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Re: Logdev size?

To: tridge@xxxxxxxxx
Subject: Re: Logdev size?
From: Seth Mos <knuffie@xxxxxxxxx>
Date: Fri, 27 Jul 2001 13:11:09 +0200 (CEST)
Cc: linux-xfs@xxxxxxxxxxx
In-reply-to: <20010727011601.6F2D345EA@lists.samba.org>
Sender: owner-linux-xfs@xxxxxxxxxxx
On Thu, 26 Jul 2001, Andrew Tridgell wrote:

> A related question about logdev sizes ...
> I've noticed that mkfs.xfs refuses to make a logdev bigger than
> 128M. Have anyone done any work on upping this limit? 

Not that I know off. I believe it's the upper limit.

> I'd like to put the logdev on a 1G ram card so that I can do really
> large netbench runs without hitting the real disks. I'm testing at the
> moment with the logdev on a ramdisk (see
> http://samba.org/ftp/unpacked/junkcode/trd/) but the 130M limit means
> those pesky disk lights come on with even quite small runs.

Yes. The log is a write through construction. Even if you have a 1G log
the linux kernel will put pressure on getting the data to the actual disk.
I am afraid that will be very hard to avoid. The log is only a
intermediate area where metadata gets stored before it is commited.

You might try making the fs on the 1G solid state disk. Using it just for
the logdev won't make the IO for the data go away.

> also, is there any work being done to use the same logdev for multiple
> filesystems? Having to know in advance how to split up a valuable nv
> ram area over lots of filsystems can be tricky.

That would be hard to do, reiserfs is stilling working on making wandering
logs so the log activity will be spread throughout the partition instead
of working on the same spot over and over. They don't have external logs
either untill v4.

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