ZFS: How to enable cache and logs.

[Alexander Leidinger]

Quoting Jeremy Chadwick <freebsd at jdc.parodius.com> (from Wed, 11 May  
2011 03:06:56 -0700):

> L2ARC devices should primarily be something with extremely fast read
> rates (e.g. SSDs).  USB1.x and 2.x memory sticks do not work well for
> this purpose given protocol and bus speed limits + overhead.  (I only
> mention them because people often think "Oh, USB flash would work great
> for this!"  I disagree.)

Using USB flash may work acceptable. It depends upon the rest of the  
system. If you have very fast harddisks (or only USB 1 hardware), USB  
flash will not give you a faster FS. If you have slow (and low-power)  
desktop disks, a fast USB flash (attention, there are also slow ones)  
connected via USB 2 (or 3) will give you a speed improvement you notice.

As a matter of fact, I have this:
  - Pentium 4
  - 1 GB RAM
  - 1 Western Digital Caviar Blue
  - 2 Seagate Barracuda 7200.10
  - an ICH5 controller (no NCQ)
  - no name cheap give-away 1 GB USB flash (so not a very fast one)

The disks are used in a RAIDZ, with the USB flash as a cache device.

My use case was connecting to a webmail system over a slow line (ADSL  
224 kilobit/s). I noticed directly when the cache was in use or not.

I also have another system, ICH 10 with NCQ, 5 disks (WD RE4 RAID) in  
RAIDZ2, Intel Xeon 4-core, 12 GB RAM. There USB flash does not make  
sense at all (and the SSD makes sense if you compare the price of the  
entire system with the price of a small or medium SSD).

For the first system, it does not make sense to spend 200 units of  
money for a SSD, the system itself is not worth much more now.  
Spending 5-10 units of money for this system is ok, and gives a speed  
improvement.

Bye,
Alexander.

-- 
Even God cannot change the past.
		-- Joseph Stalin

http://www.Leidinger.net    Alexander @ Leidinger.net: PGP ID = B0063FE7
http://www.FreeBSD.org       netchild @ FreeBSD.org  : PGP ID = 72077137