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Write speeds??


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Starting to work with unRAID and love it.  One question.  I added a second data drive this morning, formatted it with XLS and added it to the share.  My read speeds stayed consistent at about 90 mb per second.  My write speeds to the share dropped to about 30 mb per second.  Before adding the second disk my write speeds were about 90 mb per second?  Trying to figure out the issue.


Currently I have the drives attached to the motherboard sata connections.  Do i need to get a PCIe card for better throughput? 


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mr-hexen - Thanks for the quick response, very much appreciated. 


Granted I am currently using old drives, would new faster drives help the write speeds or is the bottleneck for the XOR function in with CPU?  The reason I ask is that I am looking at migrating about 750 GB of data and would like write speeds to be as smooth as possible.

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unRAID parity is updated realtime. Writing to a parity-protected disk actually reads the disk to be written, reads the parity disk, calculates the change that would be made to parity by the to-be-written data, then writes data and writes parity. The calculation is really pretty insignificant and this is mostly I/O-bound performance. Changing hardware will likely have little effect.


The slower writes is one of the reasons for the cache disk feature.

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You might be able to improve write performance with updated hardware, but as others have mentioned you are within the "normal" band.  I'm not sure what the rest of your system looks like.  Mine system is fairly old except for the drives which are larger/newer/faster.  I get write speeds in the 35MB/s - 55MB/s most of the time - sometimes faster but seldom slower.  You definitely have some older/slower drives so newer/faster drives might help somewhat, depending on what the rest of your system looks like.  You aren't going to be tripling your write speed, though - any performance gains will be more modest for the reasons mr-hexen and trurl mention.

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