The USB stack in OpenSolaris is ... complex (STREAMs based!), and
probably not the most performant or reliable portion of the system.
Furthermore, the mass storage layer, which encapsulates SCSI, is not
tuned for a high number of IOPS or low latencies, and the stack makes
different assumptions about USB media than it makes for SCSI. Further,
you will not be able to get direct DMA through this stack either, so you
wind up sucking extra CPU doing data copies. I would think long and hard
before I put too many eggs in that particular basket.
Additionally, USB has the tendency to run at high interrupt rates (1000
Hz), which can have a detrimental impact on system performance and power
consumption. Its possible that mass storage devices don't have this
attribute -- I'm not sure, I've not tried to investigate it directly.
One attribute that you can rest assured of though, is that the average
latency for USB operations cannot be less than 1 ms -- which is driven
by that 1000 Hz, because USB doesn't have a "true" interrupt mechanism
(it polls). I believe that this is considerably higher than the lowest
latency achievable with PCI and SATA or SAS devices.
Generally, eSATA flash drives would be preferable for external flash
media, I think. Additionally, the SATA framework has quite recently
inherited FMA support, so you'll benefit from closer integration of FMA
and ZFS when using SATA.
- Garrett
On 5/25/2010 8:39 AM, Edward Ned Harvey wrote:
From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
boun...@opensolaris.org] On Behalf Of Kyle McDonald
I've been thinking lately that I'm not sure I like the root pool being
unprotected, but I can't afford to give up another drive bay.
I'm guessing you won't be able to use the USB thumbs as a boot device. But
that's just a guess.
However, I see nothing wrong with mirroring your primary boot device to the
USB. At least in this case, if the OS drive fails, your system doesn't
crash. You're able to swap the OS drive and restore your OS mirror.
That led me to wonder whether partitioning out 8 or 12 GB on a 32GB
thumb drive would be beneficial as an slog??
I think the only way to find out is to measure it. I do have an educated
guess though. I don't think, even the fastest USB flash drives are able to
work quickly, with significantly low latency. Based on measurements I made
years ago, so again I emphasize, only way to find out is to test it.
One thing you could check, which does get you a lot of mileage for "free"
is: Make sure your HBA has a BBU, and enable the WriteBack. In my
measurements, this gains about 75% of the benefit that log devices would
give you.
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