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.
There are or at least have been some issues with ZFS and devices.
Here's one that is still open:
Bug 4755 - ZFS boot does not work with removable media (usb flash memory)
http://defect.opensolaris.org/bz/show_bug.cgi?id=4755
Regarding performance...USB flash drives vary significantly in
performance from one another between brands and models. Some get close
to USB 2.0 theoretical limits, others just barely exceed USB 1.1. Vista
and Windows 7 support the use of USB flash drives for ReadyBoost, a
caching system to reduce application load times. Windows tests have
shown that with enough RAM, that ReadyBoost caching offers little
additional performance (as Windows does make use of system RAM for file
caching too).
I think using good USB flash drives has the potential to improve
performance, and if you can keep mirrored flash drives on different,
dedicated USB controllers that will help performance the most. If USB
support in OpenSolaris has is poor and has weak performance, I wonder if
an iSCSI target created out of the USB device on a Linux or Windows
system on the same network might be able to offer better performance.
Even if latency goes to 2-3ms, that's still much better than the 8.5 ms
random seek times on a 7200 rpm hard disk.
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
