L2ARC with neither benchmarking nor measurement

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L2ARC with neither benchmarking nor measurement

Post by grahamperrin » Thu Dec 20, 2012 8:48 am

Spun off from Benchmark/check L2ARC performances

As much as I'd like to simply benchmark things – or have more exotic graphs of the type that are possible with some Sun and Oracle appliances – I'm guided by two key points from a 2008 introduction to L2ARC:

  • improve performance or do nothing
  • there isn't anything that should be bad.

ZFS L2ARC (Brendan Gregg) (2008-07-22, highlights)

L2ARC Screenshots (Brendan Gregg) (2009-01-30, highlights)
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a small USB flash drive as L2ARC for my home directory

Post by grahamperrin » Thu Dec 20, 2012 9:07 am

MacBookPro5,2 limited to 8 GB memory, running Mountain Lion.

disk3s2 below is from the larger of two Core Storage-encrypted partitions on the internal disk, Seagate Momentus® XT ST750LX003-1AC154 solid state hybrid.

The mixture with HFS Plus on a single disk is not recommended so I'm interested in anything that might offset that reduction in performance.

Home directory gjp22 is a ZFS file system at the root of the pool with the same name.

Code: Select all
macbookpro08-centrim:~ gjp22$ zpool status gjp22
  pool: gjp22
 state: ONLINE
 scan: scrub canceled on Mon Dec 17 20:27:30 2012
config:

   NAME                                         STATE     READ WRITE CKSUM
   gjp22                                        ONLINE       0     0     0
     GPTE_71B8BDA2-3EBA-4B91-9E1C-2AE2B1DAAD06  ONLINE       0     0     0  at disk3s2
   cache
     GPTE_64F61AFF-9EBC-4661-9520-7803CD1B8EE4  ONLINE       0     0     0  at disk6s2

errors: No known data errors


The cache vdev, currently at disk6s2, is neither large nor fast:

  • 8 GB Kinsgston DataTraveler 400 (product ID 0x1614).

Without benchmarking

For at least some things, at least some of the time, the Mac does feel a little more responsive with the L2ARC than without.

I'll work in this way for maybe a couple of weeks before summarising.

The mixture on the internal disk

Code: Select all
macbookpro08-centrim:~ gjp22$ diskutil list disk0
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *750.2 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS swap                    32.0 GB    disk0s2
   3: FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF               536.9 MB   disk0s3
   4:                  Apple_HFS spare                   671.1 MB   disk0s4
   5:          Apple_CoreStorage                         99.5 GB    disk0s5
   6:                 Apple_Boot Boot OS X               650.0 MB   disk0s6
   7:          Apple_CoreStorage                         616.3 GB   disk0s7
   8:                 Apple_Boot Boot OS X               134.2 MB   disk0s8
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Re: a small USB flash drive as L2ARC for my home directory

Post by grahamperrin » Fri Mar 22, 2013 7:09 pm

grahamperrin wrote:8 GB Kinsgston DataTraveler 400


I don't know whether people should laugh or cry at this choice of hardware for L2ARC.

Quantitatively, for maybe a day or two I experimented with secondarycache=metadata. Nothing conclusive, but output from zpool iostat suggested that not much of the 8 GB would be used in my cases (restarting the Mac more often than an average user). I could have experimented with longer warm-up periods but I decided to revert to secondarycache=all.

Qualitatively, over the past two months or so, it does sometimes feel that overall performance benefits – just a little – from attachment of this device to the MacBookPro5,2. For example: new tabs in Safari whilst the Mac was extraordinarily busy with other things – I had a sense that with the thumb drive, there'd be a little less waiting time in response to Command-T.

Using an ordinary thumb drive in this way is not something I'd recommend. I'm toying, rarely paying attention to stats …

Postscript

One of the reasons for me not bothering to benchmark or measure, in recent weeks, has been my use of pre-release builds of the OS.
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