Oracle Rewrites Linux ZCache Compression Code

Seth Jennings of IBM proposed that ZCache be moved out of the Linux kernel's staging area and be accepted officially into the mainline tree. However, that proposal is being criticized by an Oracle engineers as they have evidently "completely rewritten zcache" and will share it soon but still doesn't see a reason for the memory compression code to leave staging.

On Friday was the kernel message by Jennings that proposes zcache to leave the kernel's staging area, with the email being accompanied by four patches to make that happen. His justification for the code leaving staging is that "Based on the level of activity and contributions we're seeing from a diverse set of people and interests, I think zcache has matured to the point where it makes sense to promote this out of staging."

Zcache is the final component needed to fully support in-kernel memory compression with the Cleancache and Frontswap components already having been moved to mainline. Zcache is a back-end to Frontswap/Cleancache that accepts their pages and then compresses them, which is useful for SANs and systems where there's slow backing/swap-devices.

Seth Jennings' proposal was immediately countered by Dan Magenheimer of Oracle. His reasons are that he's completely rewritten zcache and will post that code soon, zcache belongs within the memory management directory, and Ramster heavily duplicates zcache code while his re-write addresses this duplicated code. As far as his new zcache, "I've completely rewritten zcache and will post the rewrite soon. The redesigned code fixes many of the weaknesses in zcache that makes it (IMHO) unsuitable for an enterprise distro."

Magenheimer then followed up with, "Sorry, I'm not trying to be unfair. However, I don't see the point of promoting zcache out of staging unless it is intended to be used by real users in a real distro. There's been a lot of discussion, onlist and offlist, about what needs to be fixed in zcache and not much visible progress on fixing it...So, as I said, I am still a NACK, but if there are good reasons to duplicate effort and pursue the 'third option', let's discuss them."

It looks like zcache for in-kernel memory compression on Linux won't be leaving the staging area quite so soon.