[lustre-devel] Lustre log question(s)

Degremont, Aurelien degremoa at amazon.com
Fri Jan 29 01:22:42 PST 2021


This is not totally correct.

First, LLOG is the underlying technology used to store and handle Lustre Changelogs. But LLOG is used for other Lustre mechanisms, like lustre configuration.
Second, Changelog is similar to an audit feature. Changelog only logs different filesystem change, mostly metadata change, but definitely not the file content change. They don't play a role at all in transaction or failure recovery. This is only an admin feature.

At the end, indeed ZIL cannot be used and Lustre has its own mechanisms to guarantee transaction are committed to disk and handle crash. Basicly, I/O are not acknowledge to Lustre clients before the data is actually on disk. In case of server crash, the Lustre client will replay all non-acknowledge I/Os to ensure none of them are lost.

Changelog is not needed in your case.


De : lustre-devel <lustre-devel-bounces at lists.lustre.org> au nom de Sudheendra Sampath <sudheendra.sampath at gmail.com>
Date : jeudi 28 janvier 2021 à 21:43
À : "lustre-devel at lists.lustre.org" <lustre-devel at lists.lustre.org>
Objet : [EXTERNAL] [lustre-devel] Lustre log question(s)

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I am trying to evaluate osd-zfs based MDS and OST deployment on a 2 node setup.

I have the following questions about Lustre log:
1.       Is changelog and llog both the same, in the sense are they synonymous with each other?
2.       I understand that ZIL is currently not supported in Lustre version 2.12.2.  My question is :
1.       My understanding is that transactions (in general) need some logging mechanism for it to work in 'all or none' scenarios.  Please correct me if my understanding is incorrect.   I understand that changelog has to be enabled so that filesystem changes are recorded to be replayed after a crash.  How does Lustre transactions work if there is no intent log/changelog ?
2.       Does it mean that if changelog is NOT enabled and there is a crash, we risk losing all changes/updates to the filesystem ?
Appreciate your timely response and Thank you for your help.


Sudheendra Sampath
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