zil on root devices?

Steve Costaras stevecs at chaven.com
Tue May 17 07:24:27 EDT 2011

Good question, I don't remember it ever being brought up really on the zfs forums. I would imagine it's probably a combination of:

 - separate log devices were not part of the spec until later
 - root pools (in solaris world) are very small only containing the basic 'system' itself (say 10-20GB) and data is very static as a result so no real need.
 - keeping recovery as simple as possible (generally if you have a rpool issue it's going to be a late night)
 - most systems don't have the bays for additional mirrored drives for a log. (M3K's have 4 bays, a lot of others have 2. The T5xxx can have up to 8 but generally since you are looking for redundancy primarily in enterprise systems you configure a mirror (2-way or 3-way) plus at least one hot spare for your rpool. More of a architectural design issue but probably plays into the desire of wanting it.

Though would be interested as well if anyone knows more internal details that may be a reason. 

-----Original Message-----
From: Uncle Stoatwarbler [mailto:stoatwblr at gmail.com]
Sent: Tuesday, May 17, 2011 04:58 AM
To: zfs-discuss at zfsonlinux.org
Cc: 'Steve Costaras'
Subject: Re: zil on root devices?
On 16/05/11 23:51, Steve Costaras wrote:
> root pools cannot have a seperate log device.

Yes, I know that. The question is why.

> -----Original Message-----
> *From:* Uncle Stoatwarbler [mailto:stoatwblr at gmail.com]
> *Sent:* Monday, May 16, 2011 05:31 PM
> *To:* zfs-discuss at zfsonlinux.org
> *Subject:* zil on root devices?
> I've just tried to add a ssd as write-intent log on my zfs-on-root
> installation and it won't let me do it (presumably because the ssd
> may not be available on reboot, etc etc). Is the issue
> insurmountable? AB

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