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Date: Thu, 28 Feb 2002 10:05:34 -0800
From: Andrew Morton <akpm@zip.com.au>
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To: Pavel Machek <pavel@ucw.cz>
CC: kernel list <linux-kernel@vger.kernel.org>
Subject: Re: Sync over loop devices takes ages? [2.4.17]
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Pavel Machek wrote:
> 
> Hi!
> 
> I have a script (attached). At one point it tries to do sync... That
> sync take a long time, with disk mostly unused.

When doing (say) ext2-on-loop-on-ext2 you should always ensure
that the blocksize for the topmost filesystem is the same as
the one underneath.  So probably you wanted `mkfs.ext2 -b 4096'.

If you have a 1k blocksize filesystem loop-mounted on a 4k blocksize
filesystem, every write of a 1k block requires a read of the underlying
4k block. Which is excrutiatingly slow.

Some readahead in the loop driver would help heaps.

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