ZFS concurrent random read speeds. 4-way mirror vs 2x 2-way mirror vs 4 disk stripe

E

einthusan

So what is faster for parallel random reads?

a) 4-way mirror (RAID1)
b) 2x 2-way mirror (RAID10)
c) 4 drives striped together (RAID0)
 
I seriously doubt that anyone here actually has benchmarked this, so anything would be an educated guess. I would say, theoretically a 4-way, because it can read everything from all drives, almost at any time. But theory and practice can differ, as we all know. DIY with benchmarks/bonnie++ to find out for yourself.

And I will never say that it is a good idea to stripe, but for different reasons than read performance.

/Sebulon
 
Sebulon said:
I seriously doubt that anyone here actually has benchmarked this, so anything would be an educated guess. I would say, theoretically a 4-way, because it can read everything from all drives, almost at any time. But theory and practice can differ, as we all know. DIY with benchmarks/bonnie++ to find out for yourself.

And I will never say that it is a good idea to stripe, but for different reasons than read performance.

/Sebulon
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I think you are right, 4-way should be the best for heavy concurrent reads. I was just doubting ZFS's capability to co-ordinate reads from a 4-way mirror efficiently. Thanks.
 
Actually, I did test using bonnie++, the results might be bizarre but you can see the relative difference,

4-way mirror: Read=704 MB/s Write=310 MB/s
stripe of 4 disks: Read=1542 MB/s Write=704 MB/s

The stripe is a bit more than 2x the read/write speeds compared to 4-way mirror. However, I doubt bonnie++ performs heavy concurrent random reads.
 
einthusan said:
Actually, I did test using bonnie++, the results might be bizarre but you can see the relative difference,

4-way mirror: Read=704 MB/s Write=310 MB/s
stripe of 4 disks: Read=1542 MB/s Write=704 MB/s

The stripe is a bit more than 2x the read/write speeds compared to 4-way mirror. However, I doubt bonnie++ performs heavy concurrent random reads.
Click to expand...

Are you using SSD's, or do you have compression turn on when running bonnie++?
 
t1066 said:
Are you using SSD's, or do you have compression turn on when running bonnie++?
Click to expand...

No SSD's, compression is turned on, however, the 16 GB file I used for testing with bonnie++ (2x RAM) has compress ratio of 1.0x

I know the numbers are so high probably because I concatenated a 2 GB file 8 times to make one large 16 GB file and probably ZFS detects some pattern in the file and caches some of it in memory maybe? Not sure.
 
Interesting. Did you also test 2x 2-way mirror (RAID10)?
I wonder if it is faster or slower then the 4 way mirror. I want to setup my ZIL with 4 partitions on 4 different SSDs (4 way mirror or 2x 2-way mirror) and use the rest with 4 bigger partitions (stripe) for L2 ARC. Waiting for my SSDs, might have them next week, so can't test it myself at the moment :)
 
There is a patch now in development by smh@ going to be committed soon after 10.0 release, that is expected to significantly improve mirror read performance. Unfortunately it was made too late to get into 10.0.
 
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