[dm-devel] Performance Comparison among EnhanceIO, bcache and dm-cache.

[Amit Kale]

On Tuesday 11 Jun 2013, OS Engineering wrote:
> Hi Jens,
> 
> In continuation with our previous communication, we have carried out
> performance comparison among EnhanceIO, bcache and dm-cache.
> 
> We found that EnhanceIO provides better throughput on zipf workload (with
> theta=1.2) in comparison to bcache and dm-cache for write through caches.
> However, for write back caches, we found that dm-cache had best throughput
> followed by EnhanceIO and then bcache. Dm-cache commits on-disk metadata
> every time a REQ_SYNC or REQ_FUA bio is written. If no such requests are
> made then it commits metadata once every second. If power is lost, it may
> lose some recent writes. However, EnhanceIO and bcache do not acknowledge
> IO completion until both IO and metadata hits the SSD. Hence, EnhanceIO
> and bcache provide higher data integrity at a cost of performance.

So it won't be fair to compare them with dm-cache in write-back mode since 
guarantees are different. I am sure if similar (SYNC/FUA enforcement for 
persistence) guarantees are implemented in bcache/EnhanceIO, they'll offer a 
much better performance. 

> 
> The fio config and setup information follows:
> HDD              : 100GB
> SSD              :  20GB
> Mode             : write through / write back
> Cache block_size : 4KB for bcache, EnhanceIO and 256KB for dm-cache
> 
> The other options have been left to their default values.
> 
> Note:
> 1) In case of dm-cache, we used 2 partitions of same SSD with 1GB partition
> as metadata and 20GB partition as caching device. This has been done so as
> to ensure a fair comparison as EnhanceIO and bcache do not have a separate
> metadata device.
> 
> 2) In order to ensure proper cache warm up, We have turned off sequential
> bypass in bcache. This does not impact our performance results as they are
> taken for random workload.
> 
> For each test, we first performed a warm up run using the following fio
> options: fio --direct=1 --size=90% --filesize=20G --blocksize=4k
> --ioengine=libaio --rw=rw --rwmixread=100 --rwmixwrite=0 --iodepth=8 ...
> 
> Then, we performed our actual run with the following fio options:
> fio --direct=1 --size=100% --filesize=20G --blocksize=4k --ioengine=libaio
> --rw=randrw --rwmixread=90 --rwmixwrite=10 --iodepth=8 --numjobs=4
> --random_distribution=zipf:1.2 ...

Did you experiment a little with varying iodepth and numjobs? Is this the best 
combination you found out? The numbers below appear low considering a 90% hit 
ratio for EnhanceIO. SSD baseline performance shown below appears low. However 
it should not affect this comparison. All caching solutions are being subjected 
to the same ratio of HDD/SSD performance.

> 
> ============================= Write Through ===============================
> Type      Read Latency(ms)   Write Latency(ms)    Read(MB/s)    Write(MB/s)
> ===========================================================================
> EnhanceIO      1.58              16.53               32.91       3.65
> bcache         0.58              31.05               27.17       3.02
> dm-cache       0.24              27.45               31.05       3.44

EnhanceIO shows much higher read latency here. Is that because of READFILLs ? 
Write latency, read-write throughputs are good.

> 
> ============================= Write Back ==================================
> Type      Read Latency(ms)    Write Latency(ms)    Read(MB/s)   Write(MB/s)
> ===========================================================================
> EnhanceIO      0.34               4.98               138.72      15.40
> bcache         0.95               1.76               106.82      11.85
> dm-cache       0.58               0.55               193.76      21.52

Here EnhanceIO's read latency is better compared the other two. Write latency 
is larger than the other two. 

-Amit

> 
> ============================ Base Line ====================================
> Type      Read Latency(ms)    Write Latency(ms)    Read(MB/s)   Write(MB/s)
> ===========================================================================
> HDD            6.22              27.23                13.51       1.49
> SSD            0.47               0.42               235.87      26.21
> 
> We have written scripts that aid in cache creation, deletion and
> performance run for all these caching solutions. These scripts can be
> found at:
> https://github.com/stec-inc/EnhanceIO/tree/master/performance_test
> 
> Thanks and Regards,
> sTec Team
> 
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