linux-kernel - Re: limits on raid

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Date:	Sun, 17 Jun 2007 13:46:47 -0700 (PDT)
From:	david@...g.hm
To:	dean gaudet <dean@...tic.org>
cc:	Wakko Warner <wakko@...mx.eu.org>, Neil Brown <neilb@...e.de>,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org
Subject: Re: limits on raid

On Sun, 17 Jun 2007, dean gaudet wrote:

> On Sun, 17 Jun 2007, Wakko Warner wrote:
>
>> What benefit would I gain by using an external journel and how big would it
>> need to be?
>
> i don't know how big the journal needs to be... i'm limited by xfs'
> maximum journal size of 128MiB.
>
> i don't have much benchmark data -- but here are some rough notes i took
> when i was evaluating a umem NVRAM card.  since the pata disks in the
> raid1 have write caching enabled it's somewhat of an unfair comparison,
> but the important info is the 88 seconds for internal journal vs. 81
> seconds for external journal.

if you turn on disk write caching the difference will be much larger.

> -dean
>
> time sh -c 'tar xf /var/tmp/linux-2.6.20.tar; sync'

I know that sync will force everything to get as far as the journal, will 
it force the journal to be flushed?

David Lang

>
> xfs journal     raid5 bitmap    times
> internal        none            0.18s user 2.14s system 2% cpu 1:27.95 total
> internal        internal        0.16s user 2.16s system 1% cpu 2:01.12 total
> raid1           none            0.07s user 2.02s system 2% cpu 1:20.62 total
> raid1           internal        0.14s user 2.01s system 1% cpu 1:55.18 total
> raid1           raid1           0.14s user 2.03s system 2% cpu 1:20.61 total
> umem            none            0.13s user 2.07s system 2% cpu 1:20.77 total
> umem            internal        0.15s user 2.16s system 2% cpu 1:51.28 total
> umem            umem            0.12s user 2.13s system 2% cpu 1:20.50 total
>
>
> raid5:
> - 4x seagate 7200.10 400GB on marvell MV88SX6081
> - mdadm --create --level=5 --raid-devices=4 /dev/md4 /dev/sd[abcd]1
>
> raid1:
> - 2x maxtor 6Y200P0 on 3ware 7504
> - two 128MiB partitions starting at cyl 1
> - mdadm --create --level=1 --raid-disks=2 --auto=yes --assume-clean /dev/md1 /dev/sd[fg]1
> - mdadm --create --level=1 --raid-disks=2 --auto=yes --assume-clean /dev/md2 /dev/sd[fg]2
> - md1 is used for external xfs journal
> - md2 has an ext3 filesystem for the external md4 bitmap
>
> xfs:
> - mkfs.xfs issued before each run using the defaults (aside from -l logdev=/dev/md1)
> - mount -o noatime,nodiratime[,logdev=/dev/md1]
>
> umem:
> - 512MiB Micro Memory MM-5415CN
> - 2 partitions similar to the raid1 setup
>
-
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