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Re: [PATCH] [RFC] xfs: byte range buffer dirty region tracking

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] [RFC] xfs: byte range buffer dirty region tracking
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Tue, 25 Feb 2014 18:15:25 -0800
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1393194354-12780-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1393194354-12780-1-git-send-email-david@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
Interesting.  I had a prototype of this supporting just a single
region a while ago, but never managed to get it to pass all recovery
tests.  That was before I separated the in-core from the on-disk
log items, though.

> Hence if we just track a signle region, it will almost always cover
> the entire directory buffer - if we only modify a single entry in
> the buffer, then that's a fairly large cost in terms of log space
> and CPU overhead for random individual operations. If we decide that
> we are going to use a single range, then we may as well just use the
> dirty flag and log the entire buffer every time.

Which might not be an all that bad idea given how much log bandwith
we have available.  Defintively would be interesting to instrument
and benchmark it vs the 4 regions version.

Note that we probably should also introduce a log incompat feature to
just log the range instead of converting it to the old bitmap for v5

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