2011-07-16 18:48, Tiernan OToole пишет:
thats not a typo... I was thinking 2 pools, 800gb each, and
mirrored... think i should mess around with this setup a bit more and
see what i can get working... might work better if i just move them
into a new enclosure... we see what happens...
Well, in terms of mirroring over stripes, if any component of any stripe
breaks,
the whole half of the mirror is degraded. If another drive from another
half
also breaks, you're in trouble.
Overall raid01 is assumed to be less reliable than raid10, especially if
there
are more than two couples of drives.
Thanks for the info on the USB drives... if the ZIL drive falls over,
does ZFS not recover well? do i need to reboot fully?
In theory, you should have 2 drives (slices) mirrored for a ZIL.
If one breaks, the other goes on. If all break, the pool reverts to on-disk
ZIL areas in the course of several TXG syncs (a few seconds). So the
big problem is constrained in poweroff/panic of the system in these
few seconds when no ZIL is used. But in practice it may take ZFS a
long time to detect the failure/death of the device, because it is not
ZFS itself doing it - lower-level drivers like "sd" must detect and report
the problem or the timeout...
After all, I did not try a ZIL, or not for a long enough time to be certain
of anything.
From theory again, its critical property is fast writes with low latency,
and reliability. Even in comparison with a dedicated HDD slice,
my USB stick could not compete with either. But that was only a
test setup, so the sticks were not "optimized" or "fast" (3-7Mb/s).
You might want to look at CF-IDE adapters - new professional
photo CF cards mention 600x (90Mb/s) speeds at reads and
30 to 90Mb/s writes, and some internal redundancy. Probably
not cheap though... But many small mobos include a CF port,
or a small adapter can be plugged into an IDE port. There were
some adapters which naturally "split" one IDE port into master
and slave CF ports. Or see a PCMCIA-CF port for a laptop...
Or maybe even use a USB-CF card reader thingie.
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