L2ARC

In my previous posts are links posted about the advantage of ZFS.

I will repeat in short some highlights:

-Checksums watch over data integrety on disks.

-It can repair corrupt and bad sectors. ZFS has a repair tool called “scrub” which examines and repairs Silent Corruption and other problems.

-It makes silent errors (normally repaired by the internal software of modern HD) visible

-ZFS can function as a software RAID. However, it needs to have direct contact with the discs. A hardware RAID needs to configured as JBOD (Just a bunch of discs).

One of the most interesting features, I like, is: L2ARC

Most data will be cached into RAM memory, so it will be quickly available when needed.

If you add a SSD in ZFS, you can configure it as a second cache.

The read SSD cache is called L2ARC and is similar to Level 2 CPU cache.

The write SSD cache is called the Log Device, and it is used by the ZIL (ZFS Intent Log). Read more about this on http://en.wikipedia.org/wiki/Zfs. With ZIL ZFS can watch the consistency of its filesystem and its applications. This is a log almost never read unless there is a problem like power-loss. You can put ZIL on a different SSD.  However lots of read and write cycles in this log will happen and your SSD will break down quicker.

So the last one is not really interesting (you might want to mirror ZIL on a second SSD) for a home user like me. But L2ARC is. The read performance improvement is huge with a SSD as L2ARC read cache. Image this with a RAID and you will have a fast and reliable fileserver. It funcions a bit like Intels SRT technology, Seagates HDD with a SSD cache and  recently Apples Fusions discs (which is also a HDD plus SSD).

L2ARC is also safer then e.g. Intels SRT, because Intels system cannot detect when data on a SSD cache gets corrupt. ZFS can detect corruption on a SSD cache: it will simply deactivate the SSD as read cache and read the data directly from its normal filesystem.

So here you are. ZFS RAID with L2ARC for a safe and reliable fileserver.

I will soon tell more about the filesystem. It wil be my next project: a reliable AMD Brazos home fileserver.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s