[zfs-discuss] Seeking ZFS, ZIL, and L2ARC configuration comments

Gregor Kopka gregor at kopka.net
Thu Dec 15 19:28:10 EST 2011


> It runs as a*pretty light duty*
> media server, email server, web server, and database server.
Forget about the external ZIL.
> Since it is just a small home server I don't expect to get much
> acceleration out of an external ZIL, but was thinking of dedicating 4 or
> 8 GB to it.
Would only make sense in case you expect the need to sustain plenty of 
sync writes.
It also depends on database usage. It /could/ be an option (for a home 
server) to get speed is to use an UPS and set sync=off for the database 
(and hope for the box not to reboot because of a kernel bug).
> I was thinking of 60GB or so for the L2ARC, and the rest for
> the root drive.
In case you want / on the SSD have a image ready since SSDs still tend 
to die without warning.
Since / usually is static data i would suggest to better buy 2 good 
USB-Sticks, MD mirror them and boot from this - fast enough for boot and 
reads, stuff that's usually written (/home, subset of /var) in the pool.
> The 2TB drives will be run mirrored.
Maybe get a third and run raidz instead of 2way mirror.
> I'm not quite adventurous enough to go with ZFS root, so I am planning to
> rsync or lsyncd / onto ZFS.
Does your / change often enough to have it lsynced?
For me the lazy rsync after an update does the trick.
> Since I really care about my home directory I
> was thinking of setting copies=2 for that filesystem (which gives 4
> copies) which is massive overkill, but the contents are pretty small. The
> bulk directories and downloads folders would get separate filesystems
> with more relaxed settings.
Sounds reasonable.
Maybe do a zfs send/recv to a pool on an USB Disk from time to time to 
secure against lightning.
> Questions:
>
> Is the ZIL worth it at all given that all my (small) databases will
> reside on the SSD?
Put them into the pool (to have the written data safe) and have them 
cached in RAM for speed.
> How much L2ARC should I use, if any?
Just test it with and without the SSD as cache device, in case you don't 
find a benefit you really see as worthwhile: better put the SSD as 
system disk into your /desktop/ (in case you have one) _and_ image/rsync 
it into the pool (so you can quickly recover in case the SSD dies).
> Would it make more sense (speed wise) to have /home on the SSD and lsyncd
> the contents to ZFS?
Depends on workload. Best solution still is having it cached in RAM.
> Should I go the extra mile and use ZFS root and dedicate most of the SSD
> to L2ARC?
Not having / on ZFS has also the benefit that zfs can be restarted 
without having to reboot the machine (in case something gets stuck in 
'dataset is busy', ARC outgrows your memory or whatever reason you might 
encounter).
> Or hold this SSD for another machine and buy a much smaller one for the
> L2ARC?
See above, YMMV.

Cheers,
     Gregor
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