[zfs-discuss] ZIL waste of space?

Gionatan Danti g.danti at assyoma.it
Sun Feb 4 08:59:36 EST 2018


> Hi,
> 
> While I'm looking to buy some hardware for a new ZFS machine, I find
> myself wondering about a good solution for ZIL SSD since this machine
> will offer iSCSI services to many ESXi machines (it's for a home lab
> but yet, I want a good speed since the connection will be 10Gb).
> 
> So I look at the ZEUS solution, the price is sky high - $600-800 on
> eBay. skipping.
> Then I looked at Intel 900P - great device, but won't work since it
> requires UEFI 2.3.1 and PCI 3.0, the server will be IBM/Lenovo X3650
> M3, only PCIe 2.0.
> And then I looked at Intel DC S3710 - 200GB for around $250, looks
> great.
> 
> But my main problem is waste: ZIL takes only few gigabytes, which
> means out of 200GB, ~195GB will be unusable for anything else. I know
> I can partition the drive for ZIL and L2ARC but if I understood
> correctly, it's not recommended (I'll use ZoL but looking at FreeNAS,
> it's not even possible [at least using their web interface]).
> 
> What would you recommend?

I see no problem in splitting your SSD device in two partitions to use 
it *both* for ZIL and L2ARC.
Moreover, as I tend to avoid boot on ZFS, one additional SSD partition 
can be dedicated to more traditional (and better integrated) filesystem 
for boot duties.

Some months ago I build a consumer-grade virtualization system with 4x 
2TB HDDs and 2x 500GB SSDs. The two SSDs were split in three partition: 
one for a ~60 GB MD-raid1 boot device, a 4 GB one for ZIL and ~ 250 GB 
for L2ARC. I left quite a bit of space available for overprovisioning 
(not partitioned).

Even if your workload is not synchronized-write heavy, I would almost 
always recommend a SLOG (dedicated ZIL device) to avoid excessive free 
space fragmentation (see here for example: 
http://thomas.gouverneur.name/2011/06/20110609zfs-fragmentation-issue-examining-the-zil/).

Regards.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
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