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

O. Hartmann ohartman at zedat.fu-berlin.de
Wed Nov 28 14:07:39 UTC 2012

On 11/28/12 14:38, Olivier Smedts wrote:
> Hi,
> 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
> Cheers

Good to hear.
Thanks a lot,



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