zfs discuss - Jun 2011 - Separate Log Devices

The server I have currently have only has 2GB of ram. At some point, I 
will be adding more ram to the server but I''m not sure when.

I want to add a mirrored zil. I have 2 Intel 32GB SSDSA2SH032G1GN drives

As such, I have been reading the ZFS Best Practices Guide
http://www.solarisinternals.com/wiki/index.php/ZFS_Best_Practices_Guide#Separate_Log_Devices

The guide suggests that the zil be sized to 1/2 the amount of ram in the 
server which would be 1GB.

I have a couple of questions

What happens if I oversize the zil?
If I create a 1GB slice for the zil, can I add another slice for another 
zil in the future when more ram is added?

Thanks
Karl








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> The guide suggests that the zil be sized to 1/2 the amount of ram in the 
> server which would be 1GB.
The ZFS Best Practices Guide does detail the absolute maximum
size the ZIL can grow in theory, which as you stated is 1/2 the size
of the host''s physical memory.  But in practice, the very next bullet 
point details the log device sizing equation which we have found to 
be a more relevant indicator.  Excerpt below:

"For a target throughput of X MB/sec and given that ZFS pushes 
transaction groups every 5 seconds (and have 2 outstanding), we also 
expect the ZIL to not grow beyond X MB/sec * 10 sec. So to service 
100MB/sec of synchronous writes, 1 GB of log device should be 
sufficient."
> What happens if I oversize the zil?
"Oversizing" the log device capacity has no negative repercussions
other
than the under utilization of your SSD.
> If I create a 1GB slice for the zil, can I add another slice for another 
> zil in the future when more ram is added?
If the question is if multiple disk slices can be striped to aggregate 
capacity, then the answer is yes.  Be aware with most SSDs, including 
the Intel X25-E, using a disk slice instead of the entire device will 
automatically disable the on-board write cache.

Christopher George
Founder / CTO
http://www.ddrdrive.com/
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