Why is the default on Windows 8 K page size?

Hello

I do some work setting on our databases because the Scriptures are a bit slow:
about 100 puts / deletes per second. One thing I'm getting is setting the fill factor
as recommended in the API docs and requires knowledge of the size of the page.

I found that the system on linux and windows (x 86) page size is 4 K.
However, the BDB chooses a page size default of 4K on linux and 8K on windows.
I found this by collecting data using Db::stat_print() see Windows test code below.
I have the same code that runs on both platforms.

I was expecting that BDB would choose 4K on both platforms to match to the
System page size. The docs and "The Berkeley DB Book" say or imply
that this is the case.
http://www.Oracle.com/technology/documentation/Berkeley-DB/DB/ref/am_conf/PageSize.html
http://www.Oracle.com/technology/documentation/Berkeley-DB/DB/api_cxx/db_set_pagesize.html

Can someone explain why the default page size is 8K on windows?
Should I replace the default value and set it to 4K? I am reluctant to substitute the
page size by default without knowing why a non intuitive value.

I have read several forum posts on the page size, as well as the reference setting
Guide section, API docs, etc.

I use BDB 4.6.21 on Windows XP Professional (fully patched) and linux.
I use a DB_HASH database.

My files are fixed size: the key and the data set total 40 bytes.
If the overflow pages should not be a problem, and there should be a lot of
records per page.


#include '... /.. '. "/ db-4.6.21/build_windows/db_cxx.h".
#include < Windows.h >
#include < errno.h >

void printInfo();
DbEnv * getDatabaseEnvironment();
DB allocateNewDbHandle (const char dbName);
void printSystemPageSize();

const char * dbEnvHome = "D:\\bdb\\bdbtest\\";
const char * dbToStat = "D:\\bdb\\bdbtest\\testStat.db";

DbEnv * dbEnv = NULL;
DB * dbh = NULL;

int main (int, char *)
{
printInfo();
dbEnv = getDatabaseEnvironment();
DBH = allocateNewDbHandle (dbToStat);

printf ("\n");
int rc = dbh-> stat_print (DB_FAST_STAT);
printSystemPageSize();
}

Sub printInfo()
{
int major, minor, patch;
char * bdbVersion = DbEnv::version (major, minor, and patch);
printf ("BDB version [%s] \n", bdbVersion);
}

DbEnv * getDatabaseEnvironment()
{
int rc = 0;
u_int32_t flags = DB_CREATE | DB_THREAD | DB_INIT_CDB | DB_INIT_MPOOL;

DbEnv * env = new DbEnv (DB_CXX_NO_EXCEPTIONS);
If ((rc = env-> Ouvrir (dbEnvHome, drapeaux, 0))! = 0)
{
printf ("attempt to create the environment of BDB failed: error = %d [%s] \n",)
RC, DbEnv::strerror(rc));
env-> close (0);
Remove env;
return null;
}

return the env;
}

DB allocateNewDbHandle (const char dbName)
{
int rc = 0;
DB * theDbToOpen = new Db (dbEnv, 0);

If ((rc = theDbToOpen-> Ouvrir (, dbName, NULL, DB_HASH, DB_CREATE, 0))! = 0)
{
printf ("CANNOT open db handle error = %d [%s] \n", rc)
DbEnv::strerror(rc));
theDbToOpen-> close (0);
delete theDbToOpen;
theDbToOpen = NULL;
}

Return theDbToOpen;
}

Sub printSystemPageSize()
{
SYSTEM_INFO siSysInfo;
Memset (& siSysInfo, 0, sizeof (SYSTEM_INFO));
: GetSystemInfo(&siSysInfo);
unsigned int pageSize = siSysInfo.dwPageSize;
printf ("page size \n\nSystem = %u\n\n", pageSize);
}

Hello

We use a default 8K on Windows page size because it is not easy to recover from a specific block size.

The comment in the code to set the block size is:

        /*
         * The filesystem I/O size is not easily available.  In particular,
         * the values returned by GetDiskFreeSpace() are not very helpful
         * (NTFS volumes often report 512B clusters, which are too small to
         * be a useful default).
         */

If you know a way to reliably determine the block size of system files in Windows - we can integrate it into the code.

We choose a default value of 8K on all platforms where we do not find a specific block size. This is because it is usually better - for performance reasons - to have a writing of data in chunks larger than the block size.

If you read the reference guide on page size setting, you can probably do better than our "guess". You just configure the page size you want through the DB-> set_pagesize API.

Kind regards
Alex Gorrod, Oracle Berkeley DB

Tags: Database

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