ZFS: ZIL with only one additional disk and how secure?

Olivier Smedts olivier at gid0.org
Wed Nov 28 13:38:50 UTC 2012


2012/11/28 O. Hartmann <ohartman at zedat.fu-berlin.de>:
> Hello,
> I have a naive question.
> I read about speeding up NFSv4 shared ZFS array. I use a RAIDZ1 volume
> made up from 5 times 3TB harddrives, attached to a ICH10 SATA controller
> on a FreeBSD 10.0-CURRENT box. The maximum performance of that array
> never goes beyond 45 - 51 MB/s and levels out very often at 12 - 35 MB/s
> when used as a NFSv4 share and 1 GBit LAN. The local system harddisk,
> attached to the sixth SATA port of the same controller and containig a
> UFS2 filesystem, is capable of doing tasks with 60 - 80 MB/s (peak) when
> used as a NFSv4 share with another FreeBSD 10.0-CURRENT box.
> I used several reported tweaks on the RAIDZ1 ZFS volume exporting it as
> a NFSv4 volume, so I was capable of raising the throughput from sad 3
> MB/s up to the 30 MB/s sustained.
> Also I was told that adding a dedicated ZIL drive could speed up things
> up to 90 MB/s with the mentioned construction of a RAIDZ1 over NFSv4. It
> is always suggested to add SSDs, a pair, for security reasons.
> My question in conrete is now: Do I need two (2) ZIL drives in a mirror?
> I guess this is considered due to security issues which lead to the next
> question: If it is possible to use only one ZIL drive and this drive
> gets corrupted, is the whole ZFS array corrupted, then?

You don't "need" two because if the ZIL is corrupted you'll only loose
the data since the last TXG, but no metadata. Make sure you have an
up-to-date pool.

But you'll *need* a battery cache or supercaps on the SSD(s) so that
they flush their caches in case of a power failure.

> The minor question regards to the use of SSDs: Is it possible to gain
> speedup also from an ordinary disk dedicated to the ZFS array connected
> to a additional SATA controller? The SATA controller should be fast
> enough to serve a bandwith of 90 - 100 MB/s (theoretically) over 1 GBit
> lines when using the ZFS array as a NFSv4 export (the LAN is limiting,
> so, but 80 - 90 MB/s is possible on the specific box, the limiting
> factor at the moment is the bad performance of ZFS).

I don't think so, or not much. What makes SSDs appealing for ZIL is
that they have very good access times / latencies.

> The box is a quad core system at 3 GHz (Intel Q6600) with 8 GB of RAM
> running most recent FreeBSD 10.0-CURRENT/amd64.
> Thanks in advance,
> Oliver


Olivier Smedts                                                 _
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