4 ms·

I don't think the author of this post really understands ZFS. Is that a 4 disk mirror? You'd be much faster using a raid10 there. ZFS 'cache' devices are't fo

by hoistor 11y ago

I don't think the author of this post really understands ZFS. Is that a 4 disk mirror? You'd be much faster using a raid10 there. ZFS 'cache' devices are't for write cache either, you'd want a log device for write perf reasons if that's your constraint... Even then it's unlikely to make a difference on a workstation workload with 16GB of memory..

justinsaccount 11y ago
Yeah.. it is a subtle difference between zpool create home mirror a b c d cache e and zpool create home mirror a b mirror c d cache e
- dr_zoidberg 11y ago
  Could you elaborate on the semantics of "cache"? I do understand the mirror keyword, but I know next to nothing about ZFS and don't really get what "cache" means, considering bouth your and OPs comment.
  - justinsaccount 11y ago
    You can use a fast disk like an SSD as a 2nd level cache device after ram. So reads will first check the main ram cache first, then the ssd cache disk before finally hitting the underlying storage.
    - feld 11y ago
      L2ARC is extremely well documented right in the code https://svnweb.freebsd.org/base/head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c?view=markup#l5587 https://svnweb.freebsd.org/base/head/sys/cddl/contrib/openso...
  - clord 11y ago
    a cache drive is l2arc. Brendan Gregg's post[0] explains it well, but the TLDR is that l2arc is very much like an l2 cache in a cpu. Recent and relevant items are held on a fast drive in case it will be needed in the near future. It is really an L2 cache: L1 is called ARC and is held in dram. If I recall correctly, the extra indexes and tags mean l2arc requires more memory to work efficiently. [0]: https://blogs.oracle.com/brendan/entry/test https://blogs.oracle.com/brendan/entry/test
    - semi-extrinsic 11y ago
      When your "medium-sized test server" has 128 GB RAM and half a TB worth of SSDs... in 2008! Damn.
- abrookewood 11y ago
  Correct me if Im wrong, but he just built a pool with 3 layers of parity right? The first drive with three drives mirroring it? Where as the later is effectively RAID10, a stripe across 2 mirrored vdevs.
ochoseis 11y ago
I think he might just be glossing over some details? At any rate, from what I understand ZFS has a 'zil' write cache for small log writes, and a l2arc for common reads that don't fit into memory. This is just a dev setup, but for real work I think you'd want your zil to also be mirrored so you don't lose writes if an ssd goes bad. For anyone interested this link has a few more details: http://www.45drives.com/wiki/index.php/FreeNAS_-_What_is_ZIL_%26_L2ARC http://www.45drives.com/wiki/index.php/FreeNAS_-_What_is_ZIL...

by hoistor 11y ago

justinsaccount 11y ago

Yeah.. it is a subtle difference between zpool create home mirror a b c d cache e and zpool create home mirror a b mirror c d cache e

dr_zoidberg 11y ago
Could you elaborate on the semantics of "cache"? I do understand the mirror keyword, but I know next to nothing about ZFS and don't really get what "cache" means, considering bouth your and OPs comment.
- justinsaccount 11y ago
  You can use a fast disk like an SSD as a 2nd level cache device after ram. So reads will first check the main ram cache first, then the ssd cache disk before finally hitting the underlying storage.
  - feld 11y ago
    L2ARC is extremely well documented right in the code https://svnweb.freebsd.org/base/head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c?view=markup#l5587 https://svnweb.freebsd.org/base/head/sys/cddl/contrib/openso...
- clord 11y ago
  a cache drive is l2arc. Brendan Gregg's post[0] explains it well, but the TLDR is that l2arc is very much like an l2 cache in a cpu. Recent and relevant items are held on a fast drive in case it will be needed in the near future. It is really an L2 cache: L1 is called ARC and is held in dram. If I recall correctly, the extra indexes and tags mean l2arc requires more memory to work efficiently. [0]: https://blogs.oracle.com/brendan/entry/test https://blogs.oracle.com/brendan/entry/test
  - semi-extrinsic 11y ago
    When your "medium-sized test server" has 128 GB RAM and half a TB worth of SSDs... in 2008! Damn.
abrookewood 11y ago
Correct me if Im wrong, but he just built a pool with 3 layers of parity right? The first drive with three drives mirroring it? Where as the later is effectively RAID10, a stripe across 2 mirrored vdevs.

ochoseis 11y ago

I think he might just be glossing over some details? At any rate, from what I understand ZFS has a 'zil' write cache for small log writes, and a l2arc for common reads that don't fit into memory. This is just a dev setup, but for real work I think you'd want your zil to also be mirrored so you don't lose writes if an ssd goes bad. For anyone interested this link has a few more details: http://www.45drives.com/wiki/index.php/FreeNAS_-_What_is_ZIL_%26_L2ARC http://www.45drives.com/wiki/index.php/FreeNAS_-_What_is_ZIL...