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Log device sizes

3 posts in Performance and Tuning Last posting was on 2007-07-27 11:21:27.0Z
David Posted on 2007-07-25 09:37:49.0Z
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Subject: Log device sizes
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Hi can any one tell me the best size for log devcices for
performance

we were looking at builf 10 Gb logs with database logs
striped accros them , however we we re informed that small
log devices were better as to avoid I/O conetntion

What are peoples opinion on this?


Mariano Corral Posted on 2007-07-26 14:05:20.0Z
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From: Mariano Corral
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Subject: Re: Log device sizes
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Very often, transactions are somewhat small and short in
duration. This is common in OLTP, not so in batch. With
short and small transactions, activity on the log is
concentrated on a small area, which usually belongs to
only a log device, even if log devices are small. In this
case, size doesn't impact performance.

Still working on the assumption of short and small
transactions,
what we do is to place all log devices for the same database
in the same disk. The rationale is that only one of them
will
be used at a point in time, so there's no benefit
distributing
them among several disks. Also, this maintains a homogeneous
activity on the disk, which simplifies the job of designing
how to allocate the devices to the available physical disks.

To desing the size of a log, the key point is stability: it
should be large enough to never get exhausted. Too large a
log is not advisable, because of the disk cost and the risk
of face a giant rollback. I mean, if a buggy program runs a
giant tran, eventually it will fill the log, fail and roll
back; better if this happens with a 1GB-log and not with a
10-GB, whose rollback will last hours.

Regards,
Mariano Corral

> Hi can any one tell me the best size for log devcices for
> performance
>
> we were looking at builf 10 Gb logs with database logs
> striped accros them , however we we re informed that small
> log devices were better as to avoid I/O conetntion
>
> What are peoples opinion on this?


A. M. Posted on 2007-07-27 11:21:27.0Z
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David wrote:
>
> Hi can any one tell me the best size for log devcices for
> performance

It depends.

> we were looking at builf 10 Gb logs with database logs
> striped accros them , however we we re informed that small
> log devices were better as to avoid I/O conetntion
>
> What are peoples opinion on this?

You need to provide a lot more information. Like how
many databases and logs you have. How busy these are.
What sort of hardware you're using in the i/o subsystem.
etc. etc. etc.

But, generally speaking, logging is just a sequential
write activity. Unless you have lots of design issues
that cause rollbacks, logs doesn't really benefit from
striped devices. The general pattern of sequential
writing means the logs are best suited to plain old
disk devices - usually on the slower part of a disk.
By all means spread these devices out across disks -
especially if you have more than one active database.
That will minimise disk head contention on one disk.

The only thing you should really do is mirror these
devices if its a production system. Make sure the
read policy is round robin. The write policy is up
to you.

-am © 2007