I am thinking about a new laptop. I see that there are
a number of higher-performance models (incidenatlly, they
are also marketed as "gamer" ones) which offer two SATA
2.5" bays and an SD flash card slot. Vendors usually
position the two-HDD bay part as either "get lots of
capacity with RAID0 over two HDDs, or get some capacity
and some performance by mixing one HDD with one SSD".
Some vendors go as far as suggesting a highest performance
with RAID0 over two SSDs.
Now, if I were to use this for work with ZFS on an
OpenSolaris-descendant OS, and I like my data enough
to want it mirrored, but still I want an SSD performance
boost (i.e. to run VMs in real-time), I seem to have
a number of options:
1) Use a ZFS mirror of two SSDs
- seems too pricey
2) Use a HDD with redundant data (copies=2 or mirroring
over two partitions), and an SSD for L2ARC (+maybe ZIL)
- possible unreliability if the only HDD breaks
3) Use a ZFS mirror of two HDDs
- lowest performance
4) Use a ZFS mirror of two HDDs and an SD card for L2ARC.
Perhaps add another "built-in flash card" with PCMCIA
adapters for CF, etc.
Now, there is a couple of question points for me here.
One was raised in my recent questions about CF ports in a
Thumper. The general reply was that even high-performance
CF cards are aimed for "linear" RW patterns and may be
slower than HDDs for random access needed as L2ARCs, so
flash cards may actually lower the system performance.
I wonder if the same is the case with SD cards, and/or
if anyone encountered (and can advise) some CF/SD cards
with good random access performance (better than HDD
random IOPS). Perhaps an extra IO path can be beneficial
even if random performances are on the same scale - HDDs
would have less work anyway and can perform better with
their other tasks?
On another hand, how would current ZFS behave if someone
ejects an L2ARC device (flash card) and replaces it with
another unsuspecting card, i.e. one from a photo camera?
Would ZFS automatically replace the L2ARC device and
kill the photos, or would the cache be disabled with
no fatal implication for the pools nor for the other
card? Ultimately, when the ex-L2ARC card gets plugged
back in, would ZFS automagically attach it as the cache
device, or does this have to be done manually?
Second question regards single-HDD reliability: I can
do ZFS mirroring over two partitions/slices, or I can
configure "copies=2" for the datasets. Either way I
think I can get protection from bad blocks of whatever
nature, as long as the spindle spins. Can these two
methods be considered equivalent, or is one preferred
(and for what reason)?
Also, how do other list readers place and solve their
preferences with their OpenSolaris-based laptops? ;)
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