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I have been considering bcache and flashcache.

However my friend suggested me something called ZFS. In fact it says that formating disk with ZFS can actually do the exact same thing without kernel remake or whatever.

Here are some resources I found on the web

http://constantin.glez.de/blog/2011/02/frequently-asked-questions-about-flash-memory-ssds-and-zfs

It seems that it's true though there is no tutorial on how it is actually done

...e you can come up with other great uses, but that almost always involves some sort of intelligent distinction between data that is accelerated with flash and data that is stored on regular disks. The beauty of ZFS is that it does this job automatically for you.

The beauty is exactly what I want.

Okay I am convinced. I got a /sdc as a huge hard disk and I got /sdd as an ssd.

Now how do I set things up so that the /sdd becomes the zil and l2arc and the /sdd becomes real storage?

Can't we arrange that the zil and l2arc are in the same drive?

I am using centosh. Don't know what version.

I think I have a 120GB SSD.

I would like

40GB for L2ARC 40GB for ZIL 40GB for ZIL mirror.

That seems to be reasonable. Datas are cached anyway. If someone write something and before data is written to disk will L2ARC takes it from ZIL?

user4951
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  • what os are you using? – taffer Feb 01 '13 at 08:41
  • centos I am using centos. How do I know for sure anyway>? – user4951 Feb 01 '13 at 09:03
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    How do I know? `uname -a`. ZFS is a Solaris filesystem and was ported to BSD later. It is not officially supported on Linux and there are technical and legal issues with ZFS on linux. Switch to Solaris/OpenIndiana or FreeBSD if you want to use ZFS or use bcache on Linux. Also read [this](http://unix.stackexchange.com/questions/186/zfs-under-linux-does-it-work?rq=1) and [this](http://unix.stackexchange.com/questions/62383/if-i-have-an-ssd-and-a-hard-disk-in-a-system-can-the-ssd-act-as-cache-for-the-h). – taffer Feb 01 '13 at 09:14
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    @taffer, `zfs` is available for Linux as part of the [zfsonlinux](http://zfsonlinux.org] project, and though it should be considered as experimental like btrfs, it is relatively stable (I did switch from btrfs to zfs a year ago to get better stability) – Stéphane Chazelas Feb 01 '13 at 09:59

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If I understand your last statement you only have one 120 GB ssd drive on which you want put 3 partitions of 40 GB for l2ARC and 40x2 for a Zil mirror.

There are a few things to consider :

  • Even if it would eventually work, Zfs favour the use of whole disk

  • What do you hope to gain using mirrored partition on the same disk ? Because you will not gain performance, neither redundancy.

I'll advice to put the 120GB as l2arc because :

  • you usually do way more reads than write

  • It will enhance durability of your mechanical disks by not accessing them.

Moreover you cannot be safe with a non redundant Zil while it does not matter at all for L2arc as you already have you data in your pool.

Just for information, if you had 2 ssd, there are cases where using as them as a redounded zil without l2arc can be justified : If you only access a small part of your dataset and if you have a solid amount of ram, enough to observe that you already have a great hit ratio using arcstat for exemple.

M4rty
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