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Re: xfs: add FITRIM support

To: xfs@xxxxxxxxxxx
Subject: Re: xfs: add FITRIM support
From: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Date: Thu, 6 Jan 2011 09:10:29 +0100
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Lukas Czerner <lczerner@xxxxxxxxxx>
In-reply-to: <20110105225039.GD8322@dastard>
Organization: it-management http://it-management.at
References: <20101125112304.GA4195@xxxxxxxxxxxxx> <201101052307.38379@xxxxxx> <20110105225039.GD8322@dastard>
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On Mittwoch, 5. Januar 2011 Dave Chinner wrote:
> No state or additional on-disk
> structures are needed for xfs_fsr to do it's work....

That's not exactly the same - once you defraged a file, you know it's 
done, and can skip it next time. But you dont know if the (free) space 
between block 0 and 20 on disk has been rewritten since the last trim 
run or not used at all, so you'd have to do it all again.
> The background trim is intended to enable even the slowest of
> devices to be trimmed over time, while introducing as little runtime
> overhead and complexity as possible. Hence adding complexity and
> runtime overhead to optimise background trimming tends to defeat the
> primary design goal....

It would be interesting to have real world numbers to see what's "best". 
I'd imagine a normal file or web server to store tons of files that are 
mostly read-only, while 5% of it a used a lot, as well as lots of temp 
files. For this, knowing what's been used would be great.

Also, I'm thinking of a NetApp storage, that has been setup to run 
deduplication on Sunday. It's best to run trim on Saturday and it should 
be finished before Sunday. For big storages that might be not easy to 
finish, if all disk space has to be freed explicitly.

And wouldn't it still be cheaper to keep a "written bmap" than to run 
over the full space of a (big) disk? I'd say depends on the workload.

mit freundlichen Grüssen,
Michael Monnerie, Ing. BSc

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