does vary up to the high 400 hundreds for short bursts.

 

If you have a 10g network it matters but if you are on a 1g network it will not have much effect - unless you do a lot of access locally on the server/pc with DP

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Probably because the file thats created for the test is not duplicated in time for you to see x2 read speeds if you do not have an ssd cache enabled

 

I just tested my pool which has an ssd cache and i get 800+ read and 225+ write - i have real time duplication on so that limits the write speed which is what i see in the "real world" when copying over my network although it does vary up to the high 400 hundreds for short bursts.

 

If you have a 10g network it matters but if you are on a 1g network it will not have much effect - unless you do a lot of access locally on the server/pc with DP

 

Hi,

 

I have real time duplication enabled - so it should be on both, but either way if I leave it a bit it still is no faster (we're talking like 120 MB/s speed).

 

Also, I know it's over gigabit speed (just) but I want to be futureproof, and enable the best speeds possible if I do get 10g on it in the future.

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I don't have an SSD write cache at the moment.

 

I'm doing tests both with crystal and just copying large files to and from, and it's maxing out at 120mb/s (the speed of the disk), and hardly involving the other disk, showing in the performance ui as "hold" I'd want to see it in the mid to upper 100s if using both disks

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In theory this should make it act a bit like a RAID 1 right

 

Full Stop. 

 

No. 

 

The Read Striping may improve performance, but we still have the added overhead of reading from NTFS volumes.  And the performance profile for doing so.

 

RAID 1 works at a block level, and it's able to split IO requests between the disks.  SO one disk could read the partition table information, while the second disk starts reading the actual file data. 

 

This is a vast oversimplification of what happens, but a good illustration of what happens.

 

 

So, while we may read from both disks, in parallel, there are a number of additional steps that we have to perform to do so. 

 

 

 

To be blunt, the speed is never going to rival hardware RAID.   However, between with disk switching, and the reading blocks of the file and caching into memory. 

 

 

StableBit DrivePool utilizes a number of read striping algorithms, depending on the situation.

 
For large sequential I/O, such as large file copying, read striping will utilize a block based algorithm, maximizing the use of each disk and minimizing disk context switches.
 
For random non-sequential I/O read striping always sends the request to the disk with the least outstanding requests. Because seek times can be high in this scenario, StableBit DrivePool tries to switch disk contexts often.
 
For slow non-concurrent I/O read striping passively measures the speed of each disk and dynamically switches the the fastest disk.
 
You can see this in action under the Performance UI.
 

 

 

 

 

Our read striping algorithm is actually a number of different algorithms that are automatically selected based on the situation. However, they generally do not increase performance by a factor of how many duplicated files you have. They are generally good at providing a smooth media streaming experience and balancing the performance of disks with different performance characteristics.
 
For example:
  • For large asynchronous (i.e. multi-threaded) file transfers via. SMB, read striping attempts to mimic a RAID-like striping algorithm where it will send read request in fixed block sized chunks to the disk with the least load.
  • For single-threaded transfers, media streaming applications or random access, the read striping algorithm passively monitors the performance of all the disks that contain the given file part and selects the disk with the fastest response time. For example, if you're streaming a duplicated video file from the D: drive, and all of the sudden something starts accessing the D: drive excessively, the read striping algorithm will switch to reading from another disk that is less busy.
Also I did write an informal blog post that demonstrates read striping in action:

 

 

That said, you should still see at least the native disk speeds, or a bit better. But this depends on what the disks are doing, specifically. 

 

 

 

I don't have an SSD write cache at the moment.

 

I'm doing tests both with crystal and just copying large files to and from, and it's maxing out at 120mb/s (the speed of the disk), and hardly involving the other disk, showing in the performance ui as "hold" I'd want to see it in the mid to upper 100s if using both disks

 

CrystalDiskInfo probably isn't going to get great stats because of how it tests the data.

 

At best, enable pool file duplication, so that EVERYTHING is duplicated.  Make sure Read Striping is enabled, and then test. 

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