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Date:      Wed, 04 Feb 2009 21:57:04 +0200
From:      Manolis Kiagias <sonic2000gr@gmail.com>
To:        Hugo Silva <hugo@barafranca.com>
Cc:        freebsd-questions@freebsd.org
Subject:   Re: GEOM_JOURNAL on a 550G partition - opinions ?
Message-ID:  <4989F310.4080205@gmail.com>
In-Reply-To: <4989D8B6.80505@barafranca.com>
References:  <4989D8B6.80505@barafranca.com>

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Hugo Silva wrote:
> Hi list,
>
> For a server I will be setting up, I am considering using gjournal on
> the partition that will hold all the www data.
>
> The journaled partition (mounted async) would be mostly read from,
> uploads would not be very frequent and most sites wouldn't write to
> the disk. Logs would be kept elsewhere.
>
> This server will have two hard disks, mirrored (gmirror) at the disk
> level.
>
>
>
> Here are my questions:
>
>
> - Will the fact that gmirror is underneath the journal
> (/dev/mirror/gm0s1f.journal) affect performance ? (either positively
> or negatively)
>   (* I would be keeping the journal in the same provider)

I can only tell you this works. Have not done any real measurements on
this stuff, as most of my systems are normally not under high load. 
I've done this for a friend's SAMBA server, who is storing very large
photo files all the time.  In fact, I am just preparing our local LUG
server in exactly this way.
At least in theory gmirror can be set to balance (round-robin) reads
from the disks, so read should be improved. On the other hand, the
journaling implementation in gjournal writes everything twice, so expect
to have some significant overhead there.
Ivan Voras has done some performance testing on several filesystems,
including UFS with soft updates and journaling. See the results in this
post:

http://lists.freebsd.org/pipermail/freebsd-questions/2008-December/188131.html

>
> - Would reads / writes be faster? considerably faster ? (gjournal)
> I've seen different numbers from different places, the impression I
> got is that reads should be faster while writes will be substantially
> slower - is this correct ?
>

It seems so, at least for the writes.


> - What about reliability ? From the manpage, I know that if I
> journaled the entire mirror, I would not need to sync it after an
> unclean shutdown.
>   Going from the assumption that this will not be so for a single
> journaled partition, will there be any interference between gjournal
> and gmirror ?

I haven't had any reliability problems combining gmirror and gjournal.
To my experience, gjournal syncs the gmirror almost instantly after an
unclean shutdown.

>
>
> - I've never had an UFS2 partition filled with more than 200G of data,
> so I am not sure what to expect for 550G with soft-updates (I expect
> this partition to hold close to 550G of data) - real numbers about
> this would also be helpful.
>
>
> Any personal experiences concerning gjournal or gmirror+gjournal are
> greatly appreciated!
>

As I said, I've been using both (and combined) for quite some, and
haven't faced any problems caused by the software.  I even recovered
from a serious hardware problem, without losing any data. For
performance measuring I guess you would have
to setup a test system and see by yourself if it is acceptable.



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