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cache vs log

Posted: 31 Oct 2019 18:39
by mrjacobrussell
I'm sure, it been asked a lot but I have been reading around and really don't understand what I need...

My setup One X 64 GB flash drive running my extensions and jails ( I'm hoping to replace it with an SSD later).

Tank Size 7.71TB Used 3.89TB Available 3.82TB Used 48%
x 4TB drives 64mb cache 7200 rpm
Used for media backups smallest files are mp3s largest are movies and home video.

Andoria Size 7.71TB Used 1.45TB Available 6.25TB Used 18%
5 x 2TB drive 64mb cache 7200 rpm
Used for true data backup and

Version - Omnius (revision 6766)
Compiled Wednesday June 19 14:59:46 PDT 2019
Platform OS FreeBSD 11.2-RELEASE-p10 #0 r349200M: Wed Jun 19 20:26:53 CEST 2019
Platform x64-embedded on Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz
System Gateway FX6850
16gb ram

I run 1 jail with subsonic
time machine backups to this machine from 3-4 computers but scheduled as daily( not every hour).
I stream to up to Four TVs but usually only one.

My question is should I use a log or cache disks or both to argument my system to prevent data loss and help speed up data transfers and network browsing.
NFS browsing is a little slow as well.

Re: cache vs log

Posted: 31 Oct 2019 18:41
by mrjacobrussell ... MCL=byvUua

btw I did read this and it seems my drive for backups could benefit from an SSD LOG drive and my media from a cache?

posted by raulfg3

Re: cache vs log

Posted: 05 Nov 2019 05:28
by Onichan

By cache I'm guessing you mean L2ARC. That just holds data that falls out of the ARC that ZFS thinks is useful to keep in a fast secondary ARC (normally SSD). It's for reads only and it requires keeping a table of contents in system RAM anyways so it's not without cost. Also data that falls out of the ARC isn't guaranteed to go to the L2ARC. This wouldn't help with data loss or data transfers.

By log I'm guessing you mean SLOG. That can help with writes, but it only matters for synchronous writes. All writes are buffered in RAM, synchronous just gets written to the ZIL as well, which by default is the pool. By using a separate intent log such as a fast SSD it can speed up the synchronous writes. Also latency is the most important part for the SLOG so writes can be acknowledged as quick as possible. Buying any random SSD wouldn't necessarily make much of a difference. Also it does need to be mirrored to prevent data loss. So for your needs of speeding up browsing this wouldn't help. It might help with data transfers, but that really depends. If they are transfers using SMB then no it won't help as Samba is asynchronous by default. NFS normally is synchronous so it might help with writes using that, but it wouldn't help with browsing.