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How a named cache can be too small?

3 posts in General Discussion Last posting was on 2010-09-27 21:15:35.0Z
Yaniv C. Posted on 2010-09-26 13:27:47.0Z
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From: Yaniv C.
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Subject: How a named cache can be too small?
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Hi

I have a two question regarding the following lines in the
error log:

01:00000:00075:2010/09/21 03:02:56.00 server The 16K memory
pool of named cache AdminDbData (cache id 1, cachelet id 1)
is configured too small for current demands (state 1).
Transaction progress may cease or response time may
increase.

1. What does it means?
Is it becuase there is query that needed space in the
cache and could not find place ?

Is it since the checkpoint could not keep up to remove dirty
pages to disk?

If cache works on LRU, and old pages needs to aged out, why
the
cache did not remove old pages and got the new pages?

2. Then, what?
The specific query that had no space on the cache, needs
to
do what?
wait? do I/O? is it missed query?
When the message appears,what happened to the query that
triggered this message?

Thanks,
Yaniv


Leonid Gvirtz Posted on 2010-09-27 08:07:53.0Z
From: Leonid Gvirtz <lgvirtz@yahoo.com>
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Subject: Re: How a named cache can be too small?
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Hi Yaniv

When all buffers in a cache are used and more are needed, ASE will attempt buffer replacement of any that can be
released. When none can be released, processes needing a buffer may keep retrying, resulting in the warning message
about named cache configured too small. So, the query that hit this problem will wait until sufficient number of buffers
are released from the pool and may be reused.

Why such situation may happen? I agree with you that "old" pages need to be "aged out" from the cache under LRU. But, in
some cases, it may take some time, especially if these pages are "dirty" and need to be physically written to a storage.
Of course, ASE will try to write such pages to disc in advance, when they enter the wash area. But, if the wash area is
too small then the dataserver may just not have enough time to write all pages required, especially if the storage is
overloaded. In such case, increasing of the pool in question decreases the probability of waits for available buffers in
the pool. The larger the pool and the wash area, the smaller chances we have to wait until "dirty" pages will be written
to disc.

The problem may also be caused by non-default settings of "number of oam trips" and "number of index trips"
configuration parameters. The pool may just be flooded with oam and index pages.

Hope it helps
Leonid Gvirtz
http://www.gvirtz-consulting.com

On 9/26/2010 3:27 PM, Yaniv C. wrote:
> Hi
>
> I have a two question regarding the following lines in the
> error log:
>
> 01:00000:00075:2010/09/21 03:02:56.00 server The 16K memory
> pool of named cache AdminDbData (cache id 1, cachelet id 1)
> is configured too small for current demands (state 1).
> Transaction progress may cease or response time may
> increase.
>
> 1. What does it means?
> Is it becuase there is query that needed space in the
> cache and could not find place ?
>
> Is it since the checkpoint could not keep up to remove dirty
> pages to disk?
>
> If cache works on LRU, and old pages needs to aged out, why
> the
> cache did not remove old pages and got the new pages?
>
> 2. Then, what?
> The specific query that had no space on the cache, needs
> to
> do what?
> wait? do I/O? is it missed query?
> When the message appears,what happened to the query that
> triggered this message?
>
> Thanks,
> Yaniv


Yaniv C. Posted on 2010-09-27 21:15:35.0Z
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From: Yaniv C.
Newsgroups: sybase.public.ase.general
Subject: Re: How a named cache can be too small?
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Article PK: 78845

Thanks Leonid

> Hi Yaniv
>
> When all buffers in a cache are used and more are needed,
> ASE will attempt buffer replacement of any that can be
> released. When none can be released, processes needing a
> buffer may keep retrying, resulting in the warning message
> about named cache configured too small. So, the query that
> hit this problem will wait until sufficient number of
> buffers are released from the pool and may be reused.
>
> Why such situation may happen? I agree with you that "old"
> pages need to be "aged out" from the cache under LRU. But,
> in some cases, it may take some time, especially if these
> pages are "dirty" and need to be physically written to a
> storage. Of course, ASE will try to write such pages to
> disc in advance, when they enter the wash area. But, if
> the wash area is too small then the dataserver may just
> not have enough time to write all pages required,
> especially if the storage is overloaded. In such case,
> increasing of the pool in question decreases the
> probability of waits for available buffers in the pool.
> The larger the pool and the wash area, the smaller chances
> we have to wait until "dirty" pages will be written to
> disc.
>
> The problem may also be caused by non-default settings of
> "number of oam trips" and "number of index trips"
> configuration parameters. The pool may just be flooded
> with oam and index pages.
>
> Hope it helps
> Leonid Gvirtz
> http://www.gvirtz-consulting.com
>
>
> On 9/26/2010 3:27 PM, Yaniv C. wrote:
> > Hi
> >
> > I have a two question regarding the following lines in
> > the error log:
> >
> > 01:00000:00075:2010/09/21 03:02:56.00 server The 16K
> > memory pool of named cache AdminDbData (cache id 1,
> > cachelet id 1) is configured too small for current
> > demands (state 1). Transaction progress may cease or
> > response time may increase.
> >
> > 1. What does it means?
> > Is it becuase there is query that needed space in the
> > cache and could not find place ?
> >
> > Is it since the checkpoint could not keep up to remove
> > dirty pages to disk?
> >
> > If cache works on LRU, and old pages needs to aged out,
> > why the
> > cache did not remove old pages and got the new pages?
> >
> > 2. Then, what?
> > The specific query that had no space on the cache,
> > needs to
> > do what?
> > wait? do I/O? is it missed query?
> > When the message appears,what happened to the query
> > that triggered this message?
> >
> > Thanks,
> > Yaniv
>