Benchmark/check L2ARC performances

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Benchmark/check L2ARC performances

Post by si-ghan-bi » Sun Oct 21, 2012 1:15 pm

Hello,
I attached a USB key to the USB to try the L2ARC cache and also to use the drive that otherwise useless because it shows very bad write performances.
The model is KINGSTON DATATRAVELER 2.0 and some benchmark are found here:
http://usbflashspeed.com/search/?size=& ... AVELER+2.0
I have a 8 GB model. From the graphs, for 4 KiB reads the flash drive should achieve at least 2.5 MiB/s, probably even 4 MiB/s. Compared to the my WDC green drives, able to get about 0.35 MiB/s on 4 KiB random reads (actually 2x0.35 MiB/s, since I use a mirror setup), the improvement is significant.

The usual zpool iostat output is

Code: Select all
                                                   capacity       operations       bandwidth
pool                                            alloc    free    read   write    read   write
---------------------------------------------  ------  ------  ------  ------  ------  ------
Thor                                           1024Gi  1.73Ti       6       2   162Ki  68.5Ki
  mirror                                       1024Gi  1.73Ti       6       2   162Ki  68.5Ki
    GPTE_34B525B3-6497-49AC-B8C7-8F82A9FF7B3C       -       -       1       1  81.5Ki  68.6Ki
    GPTE_2C939C1F-D9AA-4675-A633-C1DF2180E750       -       -       2       1  82.2Ki  68.6Ki
cache                                              -      -      -      -      -      -
  GPTE_A59A8BEB-DA91-457C-81DD-DC82A2F6947C    6.72Gi   430Mi       0       0  1.93Ki  72.3Ki
---------------------------------------------  ------  ------  ------  ------  ------  ------


but if I try to work for example with Lightroom and I tag photos they don't change much, as expected: the database is probably already in memory and the job involves writes, not random reads.
What I would like to know or test is the effectiveness of the L2ARC: even if it is useless, I would like to be sure that it doesn't worsen performances.

Obviously I don't need it, my load on the HD is very light, I'm just experimenting.
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Re: Benchmark/check L2ARC performances

Post by grahamperrin » Sun Oct 21, 2012 2:30 pm

si-ghan-bi wrote:… I would like to be sure that it doesn't worsen performances. … just experimenting.


"…  MLC is okay here, but TLC (think USB flash drive) can actually slow down the pool just by existing." (2012-03-13) – http://hardforum.com/showpost.php?p=1038490105

More optimistically: Poojan's Tech Blog / Flash disk cost and performance (USB, SSD, SDHC) (2011-01-08)
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Re: Benchmark/check L2ARC performances

Post by si-ghan-bi » Sun Oct 21, 2012 3:33 pm

Thank you for the link, very interesting. I would have liked some benchmark using those USB flash drives, but it seems there are none.
I must then conclude that the extremely low numbers on zpool iostat just mean I don't need a lot of random access.
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Re: Benchmark/check L2ARC performances

Post by grahamperrin » Mon Oct 22, 2012 12:53 am

As CrystalDiskMark is for Windows platforms only, I'm not sure how to interpret the figures given by Poojan Wagh.

Related: ZFS-oriented benchmarking software suggestions (not results)
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Re: Benchmark/check L2ARC performances

Post by mjt5282 » Wed Oct 24, 2012 6:51 am

I have been using a 256Gb OCZ-Agility4 SSD for L2ARC cache on my 4X3Tb raid1z zfs partition, it certainly speeds up rsync and file/directory lookups. I have currently a 16Gb Mac Pro. After reboot, the cache fills up steadily. I tried using a 60Gb SSD for ZIL but it didn't speed up anything. Here is a good writeup on SSD and ZFS : http://constantin.glez.de/blog/2011/02/ ... ds-and-zfs

Btw, the bug that caused a "initialize disk" message on reboot for cache devices is fixed, thanks Don!
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Re: Benchmark/check L2ARC performances

Post by si-ghan-bi » Wed Oct 24, 2012 8:55 am

I saw that script
http://cuddletech.com/arc_summary/
but I thought it cannot be used because it uses Perl modules meant for Solaris. I also asked a couple of times in another thread how to configure some ARC parameters, because the guides I find use tools not available on ZEVO for Mac.
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Re: Benchmark/check L2ARC performances

Post by si-ghan-bi » Wed Oct 24, 2012 4:16 pm

By the way, I found a way to test somehow the performances of my L2ARC usb stick: Windows 8 startup. I just discovered the stick gets used a lot during windows 8 usage and startup. I will measure the startup time and I will let you know.
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Re: Benchmark/check L2ARC performances

Post by si-ghan-bi » Wed Oct 24, 2012 6:17 pm

Actually, Win 8 is pretty good in placing the files in a way to reduce seek, this test doesn't work very well: during automatic defray the L2ARC gets used in the 100KiB/s to 1 MiB/s speed range, later not much anymore.
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Re: Benchmark/check L2ARC performances

Post by si-ghan-bi » Thu Nov 08, 2012 8:16 am

Grahamperrin, would you please benchmark your system with and without USB stick als L2ARC? I will do that today as well. I will be using bonnie++ 1.96, but I can try bonnie64 too if needed and useful.
If you want to suggest the test parameters (except ram size of course) to have more homogeneous results and/or tests tuned to ZFS, just let me know.
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Re: Benchmark/check L2ARC performances

Post by si-ghan-bi » Thu Nov 08, 2012 6:43 pm

I tried bonnie++, useless. It generates the file(s) and then performs too short tests. I tried the option "-x n" but it won't read again the same files, it will simply starts over.
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