Travis Tabbal wrote:
I have a few old drives here that I thought might help me a little, though not
at much as a nice SSD, for those uses. I'd like to speed up NFS writes, and
there have been some mentions that even a decent HDD can do this, though not to
the same level a good SSD will.
The 3 drives are older LVD SCSI Cheetah drives. ST318203LW. I have 2 controllers I could use, one appears to be a RAID controller with a memory module installed. An Adaptec AAA-131U2. The memory module comes up on Google as a 2MB EDO DIMM. Not sure that's worth anything to me. :)
The other controller is an Adaptec 29160. Looks to be a 64-bit PCI card, but the machine it came from is only 32-bit PCI, as is my current machine.
What say the pros here? I'm concerned that the max data rate is going to be
somewhat low with them, but the seek time should be good as they are 10K RPM (I
think). The only reason I thought to use one for L2ARC is for dedupe. It sounds
like L2ARC helps a lot there. This is for a home server, so all I'm really
looking to do is speed things up a bit while I save and look for a decent SSD
option. However, if it's a waste of time, I'd rather find out before I install
them.
I'd like to hear (or see tests of) how hard drive based ZIL/L2ARC can
help RAIDZ performance. Examples would be large RAIDZ arrays such as:
8+ drives in a single RAIDZ1
16+ drives in a single RAIDZ2
24+ drives in a single RAIDZ3
(None of these are a series of smaller RAIDZ arrays that are striped.)
From the writings I've seen, large non-striped RAIDZ arrays tend to
have poor performance that is more or less limited to the I/O capacity
of a single disk. The recommendations tend to suggest using smaller
RAIDZ arrays and then striping them together whereby the RAIDZ provides
redundancy and the striping provides reasonable performance. The
advantage of large RAIDZ arrays is you can get better protection from
drive failure (e.g. one 16 drive RAIDZ2 can lose any 2 drives vs two 8
drive RAIDZ1 striped arrays that can lose only one drive per array).
So what about using a few dedicated two or three way mirrored drives for
ZIL and/or L2ARC, in combination with the large RAIDZ arrays? The
mirrored ZIL/L2ARC would serve as a cache to the slower RAIDZ.
One model for this configuration is the cloud based ZFS test that was
done here which used local drives configured as ZIL and L2ARC to
minimize the impact of cloud latency, with respectable results:
http://blogs.sun.com/jkshah/entry/zfs_with_cloud_storage_and
The performance gap between local mirrored disks used for ZIL/L2ARC and
a large RAIDZ is not nearly as large as the gap that was addressed in
the cloud based ZFS test. Is the gap large enough to potentially
benefit from HDD based mirrored ZIL/L2ARCs? Would SSD based ZIL/L2ARCs
be necessary to see a worthwhile performance improvement?
If this theory works out in practice,useful RAIDZ array sizes may not be
as limited as much as they have been to date via best practices
guidelines. Admins may then be able to choose to have larger more
strongly redundant RAIDZ arrays while still keeping most of the
performance of smaller striped RAIDZ arrays by using mirrored ZIL/L2ARC
disks or SSDs.
-hk
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