I read the "ZFS_Best_Practices_Guide" and "ZFS_Evil_Tuning_Guide", and have
some
questions:
1. Cache device for L2ARC
Say we get a decent ssd, ~500MB/s read/write. If we have a 20 HDD zpool
setup shouldn't we be reading at least at the 500MB/s read/write range? Why
would we want a ~500MB/s cache?
2. ZFS dynamically strips along the top-most vdev's and that "performance for
1
vdev is equivalent to performance of one drive in that group". Am I correct in
thinking this means, for example, I have a single 14 disk raidz2 vdev zpool,
the
disks will go ~100MB/s each , this zpool would theoretically read/write at
~100MB/s max (how about real world average?)? If this was RAID6 I think this
would go theoretically ~1.4GB/s, but in real life I am thinking ~1GB/s (aka 10x-
14x faster than zfs, and both provide the same amount of redundancy)? Is my
thinking off in the RAID6 or RAIDZ2 numbers? Why doesn't ZFS try to dynamically
strip inside vdevs (and if it is, is there an easy to understand explanation
why
a vdev doesn't read from multiple drives at once when requesting data, or why a
zpool wouldn't make N number of requests to a vdev with N being the number of
disks in that vdev)?
Since "performance for 1 vdev is equivalent to performance of one drive in that
group" it seems like the higher raidzN are not very useful. If your using
raidzN
your probably looking for a lower than mirroring parity (aka 10%-33%), but if
you try to use raidz3 with 15% parity your putting 20 HDDs in 1 vdev which is
terrible (almost unimaginable) if your running at 1/20 the "ideal" performance.
Main Question:
3. I am updating my old RAID5 and want to reuse my old drives. I have 8 1.5TB
drives and buying new 3TB drives to fill up the rest of a 20 disk enclosure
(Norco RPC-4220); there is also 1 spare, plus the bootdrive so 22 total. I want
around 20%-25% parity. My system is like so:
Main Application: Home NAS
* Like to optimize max space with 20%(ideal) or 25% parity - would like
'decent'
reading performance
- 'decent' being max of 10GigE Ethernet, right now it is only 1 gigabit
Ethernet but hope to leave room to update in future if 10GigE becomes cheaper.
My RAID5 runs at ~500MB/s so was hoping to get at least above that with the 20
disk raid.
* 16GB RAM
* Open to using ZIL/L2ARC, but, left out for now: writing doesn't occur much
(~7GB a week, maybe a big burst every couple months), and don't really read
same
data multiple times.
What would be the best setup? I'm thinking one of the following:
a. 1vdev of 8 1.5TB disks (raidz2). 1vdev of 12 3TB disks (raidz3)?
(~200MB/s reading, best reliability)
b. 1vdev of 8 1.5TB disks (raidz2). 3vdev of 4 3TB disks (raidz)? (~400MB/s
reading, evens out size across vdevs)
c. 2vdev of 4 1.5TB disks (raidz). 3vdev of 4 3TB disks (raidz)? (~500MB/s
reading, maximize vdevs for performance)
I am leaning towards "a." since I am thinking "raidz3"+"raidz2" should provide
a
little more reliability than 5 "raidz1"'s, but, worried that the real world
read/write performance will be low (theoridical is ~200MB/s, and, since the 2nd
vdev is 3x the size as the 1st, I am probably looking at more like 133MB/s?).
The 12 disk array is also above the "9 disk group max" recommendation in the
Best Practices guide, so not sure if this affects read performance (if it is
just resilver time I am not as worried about it as long it isn't like 3x
longer)?
I guess I'm hoping "a." really isn't ~200MB/s hehe, if it is I'm leaning
towards
"b.", but, if so, all three are downgrades from my initial setup read
performance wise -_-.
Is someone able to correct my understanding if some of my numbers are off, or
would someone have a better raidzN configuration I should consider? Thanks for
any help.
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