----- Original Message -----
From: "Thomas Steinmaurer" <t...@iblogmanager.com>
To: "For discussion among Firebird Developers"
<firebird-devel@lists.sourceforge.net>
Sent: Tuesday, November 15, 2011 12:11 AM
Subject: Re: [Firebird-devel] Trace API - What's the unit for number of (reads
|fetches ...)
> Hi Vlad,
>
>>>>> as the subject says. What's the unit for the resulting values for number
>>>>> of reads, writes, fetches, marks? Just want to be sure if a particular
>>>>> result makes sense here. ;-)
>>>>
>>>> Same as for isql's statistics - number of operations. Note, we have
>>>> no
>>>> operations on group of pages, so, for ex. N writes means N times write
>>>> and every write is length of one page.
>>>
>>> So, the numbers I see is number of pages?
>>
>> Yes
>>
>>> I have the following trace output for a select count(*) on a table with
>>> ~100.000 records.
>>>
>>> -------------------------------------------------------------------------------
>>> select count(*) from accommodation
>>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>> PLAN (ACCOMMODATION NATURAL)
>>> 1 records fetched
>>> 81 ms, 7149 read(s), 214192 fetch(es)
>>>
>>>
>>> If it is pages, then this would mean ~ 800MB (4K page size) fetched
>>> from the cache? Does this make sense?
>>
>> Fetch is not a physical read.
>
> I know. I said fetched from the cache.
>
> This is more like reference. When Firebird need
>> access to page buffer (to get record, for ex.) it asks page cache to "fetch"
>> page
>> buffer. If this buffer is not present at page cache, it will be read from
>> disk first.
>>
>> In your example we see 7149 physical reads and 214192 times this pages
>> was
>> referenced by the engine. To read a record engine needs to access pointer
>> page and
>> (at least one) data page. You have ~100K records so we can explain ~200K
>> fetches.
>
> Ok, but is there a way then to tell how many pages have been fetched
> from the cache as the number above for fetched is more likely
> "referenced" and not real number of pages fetched from memory?
What do you understand under "fetched from memory" ? There is no operations
like
memmove or so on. Fetch is just :
- find buffer by given page number
- lock it
- return it address
Regards,
Vlad
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