Main references

ZFS L2ARC (Brendan Gregg) (2008-07-22) and ZFS and the Hybrid Storage Concept (Anatol Studler's Blog) (2008-11-11) include the following diagram:

a ZFS pyramid view of ARC, L2ARC, ZIL and a disk storage pool

Question

Should I interpret the vertical white line – at the SSDs layer – as a preference to use separate SSDs –

a preference to not mix L2ARC and ZIL on a single disk?

Background (response to comments)

Personally, at home I'm unlikely to use either L2ARC or ZIL with any computer that's available to me. (My everyday computer is a MacBookPro5,2 with 8 GB memory and hybrid Seagate ST750LX003-1AC154. No plans to replace the optical drive with an SSD.)

Elsewhere: at work there'll be some repurposing of kit, but I don't have a date or full details. (Xserve RAID x2 in the mix … at this time I don't imagine giving those to ZFS, but I keep an open mind.)

My curiosity about SSD best practices for both L2ARC and ZIL began whilst following performance-related discussions in the ZEVO area – in particular the topic mentioned below, where a user has both L2ARC and ZIL on a single disk.

Other references and discussions

L2ARC Screenshots (Brendan Gregg) (2009-01-30)

SLOG Screenshots (Brendan Gregg) (2009-06-26)

[zfs-discuss] ZFS root backup/"disaster" recovery, and moving root pool (2011-01-10) recommends against a mixture of three things (root pool, ZIL and L2ARC) on a single disk –

… not worth the headaches that can occur when trying to manage all 3 on the same disk. For example, if you decide to reinstall and accidentally clobber the contents of the ZIL for your data pool. Don't share disks for pool components or across pools to keep management and recovery simple. …

– I'm more interested in whether it's recommended to not mix two of those things on a single disk.

https://superuser.com/a/238744/84988 (2011-01-28) mentions "cache (L2ARC cache) and write log (ZIL) onto SSD" (singular). However as it relates to FUSE and Windows, I don't treat that answer as particularly relevant to more commonplace and performance-minded uses of ZFS.

@ChrisS mentioned ZIL and L2ARC in The Comms Room on 2011-08-16.

http://forums.macrumors.com/showpost.php?p=14248388 (2012-01-31) discusses multiple SSDs:

Something you need to understand about ZFS: It has two different kinds of cacheing, read and write (L2ARC and ZIL) that are typically housed on SSD's. The ZIL is the write cache. That's probably where this misconception comes from. The ZIL is getting hammered (assuming an active system) with every write that occurs to the zpool. The problem is that using an mlc-based SSD as a ZIL causes them to wear out and fail quite quickly. You need a (much more expensive) slc-based SSD to be used as a ZIL drive.
Having a zpool made up entirely of SSD's is not only possible, but it works quite well. It also basically eliminates the need for separate drives for the ZIL and L2ARC. Yes, you don't have TRIM support, but based on the copy-on-write nature of ZFS, that's probably a good thing.
With that said, ZFS does NOT play well with nearly full (say, 85% or higher) zpools. Performance begins to drop off significantly - regardless of whether you're using rotational magnetic media or solid-state. Lack of TRIM support would probably exacerbate that problem, but it's already a problem.

https://serverfault.com/a/397431/91969 (2012-06-11) recommends: