SSD and RaidZ2

M0dusPwnens

M0dusPwnens

I'm currently rebuilding a home server (used for NAS and a fair amount of number crunching) and having some trouble understanding the ideal usage of the HDDs under ZFS. I have six 1 TB SATA drives that I intend to use for a RAID-Z2 array and an mSATA SSD. What I don't quite understand is what would be the most beneficial use of the SSD. I've seen many people suggesting using it to store the OS, but I've also seen people suggesting that that's not very useful compared to other setups.

Like I said, I know there's probably a good answer out there already (the problem is more that there are too many answers out there), but I would appreciate any help pointing me at it.
 
FreeBSD can boot off a raidz2 RAID-Z2 vdev, so you could install the OS directly onto ZFS, and then use the SSD for L2ARC (a.k.a. cache vdev) and swap. Depending on the write speed of the SSD and your workload, you may even get some benefit from using it as a ZIL (aka log vdev).

If you are planning on adding more drives to the system later on, though, as part of a second 6-disk raidz2 RAID-Z2 vdev, you will want to install the OS to the SSD. The reason being that the BIOS/loader needs to be able to see every disk in the pool in order to boot from it. And there aren't many BIOSes out there that will enumerate 12 drives (most top out around 6 or 8).

Ideally, scrounging up another SSD would be the best solution. That way, you can create a separate OS pool using the SSDs in a mirror vdev; then use the SATA disks for a separate storage pool; and can use a partition on the SSDs for L2ARC for the storage pool. Thus, you can always boot (only need to enumerate 2 drives), you have redundancy for the OS, and the loss of an SSD won't affect the storage pool (L2ARC devices aren't redundant).
 
Thanks - I'll look at using it for the L2ARC and ZIL. Unfortunately, there's no way another SSD is fitting in this case (much less a whole second array - so no real need to worry about adding drives later) - the only reason I had this one in it in the first place was because the mSATA card is so small.
 
I also just rebuilt my NAS and used 2 x SSD for OS, ZIL and L2ARC by applying several partitions to these SSDS (60 GB OS/jails; 30 GB ZIL; 30 GB L2ARC). The storage is put on 6 x 3 TB HDDs, which gives me 10 TB effective space. Future expansion is planned by adding additional 6 x 3 TB HDDs, which I hope, then are a little bit cheaper than now, to give me 20 TB storage with a nice speed.

Without the ZIL/L2ARC I achieved about 500MB/s. With the ZIL/L2ARC I achieved about 550 MB/s. But these are just the results of bonnie++. For real results I have to migrate my data.

Regards,
Markus
 
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